Separation of scattering and absorption contributions in UV/visible spectra of resonant systems

Influence of myoglobin on the antibacterial activity of carvacrol and the binding mechanism between the two compounds

BACKGROUND

Myoglobin (MB), a pigmentation protein, can adversely affect the antibacterial activity of carvacrol (CAR) and weaken its bacteriostasis effect. This study aimed to clarify the influence of MB on the antibacterial activity of CAR and ascertain the mechanism involved in the observed influence, especially the interaction between the two compounds.

RESULTS

Microbiological analysis indicated that the presence of MB significantly suppressed the antibacterial activity of CAR against Listeria monocytogenes. Ultraviolet-visible spectrometry and fluorescence spectroscopic analysis confirmed the interaction between CAR and MB. The stoichiometric number was determined as ~0.7 via double logarithmic Stern-Volmer equation analysis, while thermodynamic analysis showed that the conjugation of the two compounds occurred as an exothermal reaction (ΔH° = -32.3 ± 11.4 kJ mol-1 and ΔS° = -75 J mol-1  K-1 ). Circular dichroism, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy showed hydrogen bonding in the carvacrol-myoglobin complex (CAR-MB). Molecular docking analysis confirmed that amino acid residues, including GLY80 and HIS82, were most likely to form hydrogen bonds with CAR, while hydrogen bonds represented the main driving force for CAR-MB formation.

CONCLUSION

CAR antibacterial activity was significantly inhibited by the presence of MB in the environment due to the notable reduction in the effective concentration of CAR caused by CAR-MB formation. © 2023 Society of Chemical Industry.

Optothermal properties of plasmonic inorganic nanoparticles for photoacoustic applications

Plasmonic systems are becoming a favourable alternative to dye molecules in the generation of photoacoustic signals for spectroscopy and imaging. In particular, inorganic nanoparticles are appealing because of their versatility. In fact, as the shape, size and chemical composition of nanoparticles are directly correlated with their plasmonic properties, the excitation wavelength can be tuned to their plasmon resonance by adjusting such traits. This feature enables an extensive spectral range to be covered. In addition, surface chemical modifications can be performed to provide the nanoparticles with designed functionalities, e.g., selective affinity for specific macromolecules. The efficiency of the conversion of absorbed photon energy into heat, which is the physical basis of the photoacoustic signal, can be accurately determined by photoacoustic methods. This review contrasts studies that evaluate photoconversion in various kinds of nanomaterials by different methods, with the objective of facilitating the researchers' choice of suitable plasmonic nanoparticles for photoacoustic applications.

Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin

Under moderate acidic conditions, the cationic (+3) complexes ions tris(1,10-phenanthroline)cobalt(III), [Co(phen)₃]³⁺, and hexamminecobalt(III), [Co(NH₃)₆]³⁺, efficiently promote the self-assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS₄) into J-aggregates. The growth kinetics have been analyzed according to a well-established autocatalytic model, in which the rate determining step is the initial formation of a nucleus containing m porphyrin units (in the range 2–3), followed by a stage whose rate constant k_c evolves as a power of time. The observed catalytic rate constants and the extent of J-aggregation increase on increasing the metal complex concentration, with the phen complex being the less active. The UV/Vis extinction spectra display quite broad envelops at the J-band, especially for the amino-complex, suggesting that electronic dipolar coupling between chromophores is operative in these species. The occurrence of spontaneous symmetry breaking has been revealed by circular dichroism and the measured dissymmetry g-factor decreases on increasing the aggregation rates. The role of these metal complexes on the growth and stabilization of porphyrin nano-assemblies is discussed in terms of the different degree of hydrophilicity and hydrogen bonding ability of the ligands present in the coordination sphere around the metal center.

Porphyrins Through the Looking Glass: Spectroscopic and Mechanistic Insights in Supramolecular Chirogenesis of New Self-Assembled Porphyrin Derivatives

The solvent driven aggregation of porphyrin derivatives, covalently linked to a L- or D-prolinate enantiomer, results in the stereospecific formation of species featuring remarkable supramolecular chirality, as a consequence of reading and amplification of the stereochemical information stored in the proline-appended group. Spectroscopic, kinetic, and topographic SEM studies gave important information on the aggregation processes, and on the structures of the final chiral architectures. The results obtained may be the seeds for the construction of stereoselective sensors aiming at the detection, for example, of novel emergent pollutants from agrochemical, food, and pharmaceutical industry.

Noncovalent Functionalization of 2D Black Phosphorus with Fluorescent Boronic Derivatives of Pyrene for Probing and Modulating the Interaction with Molecular Oxygen

We studied the chemical-physical nature of interactions involved in the formation of adducts of two-dimensional black phosphorus (2D BP) with organoboron derivatives of a conjugated fluorescent molecule (pyrene). Time-resolved fluorescence spectroscopy showed a stabilization effect of 2D BP on all derivatives, in particular for the adducts endowed with the boronic functionalities. Also, a stronger modulation of the fluorescence decay with oxygen was registered for one of the adducts compared to the corresponding organoboron derivative alone. Nuclear magnetic resonance experiments in suspension and density functional theory simulations confirmed that only noncovalent interactions were involved in the formation of the adducts. The energetic gain in their formation arises from the interaction of P atoms with both C atoms of the pyrene core and the B atom of the boronic functionalities, with a stronger contribution from the ester with respect to the acid one. The interaction results in the lowering of the band gap of 2D BP by around 0.10 eV. Furthermore, we demonstrated through Raman spectroscopy an increased stability toward oxidation in air of 2D BP in the adducts in the solid state (more than 6 months). The modification of the electronic structure at the interface between 2D BP and a conjugated organic molecule through noncovalent stabilizing interactions mediated by the B atom is particularly appealing in view of creating heterojunctions for optoelectronic, photonic, and chemical sensing applications.

Scattering and absorption differ drastically in their inner filter effects on fluorescence, resonance synchronous, and polarized resonance synchronous spectroscopic measurements

Reported herein is the finding that photon scattering and absorption differ drastically in inducing the sample IFE in SSF, RS2, and the PRS2 spectra measurements.

Use of fluorescence signals generated by elastic scattering under monochromatic incident light for determining the scattering efficiencies of various plasmonic nanoparticles

Fluorescence signals generated by elastic scattering under monochromatic incident light are useful for determining scattering efficiencies of various plasmonic nanoparticles.

Far- and near-field properties of gold nanoshells studied by photoacoustic and surface-enhanced Raman spectroscopies

Gold nanoshells, with a silica core and different core and shell dimensions, have been extensively investigated. Optical far-field properties, namely extinction and absorption, have been separately determined by means of spectrophotometry and photoacoustic spectroscopy, respectively, in the 440-900 nm range. The enhancement factor for surface-enhanced Raman scattering, which is related to near-field effects, has been measured from 568 to 920 nm. The absorption contribution to extinction decreases as the overall diameter increases. Moreover, absorption and scattering display different spectral distributions, the latter being red shifted. The Surface Enhanced Raman Scattering enhancement profile, measured using thiobenzoic acid as a Raman probe, is further shifted to the red. The latter result suggests that the enhancement is dominated by the presence of hot spots, which are possibly related to the surface roughness of gold nanoshell particles.

Extraction of absorption and scattering contribution of metallic nanoparticles toward rational synthesis and application

Noble metal nanoparticles have unique localized surface plasmon resonance (LSPR), leading to their strong absorption and scattering in the visible light range. Up to date, the common practice in the selection of nanoparticles for a specific application is still based on the measured extinction spectra. This practice may be erroneous, because the extinction spectra contain both absorption and scattering contribution that may play different roles in different applications. It would be highly desirable to develop an efficient way to obtain the absorption and scattering spectra simultaneously. Herein, we develop a method to use the experimentally measured extinction and scattering signals to extract the absorption and scattering spectra that is in excellent agreement with that simulated by discrete dipole approximation (DDA). The heating curve measurement on the three types of gold nanorods, with almost the same extinction spectra but different absorption and scattering contribution, convincingly reveals an excellent correlation between the heating effect and the absorption strength rather than the extinction strength. The result demonstrates the importance to obtain the scattering and absorption spectra to predict the potential application for different types of nanoparticles, which in turn will screen efficiently nanoparticles for a specific application.

Photoacoustic excitation profiles of gold nanoparticles

The wavelength dependence of the laser-induced photoacoustic signal amplitude has been measured for water dispersions of 10, 61, and 93 nm diameter gold nanospheres. The whole region of the localized surface plasmon resonance has been covered. This "photoacoustic excitation profile" can be overlayed with the extinction spectrum between 450 nm and 600 nm in the case of the smallest nanoparticles. At variance, the larger-sized nanoparticles display a progressive deviation from the extinction spectrum at longer wavelength, where the photoacoustic signal becomes relatively smaller. Considering that photoacoustics is intrinsically insensitive to light scattering, at least for optically thin samples, the results are in agreement with previous theoretical work predicting (i) an increasing contribution of scattering to extinction when the nanoparticle size increases and (ii) a larger scattering component at longer wavelengths. Therefore, the method has a general validity and can be applied to selectively determine light absorption by plasmonic systems.

Uncontrolled variability in the extinction spectra of C60 nanoparticle suspensions

To properly investigate the environmental transport, fate, and impact of fullerene C60 nanoparticles (nC60), it is necessary to reproducibly obtain nC60 suspensions and to accurately determine their concentration ([C60]). The results in the present study, however, clearly illustrate that the production of nC60 via extended mixing and via sonication are highly stochastic top-down processes subject to widely divergent end points. nC60 suspensions exhibit variable characteristics (e.g., [C60], average particle size, size distribution, etc.) that make it challenging, if not impossible, to acquire reproducible UV-vis extinction spectra. The mass extinction coefficient, which is the absorptivity of a suspension with [C60] = 1 mM obtained by normalizing UV-vis spectra by the mass concentration of C60 in the suspension, decreases with a given suspension's hydrodynamic diameter, whereas the particle extinction coefficient, which is the absorptivity of a suspension containing one mole of nC60 nanoparticles with the same size distribution as the target suspension and calculated based upon the suspension nanoparticle size distribution, increases with its number weighted average diameter. Other spectroscopic properties of nC60 (e.g., absorbance bandwidth, position of absorption maximum, and relative extinction intensity) also change with average particle size. As a result of the extant variability between samples, when UV-vis spectra are employed to calculate or represent [C60] for fullerene nanoparticle suspensions, extreme care must be taken and other colloidal properties of this suspension must be measured to obtain an accurate result.

Iron pyrite nanocubes: size and shape considerations for photovoltaic application

Multiple lines of recent research indicate that iron pyrite (FeS(2)) requires a {100}-terminated crystal morphology in order to maintain semiconducting properties. Additionally, the large absorption coefficient of pyrite allows for the near complete absorption of above band gap radiation in <50 nm layers. However, to our knowledge <50 nm pyrite nanocubes have yet to be isolated. Herein, we demonstrate the synthesis of ~37 nm phase pure pyrite nanocubes by manipulating the sulfur chemical potential and ligand environment of the system. Ultraviolet-visible (UV-vis) absorption spectroscopy gives a signal of resonant light scattering indicating strong electronic coupling between nanocubes, which may allow for nanocube films with superior electron mobility. The absorption spectroscopies of cubic and irregular nanocrystals are contrasted and compared with recent theoretical work in order to investigate the effect of shape on electronic properties. Specifically, nanocubes have been found to have absorption characteristics closer to theory as compared to irregular nanocrystals, especially for UV radiation: 250-350 nm. Pyrite nanocubes display an indirect band gap at ~1.1 eV in addition to two direct transitions at ~1.9 and ~3.0 eV, correlating well to theoretical values.

Spectroscopic and kinetic investigations on porphyrin J-aggregates induced by polyamines

A non-covalent supramolecular approach to build up mesoscopic fractal J-aggregates of meso-tetrakis(4-sulfonatophenyl)porphyrin by using different polyamines under mild acidic conditions is reported. Spermine-induced J-aggregates have been treated as a model system for a kinetic investigation. The kinetic analysis has been performed through two non-conventional approaches as function of a different reagent mixing protocol. In particular, an autocatalytic pathway in which the formation of aggregation nuclei is the rate-determining step is found when porphyrin is added to the reaction mixture as first reagent. In order to gain information on this early stage, a kinetic investigation has been carried out as function of different parameters, such as pH, and both spermine and porphyrin concentration. The nucleation becomes particularly evident when polyamines with fewer than three protonable nitrogen atoms are used. In these cases, a sort of "YES/NO" effect for the aggregation process has been found as a function of the reagent mixing order protocol. Most importantly, the observed kinetic behavior strongly affects the mesoscopic structure of the final aggregates.

Aggregation of meso-tetrakis(4-sulfonatophenyl)porphyrin on polyethyleneimine in aqueous solutions and on a glass surface

The aggregation behavior of meso-tetrakis(4-sulfonatophenyl)porphyrin, [ H 2( TPPS 4)]4-, in the presence of hydrated polyethyleneimine ( PEI ) in aqueous solutions (1.6 < pH < 7.6) has been investigated. The interaction leads to the formation of a variety of pH dependent species, which have been attributed to porphyrin dimers or small oligomers under neutral conditions and J-aggregates on lowering the pH. The aggregation process follows kinetics typical of self-similar systems whose rates increase steeply on increasing the matrix concentration. This finding could be explained on the basis of an increased availability of binding sites for the growing aggregates. The charged polymer has been used to electrostatically adsorb the porphyrin onto glass substrates affording multilayered films. Our results point to the presence of fully protonated species in the solid state, which rearrange into J-aggregates as a function of pH and water content. The systems in solution and on glass surfaces have been investigated through a combination of UV-vis spectroscopy, fluorescence emission and resonance light scattering techniques.

Surfactant-like behavior of short-chain alcohols in porphyrin aggregation

UV/vis absorption and time-resolved fluorescence measurements on alcoholic solutions of meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS(4)) in neutral and acid form have been performed as a function of the alcohol polarity. These solutions show a peculiar behavior that mimics porphyrin in confined water solutions. In alcohols, TPPS(4) exhibits a metastable phase showing the formation of new species in analogy with the confined water environment of AOT microemulsions. Various species have been detected at different pH values and on aging the solutions. Under neutral pH conditions, the porphyrin is present as free base monomer (S415) with a small amount of H-dimeric species (S400). On aging, the S415 leads to the formation of a new species (S423), which has been assigned as a J-type dimer of the neutral porphyrin. The species S400 and S423 are not present in bulk water solution but are typical of TPPS(4) in confined water. On decreasing pH, the S415 almost immediately converts into the diacid form (S438), which evolves toward red-shifted J-aggregates (S490) and blue-shifted H-aggregates (S420). On decreasing alcohol polarity, the kinetic evolution from fresh to aged solution and from the monomeric diacid species to H- and J-aggregates speeds up. Exploiting the amphiphilic character of short chain alcohols and widely varying their polarity, we were able to enhance in bulk conditions the peculiar behavior observed in close proximity to the microemulsion wall.

Synthesis and solvent driven self-aggregation studies of meso-"C-glycoside"-porphyrin derivatives

New types of porphyrin derivatives bearing "C-glycoside" moieties, either in 5,10,15,20- or in 5,15-meso-positions, were prepared and fully characterized. The presence of the glycosidic groups imparts to the title macrocycles, besides an amphiphilic character, a clear tendency to form chiral suprastructures upon solvent-driven self-aggregation in different aqueous-organic solvent mixtures. Supra-assembly phenomena, in terms of the size and morphology of the resulting structures, as well as their kinetics of aggregation, were studied by UV-visible, fluorescence, resonance light scattering (RLS), and CD spectroscopy, indicating that the morphology of the aggregates depends strongly on the structure of the porphyrin rings, and on the bulk conditions of aggregation.

Aggregation Properties of Hyperporphyrins with Hydroxyphenyl Substituents

Acidification of tetrakis(4-hydroxyphenyl)porphyrin (THPP) and tetrakis(3,5-dihydroxyphenyl)porphyrin (OHPP) in dichloromethane solutions has been investigated as a function of the nature of the counteranion. These porphyrins exhibit different patterns of behavior, and extended aggregates are formed displaying broad extinction features together with intense components due to resonant light scattering. Especially in the case of haloacids, J-aggregated species are obtained exhibiting large bathochromic shifts both for B- and Q-bands, which extend in the far red region. The optical characteristics of the aggregated and monomeric protonated species are strongly influenced by the nature of the counteranions. A comparison with tetrakis(4-methoxyphenyl)porphyrin (TMPP), which remains always in a monomeric form, demonstrates the key role played by the peripheral hydroxyl groups in stabilizing various porphyrin aggregates.

Counteranion Dependent Protonation and Aggregation of Tetrakis(4-sulfonatophenyl)porphyrin in Organic Solvents

The tetrabutylammonium salt of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) is soluble in dichloromethane, and the general properties of this compound have been investigated as function of various added acids HX (X = Cl, Br, I, CF(3)COO, CF(3)SO(3), TFPB) through UV-vis absorption spectroscopy, steady state fluorescence emission, and resonance light-scattering (RLS) techniques. Upon addition of HX, the initial monomeric free base TPPS is readily converted in an aggregated species, whose spectroscopic features are independent of the nature of the counteranion X. All the spectroscopic evidence suggest a J-type arrangement of chromophores in this aggregate, involving strong hydrogen bonds, electrostatic, and dispersive interactions. In the specific case of chloride and bromide, in the presence of a TBAX excess, the addition of the corresponding acid leads to a monomeric ion-pair between the TBA cations and the diacid TPPS, whose central core is strongly interacting with the halide. On further increasing the acid concentration in these latter solutions, fully protonated species are formed that eventually start to aggregate.

Spectroscopic Studies of Water-Soluble Porphyrins with Protein Encapsulated in Bis(2-ethylhexyl)sulfosuccinate (AOT) Reverse Micelles: Aggregation versus Complexation

We have investigated the interaction of two water-soluble free-base porphyrins (negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin sodium salt (TSPP) and positively charged meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMpyP)) with two drug-carrier proteins (human serum albumin (HSA) and beta-lactoglobulin (betaLG)) in bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane/water reverse micelles (RM) by using steady-state and transient-state fluorescence spectroscopy. TSPP exhibited a complex pattern of aggregation on varying the RM size and pH in the absence of the protein: at low omega0 (the ratio of water concentration to AOT concentration, the emission of H-aggregates prevails under acidic or neutral "pH(ext)" conditions. Upon formation of the water-pool, J-aggregates and monomeric diacid species dominate at low "pH(ext)" but only monomer is detected at neutral "pH(ext)". The aggregation number increases with omega0 and the presence of the protein does not seem to contribute to further growth of the aggregate. The presence of protein leads to H-deaggregation but promotes J-aggregation up to a certain protein/porphyrin ratio above which, complexation with the monomer bound to a hydrophobic site of the protein prevails. The effective complex binding constants are smaller than in free aqueous solution; this indicates a weaker binding in these RM probably due to some conformational changes imposed by encapsulation. Only a weak quenching of TMpyP fluorescence is detected due to the presence of protein in contrast to the negative porphyrin.

Supramolecular chirality control by solvent changes. Solvodichroic effect on chiral porphyrin aggregation

The mode of aggregation of amphiphilised porphyrin derivatives bearing a chiral functionality is strongly affected by solvent composition; this results in the tuning of the supramolecular chirality of the porphyrin aggregates.

Protein analysis with tetra-substituted sulfonated cobalt phthalocyanine by the technique of Rayleigh light scattering

This is the first report of cobalt-tetrasulfonatophthalocyanine (CoTSPc) as a probe of Rayleigh light scattering (RLS) to determine proteins at nanogram levels. A highly sensitive method has been developed for the determination of proteins by the light scattering technique on a common spectrofluorimeter, based on the fact that the weak RLS of CoTSPc can be greatly enhanced in the presence of proteins. Under optimum conditions, the linear ranges of the calibration curves were 0.10-34.3 microg x mL(-1) for both human serum albumin and bovine serum albumin, with detection limits of 15.5 and 13.9 ng x mL(-1), respectively. Moreover, there is almost no interference of any amino acids and metal ions. The method has been applied to the direct determination of total proteins in human serum samples, and the results were satisfactory with clinical data provided.

Determination of proteins with Fast Red VR by a corrected resonance light-scattering technique

A simple corrected resonance light-scattering (CRLS) technique was established to correct for any distortion of the resonance light scattering (RLS) spectra resulting from molecular absorption. By using an absorption cell holder to change the propagation direction of the incident light beam of a common spectrofluorometer, the molecular absorption was directly measured through a spectrofluorometer. With measurements of the CRLS signals of the interaction of Fast Red VR (FRV) and proteins, we proved that the present correction for the RLS spectra in terms of the molecular absorption of excitation and scattering radiation can improve the detection sensitivity by about two fold.