Customizable Porphyrin Platform Enables Folate Receptor PET Imaging Using Copper-64

Folate receptors including folate receptor α (FRα) are overexpressed in up to 90% of ovarian cancers. Ovarian cancers overexpressing FRα often exhibit high degrees of drug resistance and poor outcomes. A porphyrin chassis has been developed that is readily customizable according to the desired targeting properties. Thus, compound O5 includes a free base porphyrin, two water-solubilizing groups that project above and below the macrocycle plane, and a folate targeting moiety. Compound O5 was synthesized (>95% purity) and exhibited aqueous solubility of at least 0.48 mM (1 mg/mL). Radiolabeling of O5 with 64Cu in HEPES buffer at 37 °C gave a molar activity of 1000 μCi/μg (88 MBq/nmol). [64Cu]Cu-O5 was stable in human serum for 24 h. Cell uptake studies showed 535 ± 12% bound/mg [64Cu]Cu-O5 in FRα-positive IGROV1 cells when incubated at 0.04 nM. Subcellular fractionation showed that most radioactivity was associated with the cytoplasmic (39.4 ± 2.7%) and chromatin-bound nuclear (53.0 ± 4.2%) fractions. In mice bearing IGROV1 xenografts, PET imaging studies showed clear tumor uptake of [64Cu]Cu-O5 from 1 to 24 h post injection with a low degree of liver uptake. The tumor standardized uptake value at 24 h post injection was 0.34 ± 0.16 versus 0.06 ± 0.07 in the blocking group. In summary, [64Cu]Cu-O5 was synthesized at high molar activity, was stable in serum, exhibited high binding to FRα-overexpressing cells with high nuclear translocation, and gave uptake that was clearly visible in mouse tumor xenografts.

Extension of nature's NIR-I chromophore into the NIR-II region

The development of chromophores that absorb in the near-infrared (NIR) region beyond 1000 nm underpins numerous applications in medical and energy sciences, yet also presents substantial challenges to molecular design and chemical synthesis. Here, the core bacteriochlorin chromophore of nature's NIR absorbers, bacteriochlorophylls, has been adapted and tailored by annulation in an effort to achieve absorption in the NIR-II region. The resulting bacteriochlorin, Phen2,1-BC, contains two annulated naphthalene groups spanning meso,β-positions of the bacteriochlorin and the 1,2-positions of the naphthalene. Phen2,1-BC was prepared via a new synthetic route. Phen2,1-BC is an isomer of previously examined Phen-BC, which differs only in attachment via the 1,8-positions of the naphthalene. Despite identical π-systems, the two bacteriochlorins have distinct spectroscopic and photophysical features. Phen-BC has long-wavelength absorption maximum (912 nm), oscillator strength (1.0), and S1 excited-state lifetime (150 ps) much different than Phen2,1-BC (1292 nm, 0.23, and 0.4 ps, respectively). These two molecules and an analogue with intermediate characteristics bearing annulated phenyl rings have unexpected properties relative to those of non-annulated counterparts. Understanding the distinctions requires extending concepts beyond the four-orbital-model description of tetrapyrrole spectroscopic features. In particular, a reduction in symmetry resulting from annulation results in electronic mixing of x- and y-polarized transitions/states, as well as vibronic coupling that together reduce oscillator strength of the long-wavelength absorption manifold and shorten the S1 excited-state lifetime. Collectively, the results suggest a heuristic for the molecular design of tetrapyrrole chromophores for deep penetration into the relatively unutilized NIR-II region.

On the Pelletier and Caventou (1817, 1818) papers on chlorophyll and beyond

The first use of the word 'chlorophyll' (chlorophile or chlorophyle in the French original) appeared in two papers by Pierre-Joseph Pelletier and Joseph Bienaimé Caventou, pharmacists in Paris who isolated and studied the green pigment from plants. Here, we provide English translations of their 1818 note and the slightly longer 1817 paper. Historical context is provided including a timeline of key discoveries in chlorophyll chemistry pertaining to photosynthesis.

Tolyporphins-Exotic Tetrapyrrole Pigments in a Cyanobacterium-A Review

Tolyporphins were discovered some 30 years ago as part of a global search for antineoplastic compounds from cyanobacteria. To date, the culture HT-58-2, comprised of a cyanobacterium-microbial consortium, is the sole known producer of tolyporphins. Eighteen tolyporphins are now known-each is a free base tetrapyrrole macrocycle with a dioxobacteriochlorin (14), oxochlorin (3), or porphyrin (1) chromophore. Each compound displays two, three, or four open β-pyrrole positions and two, one, or zero appended C-glycoside (or -OH or -OAc) groups, respectively; the appended groups form part of a geminal disubstitution motif flanking the oxo moiety in the pyrroline ring. The distinct structures and repertoire of tolyporphins stand alone in the large pigments-of-life family. Efforts to understand the cyanobacterial origin, biosynthetic pathways, structural diversity, physiological roles, and potential pharmacological properties of tolyporphins have attracted a broad spectrum of researchers from diverse scientific areas. The identification of putative biosynthetic gene clusters in the HT-58-2 cyanobacterial genome and accompanying studies suggest a new biosynthetic paradigm in the tetrapyrrole arena. The present review provides a comprehensive treatment of the rich science concerning tolyporphins.

Synthesis of a BC-Dihydrodipyrrin Building Block of Bacteriochlorophyll a

A strategy for the synthesis of bacteriochlorophyll a relies on joining AD and BC halves that contain the requisite stereochemical configurations of the target macrocycle. The BC half (1) is a dihydrodipyrrin bearing a dimethoxymethyl group at the 1-position, a β-ketoester at the 8-position, and (R)-2-methyl and (R)-3-ethyl substituents in the pyrroline ring. An established route to AD-dihydrodipyrrins (Pd-mediated coupling of a 2-halopyrrole with a chiral 4-pentynoic acid followed by Petasis methenylation, acidic hydrolysis, Paal-Knorr ring closure, and Riley oxidation) proved to be unviable for BC-dihydrodipyrrins given the presence of the β-ketoester unit. A route presented here entails Pd-mediated coupling of a 2-halopyrrole (2) with (3R,4R)-4-ethyl-1,1-dimethoxy-3-methylhex-5-yn-2-one (3), anti-Markovnikov hydration of the alkyne to give the 1,4-diketone, and Paal-Knorr ring closure. Compound 3 was prepared by Schreiber-modified Nicholas reaction beginning with (S)-4-isopropyl-3-propionyloxazolidin-2-one and the hexacarbonyldicobalt complex of (±) 3-methoxy-1-(trimethylsilyl)pentyne followed by transformation of the aldehyde derived therefrom to the 1,1-dimethoxymethylcarbonyl motif. The absolute stereochemical configuration of the Schreiber-Nicholas alkylation product was confirmed by single-crystal X-ray diffraction, whereas the BC half (1) by 1H NMR spectroscopy showed a J value of 2.9 Hz consistent with the trans-configuration. Taken together, the route provides a key chiral building block for the synthesis of photosynthetic tetrapyrroles and analogues.

Tethered Indoxyl-Glucuronides for Enzymatically Triggered Cross-Linking

Indoxyl-glucuronides, upon treatment with β-glucuronidase under physiological conditions, are well known to afford the corresponding indigoid dye via oxidative dimerization. Here, seven indoxyl-glucuronide target compounds have been prepared along with 22 intermediates. Of the target compounds, four contain a conjugatable handle (azido-PEG, hydroxy-PEG, or BCN) attached to the indoxyl moiety, while three are isomers that include a PEG-ethynyl group at the 5-, 6-, or 7-position. All seven target compounds have been examined in indigoid-forming reactions upon treatment with β-glucuronidase from two different sources and rat liver tritosomes. Taken together, the results suggest the utility of tethered indoxyl-glucuronides for use in bioconjugation chemistry with a chromogenic readout under physiological conditions.

Phyllobilins - Bioactive Natural Products Derived from Chlorophyll - Plant Origins, Structures, Absorption Spectra, and Biomedical Properties

Phyllobilins are open-chain products of the biological degradation of chlorophyll a in higher plants. Recent studies reveal that phyllobilins exert anti-oxidative and anti-inflammatory properties, as well as activities against cancer cells, that contribute to the human health benefits of numerous plants. In general, phyllobilins have been overlooked in phytochemical analyses, and - more importantly - in the analyses of medicinal plant extracts. Nevertheless, over the past three decades, > 70 phyllobilins have been identified upon examination of more than 30 plant species. Eight distinct chromophoric classes of phyllobilins are known: phyllolumibilins (PluBs), phylloleucobilins (PleBs), phylloxanthobilins (PxBs), and phylloroseobilins (PrBs)-each in type-I or type-II groups. Here, we present a database of absorption and fluorescence spectra that has been compiled of 73 phyllobilins to facilitate identification in phytochemical analyses. The spectra are provided in digital form and can be viewed and downloaded at www.photochemcad.com. The present review describes the plant origin, molecular structure, and absorption and fluorescence features of the 73 phyllobilins, along with an overview of key medicinal properties. The review should provide an enabling tool for the community for the straightforward identification of phyllobilins in plant extracts, and the foundation for deeper understanding of these ubiquitous but underexamined plant-derived micronutrients for human health.

Digital Database of Absorption Spectra of Diverse Flavonoids Enables Structural Comparisons and Quantitative Evaluations

Flavonoids play diverse roles in plants, comprise a non-negligible fraction of net primary photosynthetic production, and impart beneficial effects in human health from a plant-based diet. Absorption spectroscopy is an essential tool for quantitation of flavonoids isolated from complex plant extracts. The absorption spectra of flavonoids typically consist of two major bands, band I (300-380 nm) and band II (240-295 nm), where the former engenders a yellow color; in some flavonoids the absorption tails to 400-450 nm. The absorption spectra of 177 flavonoids and analogues of natural or synthetic origin have been assembled, including molar absorption coefficients (109 from the literature, 68 measured here). The spectral data are in digital form and can be viewed and accessed at http://www.photochemcad.com. The database enables comparison of the absorption spectral features of 12 distinct types of flavonoids including flavan-3-ols (e.g., catechin, epigallocatechin), flavanones (e.g., hesperidin, naringin), 3-hydroxyflavanones (e.g., taxifolin, silybin), isoflavones (e.g., daidzein, genistein), flavones (e.g., diosmin, luteolin), and flavonols (e.g., fisetin, myricetin). The structural features that give rise to shifts in wavelength and intensity are delineated. The availability of digital absorption spectra for diverse flavonoids facilitates analysis and quantitation of these valuable plant secondary metabolites. Four examples are provided of calculations─multicomponent analysis, solar ultraviolet photoprotection, sun protection factor (SPF), and Förster resonance energy transfer (FRET)─for which the spectra and accompanying molar absorption coefficients are sine qua non.

Porphyrin building blocks bearing two or four divergent ethynes

Tetrapyrrole building blocks are invaluable constituents in the construction of molecular architectures for use in biomimicry, functional materials, and biomedicine. The reaction of dipyrromethane and the triisopropylsilyl-protected 3,5-diethynylbenzaldehyde afforded the corresponding trans-A2-porphyrin (free base) bearing four ethynes. Subsequent meso-bromination, Suzuki coupling, and protecting group removal afforded a porphyrin building block bearing four ethynes and one benzylamine. The reaction of dipyrromethane and 3,5-bis(propargyloxy)benzaldehyde afforded the corresponding trans-A2-porphyrin (free base) bearing four ethynes. The reaction of 5-(3,5-bis(propargyloxy)phenyl)dipyrromethane and the Eschenmoser (1,9-dimethylaminomethyl) derivative of a 5-([Formula: see text]-substituted aryl)dipyrromethane was used to create two trans-AB-porphyrins (zinc chelates). The [Formula: see text]-substituent of the aryl group was cyano or an acetal moiety. Hydrolysis of the acetal and a click reaction with m-PEG24-azide gave the bis(PEGylated)porphyrin-carboxaldehyde. The porphyrins present readily derivatizable functional groups in a compact architecture.

Four Routes to 3-(3-Methoxy-1,3-dioxopropyl)pyrrole, a Core Motif of Rings C and E in Photosynthetic Tetrapyrroles

The photosynthetic tetrapyrroles share a common structural feature comprised of a β-ketoester motif embedded in an exocyclic ring (ring E). As part of a total synthesis program aimed at preparing native structures and analogues, 3-(3-methoxy-1,3-dioxopropyl)pyrrole was sought. The pyrrole is a precursor to analogues of ring C and the external framework of ring E. Four routes were developed. Routes 1-3 entail a Pd-mediated coupling process of a 3-iodopyrrole with potassium methyl malonate, whereas route 4 relies on electrophilic substitution of TIPS-pyrrole with methyl malonyl chloride. Together, the four routes afford considerable latitude. A long-term objective is to gain the capacity to create chlorophylls and bacteriochlorophylls and analogues thereof by facile de novo means for diverse studies across the photosynthetic sciences.

Investigation of a bacteriochlorin-containing pentad array for panchromatic light-harvesting and charge separation

A new pentad array designed to exhibit panchromatic absorption and charge separation has been synthesized and characterized. The array is composed of a triad panchromatic absorber (a bis(perylene-monoimide)-porphyrin) to which are appended an electron acceptor (perylene-diimide) and an electron donor/hole acceptor (bacteriochlorin) in a crossbar arrangement. The motivation for incorporation of the bacteriochlorin versus a free-base or zinc chlorin utilized in prior constructs was to facilitate hole transfer to this terminal unit and thereby achieve a higher yield of charge separation across the array. The intense S₀ → S₁ (Q_y) band of the bacteriochlorin also enhances absorption in the near-infrared spectral region. Due to synthetic constraints, a phenylethyne linker was used to join the bacteriochlorin to the core porphyrin of the panchromatic triad rather than the diphenylethyne linker employed for the prior chlorin-containing pentads. Static and time-resolved photophysical studies reveal enhanced excited-state quenching for the pentad in benzonitrile and dimethyl sulfoxide compared to the prior chlorin-containing analogues. Success was only partial, however, as a long-lived charge separated state was not observed despite the improved energetics for the final ground-state hole/electron-shift reaction. The apparent reason is more facile competing charge-recombination due to the shorter bacteriochlorin - porphyrin linker that increases electronic coupling for this process. The studies highlight design criteria for balancing panchromatic absorption and long-lived charge separation in molecular architectures for solar-energy conversion.

Dyads with tunable near-infrared donor-acceptor excited-state energy gaps: molecular design and Förster analysis for ultrafast energy transfer

Bacteriochlorophylls, nature's near-infrared absorbers, play an essential role in energy transfer in photosynthetic antennas and reaction centers. To probe energy-transfer processes akin to those in photosynthetic systems, nine synthetic bacteriochlorin-bacteriochlorin dyads have been prepared wherein the constituent pigments are joined at the meso-positions by a phenylethyne linker. The phenylethyne linker is an unsymmetric auxochrome, which differentially shifts the excited-state energies of the phenyl- or ethynyl-attached bacteriochlorin constituents in the dyad. Molecular designs utilized known effects of macrocycle substituents to engineer bacteriochlorins with S₀ → S₁ (Q_y) transitions spanning 725-788 nm. The design-predicted donor-acceptor excited-state energy gaps in the dyads agree well with those obtained from time dependent density functional theory calculations and with the measured range of 197-1089 cm^-1. Similar trends with donor-acceptor excited-state energy gaps are found for (1) the measured ultrafast energy-transfer rates of (0.3-1.7 ps)^-1, (2) the spectral overlap integral (J) in Förster energy-transfer theory, and (3) donor-acceptor electronic mixing manifested in the natural transition orbitals for the S₀ → S₁ transition. Subtle outcomes include the near orthogonal orientation of the π-planes of the bacteriochlorin macrocycles, and the substituent-induced shift in transition-dipole moment from the typical coincidence with the NH-NH axis; the two features together afforded the Förster orientation term κ² ranging from 0.55-1.53 across the nine dyads, a value supportive of efficient excited-state energy transfer. The molecular design and collective insights on the dyads are valuable for studies relevant to artificial photosynthesis and other processes requiring ultrafast energy transfer.

Balancing Panchromatic Absorption and Multistep Charge Separation in a Compact Molecular Architecture

A panchromatic triad and a charge-separation unit are joined in a crossbar architecture to capture solar energy. The panchromatic-absorber triad (T) is comprised of a central free-base porphyrin that is strongly coupled via direct ethyne linkages to two perylene-monoimide (PMI) groups. The charge-separation unit incorporates a free-base or zinc chlorin (C or ZnC) as a hole acceptor (or electron donor) and a perylene-diimide (PDI) as an electron acceptor, both attached to the porphyrin via diphenylethyne linkers. The free-base porphyrin is common to both light-harvesting and charge-separation motifs. The chlorin and PDI also function as ancillary light absorbers, complementing direct excitation of the panchromatic triad to produce the discrete lowest excited state of the array (T*). Attainment of full charge separation across the pentad entails two steps: (1) an initial excited-state hole/electron-transfer process to oxidize the chlorin (and reduce the panchromatic triad) or reduce the PDI (and oxidize the panchromatic triad); and (2) subsequent ground-state electron/hole migration to produce oxidized chlorin and reduced PDI. Full charge separation for pentad ZnC-T-PDI to generate ZnC⁺-T-PDI^- occurs with a quantum yield of ∼30% and mean lifetime ∼1 μs in dimethyl sulfoxide. For C-T-PDI, initial charge separation is followed by rapid charge recombination. The molecular designs and studies reported here reveal the challenges of balancing the demands for charge separation (linker length and composition, excited-state energies, redox potentials, and medium polarity) with the constraints for panchromatic absorption (strong electronic coupling of the porphyrin and two PMI units) for integrated function in solar-energy conversion.

Tailoring the AIE Chromogen 2-(2-Hydroxyphenyl)benzothiazole for Use in Enzyme-Triggered Molecular Brachytherapy

A targeted strategy for treating cancer is antibody-directed enzyme prodrug therapy, where the enzyme attached to the antibody causes conversion of an inactive small-molecule prodrug into an active drug. A limitation may be the diffusion of the active drug away from the antibody target site. A related strategy with radiotherapeutics entails enzymatically promoted conversion of a soluble to insoluble radiotherapeutic agent, thereby immobilizing the latter at the target site. Such a molecular brachytherapy has been scarcely investigated. In distinct research, the advent of molecular designs for aggregation-induced emission (AIE) suggests translational use in molecular brachytherapy. Here, several 2-(2-hydroxyphenyl)benzothiazole substrates that readily aggregate in aqueous solution (and afford AIE) were elaborated in this regard. In particular, (1) the 2-(2-hydroxyphenyl) unit was derivatized to bear a pegylated phosphodiester that imparts water solubility yet undergoes enzymatic cleavage, and (2) a p-phenol unit was attached to the benzo moiety to provide a reactive site for final-step iodination (here examined with natural abundance iodide). The pegylated phosphodiester-iodinated benzothiazole undergoes conversion from aqueous-soluble to aqueous-insoluble upon treatment with a phosphatase or phosphodiesterase. The aggregation is essential to molecular brachytherapy, whereas the induced emission of AIE is not essential but provides a convenient basis for research development. Altogether, 21 compounds were synthesized (18 new, 3 known via new routes). Taken together, blending biomedical strategies of enzyme prodrug therapy with materials chemistry concerning substances that undergo AIE may comprise a step forward on the long road toward molecular brachytherapy.

Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment

The syntheses of two triads are reported. Each triad is composed of two perylene-monoimides linked to a porphyrin via an ethyne unit, which bridges the perylene 9-position and a porphyrin 5- or 15-position. Each triad also contains a single tether composed of an alkynoic acid or an isophthalate unit. Each triad provides panchromatic absorption (350–700 nm) with fluorescence emission in the near-infrared region (733 or 743 nm; fluorescence quantum yield ~0.2). The syntheses rely on the preparation of trans-AB-porphyrins bearing one site for tether attachment (A), an aryl group (B), and two open meso-positions. The AB-porphyrins were prepared by the condensation of a 1,9-diformyldipyrromethane and a dipyrromethane. The installation of the two perylene-monoimide groups was achieved upon the 5,15-dibromination of the porphyrin and the subsequent copper-free Sonogashira coupling, which was accomplished before or after the attachment of the tether. The syntheses provide relatively straightforward access to a panchromatic absorber for use in bioconjugation or surface-attachment processes.

Probing the Effects of Electronic-Vibrational Resonance on the Rate of Excited-State Energy Transfer in Bacteriochlorin Dyads

The impact of vibrational-electronic resonances on the rate of excited-state energy transfer is examined in a set of bacteriochlorin dyads that employ the same phenylethyne linker. The donor/acceptor excited-state energy gap is tuned from ∼200 to ∼1100 cm^-1 using peripheral substituents on the donor and acceptor bacteriochlorin macrocycles. Ultrafast energy transfer is observed with rate constants of (0.3 ps)^-1 to (1.7 ps)^-1, which agree with those predicted by Förster theory to within a factor of 2. Furthermore, the measured rates follow a trend-line with only small deviations that do not correlate with the density of vibrations at the donor/acceptor excited-state energy gap. Thus, if vibrational-electronic resonances occur in any of these dyads, which seems likely, the impact on the rate of energy transfer is small.

Simple Dipyrrin Analogues of Prodigiosin for Use as Colistin Adjuvants

Multidrug resistant (MDR) bacteria are an increasing public health problem. One promising alternative to the development of new antibiotics is the use of antibiotic adjuvants, which would allow the continued use of FDA-approved antibiotics that have been rendered ineffective due to resistance. Herein, we report a series of dipyrrins and pyrrole derivatives designed as analogues of prodigiosin and obatoclax, several of which potentiate the activity of colistin against Klebsiella pneumoniae, with lead compounds also potentiating colistin against Acinetobacter baumannii and Pseudomonas aeruginosa.

Syntheses and Properties of Metalated Tetradehydrocorrins

The monoanionic tetrapyrrolic macrocycle B,C-tetradehydrocorrin (TDC) resides chemically between corroles and corrins. This chemical space remains largely unexplored due to a lack of reliable synthetic strategies. We now report the preparation and characterization of Co(II)- and Ni(II)-metalated TDC derivatives ([Co-TDC]⁺ and [Ni-TDC]⁺, respectively) with a combination of crystallographic, electrochemical, computational, and spectroscopic techniques. [Ni-TDC]⁺ was found to undergo primarily ligand-centered electrochemical reduction, leading to hydrogenation of the macrocycle under cathodic electrolysis in the presence of acid. Transient absorption (TA) spectroscopy reveals that [Ni-TDC]⁺ and the two-electron-reduced [Ni-TDC]^- possess long-lived excited states, whereas the excited state of singly reduced [Ni-TDC] exhibits picosecond dynamics. The Co(I) compound [Co-TDC] is air stable, highlighting the notable property of the TDC ligand to stabilize low-valent metal centers in contradistinction to other tetrapyrroles such as corroles, which typically stabilize metals in higher oxidation states.

Synthesis of bacteriochlorins bearing diverse β-substituents

Eleven bacteriochlorins have been prepared for surface attachment, bioconjugation, water-solubilization, vibrational studies, and elaboration into multichromophore arrays.

Meso bromination and derivatization of synthetic bacteriochlorins

Twelve bacteriochlorin building blocks featuring meso-substitution have been prepared including a set with finely tuned long-wavelength absorption (725–757 nm) for studies in photonics.

Electronic Structure and Excited-State Dynamics of Rylene-Tetrapyrrole Panchromatic Absorbers

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (τ_S) and increased fluorescence quantum yield (Φ_f) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a π-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Φ_f and shortened τ_S values. The PMI-P-TMI triad in toluene exhibits Φ_f = 0.038 and τ_S = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φ_f = 0.26 and τ_S = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.

A perspective on the redox properties of tetrapyrrole macrocycles

Tetrapyrrole macrocycles serve a multitude of roles in biological systems, including oxygen transport by heme and light harvesting and charge separation by chlorophylls and bacteriochlorophylls. Synthetic tetrapyrroles are utilized in diverse applications ranging from solar-energy conversion to photomedicine. Nevertheless, students beginning tetrapyrrole research, as well as established practitioners, are often puzzled when comparing properties of related tetrapyrroles. Questions arise as to why optical spectra of two tetrapyrroles often shift in wavelength/energy in a direction opposite to that predicted by common chemical intuition based on the size of a π-electron system. Gouterman's four-orbital model provides a framework for understanding these optical properties. Similarly, it can be puzzling as to why the oxidation potentials differ significantly when comparing two related tetrapyrroles, yet the reduction potentials change very little or shift in the opposite direction. In order to understand these redox properties, it must be recognized that structural/electronic alterations affect the four frontier molecular orbitals (HOMO, LUMO, HOMO-1 and LUMO+1) unequally and in many cases the LUMO+1, and not the LUMO, may track the HOMO in energy. This perspective presents a fundamental framework concerning tetrapyrrole electronic properties that should provide a foundation for rational molecular design in tetrapyrrole science.

Synthesis of AD-Dihydrodipyrrins Equipped with Latent Substituents of Native Chlorophylls and Bacteriochlorophylls

Native chlorophylls and bacteriochlorophylls share a common trans-substituted pyrroline ring D (17-propionic acid, 18-methyl), whereas diversity occurs in ring A particularly at the 3-position. Two dihydrodipyrrins equipped with native-like D-ring substituents and tailorable A-ring substituents have been synthesized. The synthesis relies on a Schreiber-modified Nicholas reaction to construct the stereochemically defined precursor to ring D, a dialkyl-substituted pent-4-ynoic acid. The carboxylic acid group of the intact propionic acid proved unworkable, whereupon protected propionate (-CO₂^tBu) and several latent propyl ethers were examined. The tert-butyldiphenylsilyl-protected propanol substituent proved satisfactory for reaction of the chiral N-acylated oxazolidinone, affording (2S,3S)-2-(3-((tert-butyldiphenylsilyl)oxy)propyl)-3-methylpent-4-ynoic acid in ∼30% yield over 8 steps. Two variants for ring A, 2-tert-butoxycarbonyl-3-Br/H-5-iodo-4-methylpyrrole, were prepared via the Barton-Zard route. Dihydrodipyrrin formation from the pyrrole and pentynoic acid entailed Jacobi Pd-mediated lactone formation, Petasis methenylation, and Paal-Knorr-type pyrroline formation. The two AD-dihydrodipyrrins bear the D-ring methyl and protected propanol groups with a stereochemical configuration identical to that of native (bacterio)chlorophylls, and a bromine or no substitution in ring A corresponding to the 3-position of (bacterio)chlorophylls. The analogous β-position of a lactone-pyrrole intermediate on the path to the dihydrodipyrrin also was successfully brominated, opening opportunities for late-stage diversification in the synthesis of (bacterio)chlorophylls.

Identification of Putative Biosynthetic Gene Clusters for Tolyporphins in Multiple Filamentous Cyanobacteria

Tolyporphins A-R are unusual tetrapyrrole macrocycles produced by the non-axenic filamentous cyanobacterium HT-58-2. A putative biosynthetic gene cluster for biosynthesis of tolyporphins (here termed BGC-1) was previously identified in the genome of HT-58-2. Here, homology searching of BGC-1 in HT-58-2 led to identification of similar BGCs in seven other filamentous cyanobacteria, including strains Nostoc sp. 106C, Nostoc sp. RF31YmG, Nostoc sp. FACHB-892, Brasilonema octagenarum UFV-OR1, Brasilonema octagenarum UFV-E1, Brasilonema sennae CENA114 and Oculatella sp. LEGE 06141, suggesting their potential for tolyporphins production. A similar gene cluster (BGC-2) also was identified unexpectedly in HT-58-2. Tolyporphins BGCs were not identified in unicellular cyanobacteria. Phylogenetic analysis based on 16S rRNA and a common component of the BGCs, TolD, points to a close evolutionary history between each strain and their respective tolyporphins BGC. Though identified with putative tolyporphins BGCs, examination of pigments extracted from three cyanobacteria has not revealed the presence of tolyporphins. Overall, the identification of BGCs and potential producers of tolyporphins presents a collection of candidate cyanobacteria for genetic and biochemical analysis pertaining to these unusual tetrapyrrole macrocycles.

Fluorescence Assay for Tolyporphins Amidst Abundant Chlorophyll in Crude Cyanobacterial Extracts

Tolyporphins are distinctive tetrapyrrole natural products found singularly in a filamentous cyanobacterial-microbial holobiont (termed HT-58-2) from Micronesia. The absorption and fluorescence features of tolyporphins resemble those of chlorophyll a, complicating direct analysis of culture samples. Treatment of the crude (unfractionated) organic extract (CH₂ Cl₂ /2-propanol, 1:1) of HT-58-2 cultures with NaBH₄ in methanol causes reduction of the peripheral ketone auxochromes, whereupon tolyporphins (predominantly 7,17-dioxobacteriochlorins) exhibit a bathochromic shift (λ_abs ~ 676 → ~ 700 nm) and chlorophyll a (a 13¹ -oxochlorin) exhibits a hypsochromic shift (λ_abs 665 → 634 nm). Fluorescence excitation spectroscopy (at 368 and 491 nm with λ_em 710 nm) enabled detection of reduced tolyporphins amidst abundant reduced chlorophyll a (1:19 ratio), a detection sensitivity >5 times that without reduction. The resulting assay combines simple sample preparation from non-axenic cultures at microscale quantities (2 mL, 2 μm), absence of any fractionation procedures, and fluorescence detection. Tolyporphins were readily detected in cultures of HT-58-2 at reasonable growth periods in the absence of environmental stressors, which was not possible previously.

In Situ, Protein-Mediated Generation of a Photochemically Active Chlorophyll Analogue in a Mutant Bacterial Photosynthetic Reaction Center

All possible natural amino acids have been substituted for the native LeuL185 positioned near the B-side bacteriopheophytin (H_B) in the bacterial reaction center (RC) from Rhodobacter sphaeroides. Additional mutations that enhance electron transfer to the normally inactive B-side cofactors are present. Approximately half of the isolated RCs with Glu at L185 contain a magnesium chlorin (C_B) in place of H_B. The chlorin is not the common BChl a oxidation product 3-desvinyl-3-acetyl chlorophyll a with a C-C bond in ring D and a C═C bond in ring B but has properties consistent with reversal of these bond orders, giving 17,18-didehydro BChl a. In such RCs, charge-separated state P⁺C_B^- forms in ∼5% yield. The other half of the GluL185-containing RCs have a bacteriochlorophyll a (BChl a) denoted β_B in place of H_B. Residues His, Asp, Asn, and Gln at L185 yield RCs with ≥85% β_B in the H_B site, while most other amino acids result in RCs that retain H_B (≥95%). To the best of our knowledge, neither bacterial RCs that harbor five BChl a molecules and one chlorophyll analogue nor those with six BChl a molecules have been reported previously. The finding that altering the local environment within a cofactor binding site of a transmembrane complex leads to in situ generation of a photoactive chlorin with an unusual ring oxidation pattern suggests new strategies for amino acid control over pigment type at specific sites in photosynthetic proteins.

Tolyporphins A–R, unusual tetrapyrrole macrocycles in a cyanobacterium from Micronesia, assessed quantitatively from the culture HT-58-2

Tolyporphins A–R are the newest additions to the family of native tetrapyrroles. LC-MS-dMRM and absorption spectroscopy have been employed for analysis of mixtures containing the 18 distinctive natural products.

Synthesis of model bacteriochlorophylls containing substituents of native rings A, C and E

An established route to the bacteriochlorophyll skeleton from two dihydrodipyrrin halves has been extended to accommodate several substituents characteristic of the native bacteriochlorophyll a.

Study of conditions for streamlined assembly of a model bacteriochlorophyll from two dihydrodipyrrin halves

Knoevenagel condensation followed by double-ring closure (Nazarov cyclization, electrophilic aromatic substitution, elimination of methanol) and optional zinc insertion smoothly afford models of the native bacteriochlorophylls.

Considerations of the biosynthesis and molecular diversity of tolyporphins

Tolyporphins, relatively new members of the pigments of life family found in a cyanobacterium, differ in the chromophores, pyrroline substituents, and stereochemistry, yet likely all derive from uroporphyrinogen III.

Absorption and Fluorescence Spectral Database of Chlorophylls and Analogues

Absorption spectra and fluorescence spectra are essential for use across the photosciences, yet such spectra along with the all-important values for molar absorption coefficient (ε) and fluorescence quantum yield (Φ_f ) often are found with great difficulty. Here, a literature survey concerning the vital class of chlorophyll compounds has led to identification of spectra for 150 members. Spectra in print form have been digitized (with baseline corrections) and assembled into a database along with literature references, solvent identity and values for ε and Φ_f (where available). The database encompasses photosynthetic tetrapyrroles wherein the chromophore is a porphyrin (e.g. chlorophyll c₁ , protochlorophyll a), chlorin (e.g. chlorophyll a, bacteriochlorophyll c) or bacteriochlorin (e.g. bacteriochlorophyll a). Altogether, the database contains 305 absorption spectra (from 19 porphyrins, 109 chlorins and 22 bacteriochlorins) and 72 fluorescence spectra (from 10 porphyrins, 30 chlorins and 4 bacteriochlorins). The spectral database should facilitate comparisons and quantitative calculations. All spectra are available in print form in the Supporting Information. The entire database in digital form is available with the PhotochemCAD program for free downloading and further use at http://www.photochemcad.com.

Photophysical Properties and Electronic Structure of Zinc(II) Porphyrins Bearing 0-4 meso-Phenyl Substituents: Zinc Porphine to Zinc Tetraphenylporphyrin (ZnTPP)

Six zinc(II) porphyrins bearing 0-4 meso-phenyl substituents have been examined spectroscopically and theoretically. Comparisons with previously examined free base analogues afford a deep understanding of the electronic and photophysical effects of systematic addition of phenyl groups in porphyrins containing a central zinc(II) ion versus two hydrogen atoms. Trends in the wavelengths and relative intensities of the absorption bands are generally consistent with predictions from time-dependent density functional theory calculations and simulations from Gouterman's four-orbital model. These trends derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital. The calculations reveal additional insights, such as a progressive increase in oscillator strength in the violet-red (B-Q) absorption manifold with increasing number of phenyls. Progressive addition of 0-4 phenyl substituents to the zinc porphyrins in O₂-free toluene engenders a reduction in the measured lifetime of the lowest singlet excited state (2.5-2.1 ns), an increase in the S₁ → S₀ fluorescence yield (0.022-0.030), a decrease in the yield of S₁ → T₁ intersystem crossing (0.93-0.88), and an increase in the yield of S₁ → S₀ internal conversion (0.048-0.090). The derived rate constants for S₁ decay reveal significant differences in the photophysical properties of the zinc chelates versus free base forms. The unexpected finding of a larger rate constant for internal conversion for zinc chelates versus free bases is particularly exemplary. Collectively, the findings afford fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins, which are widely used as benchmarks for tetrapyrrole-based architectures in solar energy and life sciences research.

Fourfold alkyl wrapping of a copper(II) porphyrin thwarts macrocycle π-π stacking in a compact supramolecular package

Porphyrins are valuable constituents in optoelectronic, catalytic, and other applications, yet control of intermolecular π-π stacking is invariably essential to attain the desired properties. Superstructures built onto the porphyrin, often via meso-aryl groups, can afford facial encumbrance that suppresses π-π stacking, although some molecular designs have provided insufficient facial coverage and many have entailed cumbersome syntheses. In this study, a copper(II) porphyrin bearing four meso substituents, namely, {10,20-bis[2,6-bis(octyloxy)phenyl]-5,15-dibromoporphinato}copper(II), [Cu(C₆₄H₈₂Br₂N₄O₄)], was prepared by metalation of the corresponding free-base porphyrin and was characterized by single-crystal X-ray diffraction. The crystal structure reveals a dihedral angle of 111.1 (2)° for the plane of the meso-aryl group relative to the plane of the porphyrin, with both aryl groups tilted in the same direction. Each of the four octyloxy groups exhibits a gauche conformation for the -OCH₂CH₂- unit but is extended with four or five anti (-CH₂CH₂-/H) conformations thereafter, causing each octyl group to span the dimension of the macrocycle. In a global frame of reference where the two Br atoms define the north/south poles and the two aryl groups are at antipodes on the equator, two octyl groups of one aryl unit project over the northern hemisphere (covering pyrroles A and B), whereas those of the other aryl unit project over the southern hemisphere (covering pyrroles C and D). Together, the four octyl groups ensheath the two faces of the porphyrin in a self-wrapped assembly. The closest approach of the Cu atom to an octyl methylene C atom (position 6) is 3.5817 (18) Å, the mean separations of neighboring porphyrin planes are 8.059 (4) and 4.693 (8) Å along the a and c axes, respectively, and the center-to-center distances between the Cu atoms of neighboring porphyrins are 10.2725 (4), 12.2540 (6), and 12.7472 (6) Å along the a, b, and c axes, respectively. The Hirshfeld surface analysis and two-dimensional (2D) fingerprint plots provide information concerning contact interactions in the supramolecular assembly of the solid crystal.

Riley Oxidation of Heterocyclic Intermediates on Paths to Hydroporphyrins-A Review

Riley oxidation of advanced heterocyclic intermediates (dihydrodipyrrins and tetrahydrodipyrrins) is pivotal in routes to synthetic hydroporphyrins including chlorins, bacteriochlorins, and model (bacterio)chlorophylls. Such macrocycles find wide use in studies ranging from energy sciences to photomedicine. The key transformation (–CH₃ → –CHO) is often inefficient, however, thereby crimping the synthesis of hydroporphyrins. The first part of the review summarizes 12 representative conditions for Riley oxidation across diverse (non-hydrodipyrrin) substrates. An interlude summarizes the proposed mechanisms and provides context concerning the nature of various selenium species other than SeO₂. The second part of the review comprehensively reports the conditions and results upon Riley oxidation of 45 1-methyltetrahydrodipyrrins and 1-methyldihydrodipyrrins. A comparison of the results provides insights into the tolerable structural features for Riley oxidation of hydrodipyrrins. In general, Riley oxidation of dihydrodipyrrins has a broad scope toward substituents, but proceeds in only modest yield. Too few tetrahydrodipyrrins have been examined to draw conclusions concerning scope. New reaction conditions or approaches will be required to achieve high yields for this critical transformation in the synthesis of hydroporphyrins.

Bioconjugatable synthetic chlorins rendered water-soluble with three PEG-12 groups via click chemistry

Chlorins provide many ideal features for use as red-region fluorophores but require molecular tailoring for solubilization in aqueous solution. A chlorin building-block bearing 18,18-dimethyl, 15-bromo and 10-[2,4,6-tris(propargyloxy)phenyl] substituents has been transformed via click chemistry with CH3(OCH2CH[Formula: see text]-N3 followed by Suzuki coupling with 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)propanoic acid, thereby installing a water-solubilization motif and a bioconjugatable handle, respectively. In toluene, [Formula: see text]-dimethylformamide (DMF) or water, the resulting facially encumbered free base chlorin exhibits characteristic chlorin absorption ([Formula: see text] [Formula: see text]412, 643 nm) and fluorescence ([Formula: see text] [Formula: see text]645 nm) spectra with only modest variation in fluorescence quantum yield ([Formula: see text] values (0.24, 0.25 and 0.19, respectively). The zinc chlorin derived therefrom exhibits similar spectral constancy ([Formula: see text] [Formula: see text]414 and 613 nm, [Formula: see text] [Formula: see text]616 nm) and [Formula: see text] 0.094, 0.10 and 0.086 in the three solvents. The results together indicate the viability of the molecular design and synthetic methodology to create red-region fluorophores for use in diverse applications.

Bacteriochlorin-bis(spermine) conjugate affords an effective photodynamic action to eradicate microorganisms

A novel bacteriochlorin bearing two spermine units (BCS) was synthesized from 3,13-dibromo-8,8,18,18-tetramethylbacteriochlorin (BC-Br ^3,13 ). The synthesis involved the Suzuki coupling of BC-Br ^3,13 to obtain a bacteriochlorin-dibenzaldehyde (BCA), which was subjected to reductive amination with spermine. The resulting bacteriochlorin BCS presents a strong near-infrared absorption band at 747 nm, emits at 750 nm with fluorescence quantum yield of 0.14, and generates singlet molecular oxygen, O₂ (¹ Δ_g ), with a quantum yield of 0.27. Photokilling capacities mediated by BCS were evaluated in microbial cells. The viability of Staphylococcus aureus decreased 7 logs when cells were incubated with 1 μM BCS and irradiated for 15 minutes. Comparable photocytotoxic effect was obtained with Escherichia coli, when cells were treated for 30 minutes with visible light. BCS was also an effective photosensitizer to inactivate Candida albicans. In addition, this bacteriochlorin was able to eradicate bacteria at short incubation times. The structure of BCS contains eight basic amino groups that, when protonated in water, increase the binding to the cell envelope. In summary, the readily accessible bacteriochlorin BCS was highly effective at low concentrations as a broad-spectrum antimicrobial photosensitizer.

Enzymatically triggered chromogenic cross-linking agents under physiological conditions

Oxidative dimerization of an indoxyl moiety, released by glycosidase action in aqueous solution, yields an indigoid dye in formats that enable bioconjugation and molecular cross-linking.

Aqueous solubilization of hydrophobic tetrapyrrole macrocycles by attachment to an amphiphilic single-chain nanoparticle (SCNP)

Snapping a heterotelechelic amphiphilic polymer onto a tetrapyrrole imparts aqueous solubility to the otherwise hydrophobic macrocycle as demonstrated for a chlorin, bacteriochlorin and phthalocyanine.

Chromogenic agents built around a multifunctional double-triazine framework for enzymatically triggered cross-linking under physiological conditions

A molecular architecture designed for bioconjugation and internal absorption ratiometry undergoes enzymatically triggered cleavage of glucosyl groups and subsequent oxidative dimerization in aqueous solution to yield indigoid-containing scaffolds.

Engineering of an archaeal phosphodiesterase to trigger aggregation-induced emission (AIE) of synthetic substrates

Aggregation-induced emission (AIE) probes that can be triggered by enzymatic activity are valuable for applications across the life sciences.

Use of the Nascent Isocyclic Ring to Anchor Assembly of the Full Skeleton of Model Chlorophylls

The chlorophyll skeleton contains a chlorin macrocycle and an annulated fifth (or isocyclic) ring bearing 13¹-oxo and 13²-carbomethoxy substituents. The isocyclic ring has traditionally been constructed by annulation of an intact tetrapyrrole macrocycle. Here, a complementary route employs reaction of a gem-dimethyl-substituted dihydrodipyrrin-carboxaldehyde (AD half) and a dipyrromethane bearing a 3-methoxy-1,3-dioxopropyl group (BC half). A McMurry-like reaction of a 2-(2-nitro-5-oxohexyl)pyrrole was employed to construct the second pyrrole ring in one of three BC halves, whereas the other two were prepared by known routes. An AD half and a BC half were joined by Knoevenagel condensation at room temperature, affording the AD,BC-substituted 2-methoxycarbonyl-2-propenone. The subsequent reaction of three AD, BC-propenones (mixture of Z,E-isomers) in CH₃CN containing InCl₃ and In(OTf)₃ at 80 °C afforded the chlorophyll skeleton as the chloroindium(III) chelate; the reaction proceeds via Nazarov cyclization (to form the isocyclic ring), S_EAr (to construct the macrocycle), and 2e^-,2H⁺ oxidation (to give the aromatic chromophore). The absorption spectra of the complexes closely resemble that of chlorophyll a. The present work exploits the nascent isocyclic ring as an anchor for directed assembly of the AD and BC halves, forming both the chlorin macrocycle and the isocyclic ring in a single-flask transformation.

Cellular localization of tolyporphins, unusual tetrapyrroles, in a microbial photosynthetic community determined using hyperspectral confocal fluorescence microscopy

The cyanobacterial culture HT-58-2, composed of a filamentous cyanobacterium and accompanying community bacteria, produces chlorophyll a as well as the tetrapyrrole macrocycles known as tolyporphins. Almost all known tolyporphins (A-M except K) contain a dioxobacteriochlorin chromophore and exhibit an absorption spectrum somewhat similar to that of chlorophyll a. Here, hyperspectral confocal fluorescence microscopy was employed to noninvasively probe the locale of tolyporphins within live cells under various growth conditions (media, illumination, culture age). Cultures grown in nitrate-depleted media (BG-11₀ vs. nitrate-rich, BG-11) are known to increase the production of tolyporphins by orders of magnitude (rivaling that of chlorophyll a) over a period of 30-45 days. Multivariate curve resolution (MCR) was applied to an image set containing images from each condition to obtain pure component spectra of the endogenous pigments. The relative abundances of these components were then calculated for individual pixels in each image in the entire set, and 3D-volume renderings were obtained. At 30 days in media with or without nitrate, the chlorophyll a and phycobilisomes (combined phycocyanin and phycobilin components) co-localize in the filament outer cytoplasmic region. Tolyporphins localize in a distinct peripheral pattern in cells grown in BG-11₀ versus a diffuse pattern (mimicking the chlorophyll a localization) upon growth in BG-11. In BG-11₀, distinct puncta of tolyporphins were commonly found at the septa between cells and at the end of filaments. This work quantifies the relative abundance and envelope localization of tolyporphins in single cells, and illustrates the ability to identify novel tetrapyrroles in the presence of chlorophyll a in a photosynthetic microorganism within a non-axenic culture.

Synthesis of the Ring C Pyrrole of Native Chlorophylls and Bacteriochlorophylls

As part of a program to develop practical syntheses of members of the family of (bacterio)chlorophylls, two routes to 2-iodo-3-methyl-4-(3-methoxy-1,3-dioxopropyl)pyrrole, a precursor of the universal ring C, have been developed. The β-ketoester of ring C is expected to give rise to ring E upon Knoevenagel condensation and Nazarov cyclization with a ring D constituent as demonstrated in an analogue synthesis. Two viable routes were developed beginning with N-TIPS-pyrrole or with 4-oxo-2-pentene and TosMIC, affording multi-gram-quantities of this ostensibly simple pyrrole.

Total synthesis campaigns toward chlorophylls and related natural hydroporphyrins - diverse macrocycles, unrealized opportunities

Covering: up to 2018 Chlorophylls, bacteriochlorophylls and related hydroporphyrins constitute invaluable natural products but have largely remained outside the scope of viable syntheses. The campaign toward chlorophyll a by Woodward and coworkers is a deservedly celebrated landmark in organic synthesis yet the route entailed 49 steps, relied on semisynthetic replenishment of advanced intermediates, and then pointed to (but did not implement) uncertain literature procedures for the final transformations. Indeed, the full synthesis at any scale of any (bacterio)chlorophylls - conversion of small-molecule starting materials to the product - has never been accomplished. Herein, the reported syntheses of (±)-bonellin dimethyl ester (0.93 mg) and tolyporphin A O,O-diacetate (0.38 mg), as well as the never-fully traversed route to chlorophyll a, have been evaluated in a quantitative manner. Bonellin and tolyporphin A are naturally occurring chlorin and bacteriochlorin macrocycles, respectively, that lack the characteristic fifth ring of (bacterio)chlorophylls. A practical assessment is provided by the cumulative reaction mass efficiency (cRME) of the entire synthetic process. The cRME for the route to chlorophyll a would be 4.3 × 10-9 (230 kg of all reactants and reagents in total would yield 1.0 mg of chlorophyll a), whereas that for (±)-bonellin dimethyl ester or tolyporphin A O,O-diacetate is approximately 6.4 × 10-4 or 3.6 × 10-5, respectively. Comparison of the three syntheses reveals insights for designing hydroporphyrin syntheses. Development of syntheses with cRME > 10-5 (if not 10-4), as required to obtain 10 mg quantities of hydroporphyrin for diverse physicochemical, biochemical and medicinal chemistry studies, necessitates significant further advances in tetrapyrrole chemistry.

Heuristics from Modeling of Spectral Overlap in Förster Resonance Energy Transfer (FRET)

Among the photophysical parameters that underpin Förster resonance energy transfer (FRET), perhaps the least explored is the spectral overlap term ( J). While by definition J increases linearly with acceptor molar absorption coefficient (ε_(A) in M^-1 cm^-1), is proportional to wavelength (λ⁴), and depends on the degree of overlap of the donor fluorescence and acceptor absorption spectra, the question arose as to the value of J for the case of perfect spectral overlap versus that for representative fluorophores with incomplete spectral overlap. Here, Gaussian distributions of absorption and fluorescent spectra have been modeled that encompass varying degrees of overlap, full-width-at-half-maximum (fwhm), and Stokes shift. For ε_(A) = 10⁵ M^-1 cm^-1 and perfect overlap, the J value (in M^-1 cm^-1 nm⁴) ranges from 1.15 × 10¹⁴ (200 nm) to 7.07 × 10¹⁶ (1000 nm), is almost linear with λ⁴ (average of λ_abs and λ_flu), and is nearly independent of fwhm. For visible-region fluorophores with perfectly overlapped Gaussian spectra, the resulting value of J ( J_G-0) is ∼0.71 ε_(A)λ⁴ (M^-1 cm^-1 nm⁴). The experimental J values for homotransfer, as occurs in light-harvesting antennas, were calculated with spectra from a static database of 60 representative compounds (12 groups, 5 compounds each) and found to range from 4.2 × 10¹⁰ ( o-xylene) to 5.3 × 10¹⁶ M^-1 cm^-1 nm⁴ (a naphthalocyanine). The degree of overlap, defined by the ratio of the experimental J to the model J_G-0 for perfectly overlapped spectra, ranges from ∼0.5% (coumarin 151) to 77% (bacteriochlorophyll a). The results provide insights into how a variety of factors affect the resulting J values. The high degree of spectral overlap for (bacterio)chlorophylls prompts brief conjecture concerning the relevance of energy transfer to the question "why chlorophyll".