Fluorescence modulation of excited state intramolecular proton transfer (ESIPT) probe 3-formyl-4-hydroxy benzoic acid encapsulated in the protein binding domain of serum albumins: a combined spectroscopic and molecular docking study

Dual Excited State Proton Transfer Pathways in the Bifunctional Photoacid 6-Amino-2-naphtol

This study investigates the photoacidity and excited state proton transfer (ESPT) pathways of a bifunctional molecule, 6-amino-2-naphthol (6N2OH), using absorption, steady-state fluorescence, time-resolved fluorescence, and theoretical calculations. 6N2OH attains four different prototropic forms in the excited state (cation, neutral, anion, or zwitterion) depending on pH of the solution. Interestingly, ESPT at the OH site of the molecule can be controlled by the protonation state of the amino substituent. Conversion of the electron donating NH2 group to the electron withdrawing NH3+ group brings about a reduction of more than 7 pKa units for the deprotonation of OH in the excited state. Further, the position of the NH2 substituent on the naphthalene framework is found to play an important role in dictating the ESPT pathways of aminonaphthols. Unlike most aminonaphthol derivatives that undergo ESPT only at the OH site, akin to substituted naphthols, 6N2OH undergoes ESPT at both OH and NH3+ sites, indicating its similarity to substituted naphthols and substituted naphthylamines. ESPT at the NH3+ site resulting in cation ↔ neutral equilibrium of 6N2OH in the excited state is well-corroborated by comparative studies with another reference photoacid, 6-amino-2-methoxynaphthalene (6N2M). Correlation of the acidity constants of 6N2OH with the σp parameters according to the Hammett model reveals that while 6N2OH can be treated either as naphthol or as naphthylamine in the ground state, the structure-function correlation cannot be extrapolated directly in the excited state, thus highlighting the rich and complex photophysics of bifunctional photoacids.

Nature of Fast Relaxation Processes and Spectroscopy of a Membrane-Active Peptide Modified with Fluorescent Amino Acid Exhibiting Excited State Intramolecular Proton Transfer and Efficient Stimulated Emission

A fluorescently labeled peptide that exhibited fast excited state intramolecular proton transfer (ESIPT) was synthesized, and the nature of its electronic properties was comprehensively investigated, including linear photophysical and photochemical characterization, specific relaxation processes in the excited state, and its stimulated emission ability. The steady-state absorption, fluorescence, and excitation anisotropy spectra, along with fluorescence lifetimes and emission quantum yields, were obtained in liquid media and analyzed based on density functional theory quantum-chemical calculations. The nature of ESIPT processes of the peptide's chromophore moiety was explored using a femtosecond transient absorption pump-probe technique, revealing relatively fast ESIPT velocity (∼10 ps) in protic MeOH at room temperature. Efficient superluminescence properties of the peptide were realized upon femtosecond excitation in the main long-wavelength absorption band with a corresponding threshold of the pump pulse energy of ∼1.5 μJ. Quantum-chemical analysis of the electronic structure of the peptide was performed using the density functional theory/time-dependent density functional theory level of theory, affording good agreement with experimental data.

G-Quadruplex binding of cavity-containing anthraquinonesulfonyl-β-cyclodextrin conjugate. Effect of encapsulation of ethidium bromide and berberine

An anthraquinonesulfonyl derivative of β-cyclodextrin is prepared and characterized employing spectroscopic techniques. The binding interactions of the compound with ethidium bromide, berberine, calf-thymus DNA, quadruplex DNAs viz., kit22, telo24, and myc22 are investigated by ultraviolet-visible, and fluorescence spectroscopic methods. Anthraquinonesulfonyl-β-cyclodextrin conjugate acts as a host molecule and enhances ethidium bromide and berberine fluorescence due to their encapsulation in cyclodextrin's cavity. The binding constant values are 9.0 × 10⁵ mol^-1 dm³ and 5.7 × 10⁴ mol^-1 dm³ for the formation of host: guest complexes of the β-CD derivative with ethidium bromide and berberine respectively. The proximity of the protons of ethidium bromide and berberine protons with those of the internal cavity of β-CD in the anthraquinonesulfonyl-β-CD conjugate is confirmed by two-dimensional rotating-frame Overhauser effect spectroscopy. The conjugate displays a quenching of fluorescence selectively to the quadruplexes kit22 and telo24 that is contrast to the spectral behavior with duplex DNA. ctDNA and myc22 exhibit different absorption and emission profiles with ethidium bromide on encapsulation by β-CD. The encapsulation of berberine leads to a fluorescence enhancement on binding to ctDNA, telo24, and myc22 with binding constants of 5.6 × 10⁵, 3.3 × 10⁵ mol^-1 dm³, and 1.5 × 10⁵ mol^-1 dm³ respectively. In contrast, kit22 leads to fluorescence quenching on berberine encapsulated-anthraquinonesulfonyl-β-cyclodextrin conjugate with a Stern-Volmer constant of 3.3 × 10⁵ mol^-1 dm³.Communicated by Ramaswamy H. Sarma.

Molecular encapsulation of berberine and ethidium bromide in anthraquinonecarboxamido-β-cyclodextrin conjugate: supramolecular association with DNA duplex and G-quadruplexes

G-quadruplex DNA in recognized as a potential target for anti-cancer drugs. In this work, an anthraquinonecarboxamido derivative of β-cyclodextrin (AQCC) is synthesized as a novel DNA binder that further can deliver an additional molecule at the target, carrying it in the cavity of modified cyclodextrin. The binding of AQCC with ethidium bromide (EtBr), berberine (Ber), duplex calf-thymus DNA (CT-DNA), quadruplexes (G4) viz., kit22, myc22, and telo24 are studied. The compound acts as a host molecule for the encapsulation of DNA binders viz., EtBr, Ber and enhances their fluorescence due to the encapsulation in its AQCC's cyclodextrin cavity. The binding constant of the host: guest complex of EtBr and Ber with AQCC's cavity are 6.4 × 10⁵ and 3.3 × 10⁶ mol^-1 dm³, respectively. The proximity of the protons of the guest and host molecules is confirmed by two-dimensional rotating-frame Overhauser effect spectroscopy (2D ROESY). The conjugate displays a quenching of fluorescence selectively on the association with CT-DNA and quadruplex kit22 that is contrast to the spectral behavior with quadruplex myc22 and telo24. CT-DNA exhibits dissimilar fluorescence spectra in free- and EtBr-bound forms. In addition, kit22 exhibit dissimilar emission profile when AQCC encapsulates Ber. Therefore, the Ber-loaded complexes and the AQCC molecule bind to different G-quadruplexes with different binding strengths. In addition, the effect of Ber in binding to the target DNAs is pronounces since the Ber molecule has more affinity to bind to quadruplexes than the duplex.

Multispectral and molecular docking investigations on the interaction of primethamine/trimethoprim with BSA/HSA

Primethamine (PMA) and trimethoprim (TMP) were investigated as traditional coccidiostats on the binding of bovine serum albumin (BSA) and human serum albumin (HSA) by multispectral and molecular docking techniques. The Stern-Volmer plots and time-resolved fluorescence measurement declared that PMA/TMP quenching the intrinsic fluorescence of BSA/HSA was static quenching process. The binding constants (K_a) and binding sites (n) were calculated at different temperatures. Meanwhile, thermodynamic parameters showed electrostatic forces played a leading role in the interaction of PMA/TMP with BSA/HSA. Some metal ions such as K⁺, Mg²⁺, Cu²⁺, Ca²⁺, Zn²⁺ and Fe³⁺ had no effects on the binding system. The UV-vis absorption spectra confirmed that the interaction between PMA/TMP and BSA/HSA did happen. The analyses of synchronous fluorescence, FT-IR and circular dichroism spectra illustrated that PMA/TMP changed the secondary structures of BSA/HSA. According to Förster non-radiative energy transfer theory, the binding distance between PMA/TMP and BSA/HSA was calculated. The binding location of PMA/TMP to BSA/HSA was identified as sub-domain IIA, which was further confirmed by molecular docking.Communicated by Ramaswamy H. Sarma.

Fluorescence solvatochromism and modulated anticholinergic activity of novel coumarin compounds sequestered in human serum albumin nanocavities

The antagonistic effect of the HSA matrix towards the acetylcholinesterase inhibition potencies of novel coumarin compounds is probed by multi-spectroscopic techniques.