A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Molecular Insights into the Conformational and Binding Behaviors of Human Serum Albumin Induced by Surface-Active Ionic Liquids

Extensive research has been carried out to investigate the stability and function of human serum albumin (HSA) when exposed to surface-active ionic liquids (SAILs) with different head groups (imidazolium, morpholinium, and pyridinium) and alkyl chain lengths (ranging from decyl to tetradecyl). Analysis of the protein fluorescence spectra indicates noticeable changes in the secondary structure of HSA with varying concentrations of all SAILs tested. Helicity calculations based on the Fourier transform infrared (FTIR) data show that HSA becomes more organized at the micellar concentration of SAILs, leading to an increased protein activity at this level. Small-angle neutron scattering (SANS) data confirm the formation of a bead-necklace structure between the SAILs and HSA. Atomistic molecular dynamics (MD) simulation results identify several hotspots on the protein surface for interaction with SAIL, which results in the modulation of protein conformational fluctuation and stability. Furthermore, fluorescence resonance energy transfer (FRET) experiments with the intramolecular charge transfer (ICT) probe trans-ethyl p-(dimethylamino) cinnamate (EDAC) demonstrate that higher alkyl chain lengths and SAIL concentrations result in a significantly increased energy transfer efficiency. The findings of this study provide a detailed molecular-level understanding of how the protein structure and function are affected by the presence of SAILs, with potential implications for a wide range of applications involving protein-SAIL composite systems.

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

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.

Hydrogen bond dynamics governs the effective photoprotection mechanism of plant phenolic sunscreens

Sinapic acid derivatives are important sunscreen species in natural plants, which could provide protection from solar UV radiation. Using a combination of ultrafast excited state dynamics, together with classical molecular dynamics studies, we demonstrate that there is direct coupling of hydrogen bond motion with excited state photoprotection dynamics as part of the basic mechanism in solution. Beyond the intra-molecular degree of freedom, the inter-molecular motions on all timescales are potentially important for the photochemical or photophysical events, ranging from the ultrafast hydrogen bond motion to solvent rearrangements. This provides not only an enhanced understanding of the anomalous experimental spectroscopic results, but also the key idea in the development of sunscreen agents with improved photo-chemical properties. We suggest that the hydrogen bond dynamics coupled excited state photoprotection mechanism may also be possible in a broad range of bio-related molecules in the condensed phase.

Ultrafast Photoprotecting Sunscreens in Natural Plants

We explore the ultrafast photoprotective properties of a series of sinapic acid derivatives in a range of solvents, utilizing femtosecond transient electronic absorption spectroscopy. We find that a primary relaxation mechanism displayed by the plant sunscreen sinapoyl malate and other related molecular species may be understood as a multistep process involving internal conversion of the initially photoexcited 1(1)ππ* state along a trans-cis photoisomerization coordinate, leading to the repopulation of the original trans ground-state isomer or the formation of a stable cis isomer.

Time-dependent density functional theory study on excited-state spectral and dynamic properties of hydrogen-bonded complexes formed by DMACA and water

Excited-state spectral and dynamic properties of hydrogen-bonded complexes formed by 4-(dimethylamino)cinnamic acid with water molecules were studied using the TDDFT method.

A highly efficient and selective coumarin based fluorescent probe for colorimetric detection of Fe3+and fluorescence dual sensing of Zn2+and Cu2+

A new coumarin based Schiff-base chemosensor was synthesized and evaluated. Based on Job’s plot,¹H NMR titration, ESI-MS and B-H plot, the structure of ligand and its metal ion is 1 : 1 stoichiometry. It also exhibits a binary logic function.

5-Arylvinyl-2,2'-bipyridyls: Bright "push-pull" dyes as components in fluorescent indicators for zinc ions

This article reviews the zinc(II)-dependent photophysical properties of arylvinylbipyridines (AVBs), a class of fluoroionophores in which 2,2'-bipyridyl and an aryl moiety are electronically conjugated. Zinc(II) binding of an AVB may lead to an emission bathochromic shift of the fluoroionophore without diminishing its fluorescence quantum yield. This observation can be explained using the excited state model of electron donor-π bridge-electron acceptor "push-pull" fluorophores, in which the bipy moiety acts as an electron acceptor, and zinc(II)-coordination strengthens its electron affinity. The spectral sensitivity of bipy-containing fluoroionophores, such as AVBs, to zinc(II) can be exploited to prepare fluorescent indicators for this ion. In several cases, AVB moieties are incorporated in fluorescent heteroditopic ligands, so that the variation of zinc(II) concentration over a relatively large range can be correlated to fluorescence changes in either intensity or color. AVB fluoroionophores are also used to introduce an intramolecular Förster resonance energy transfer (FRET) strategy for creating zinc(II) indicators with high photostability and a narrow emission band, two desired characteristics of dyes used in fluorescence microscopy.

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-Phenylenediamine in presence of bile acid host

Fluorescence properties of Schiff base - N,N'-bis(salicylidene) - 1,2-phenylenediamine (LH2) is used to study the micelles formed by aggregation of different important bile acids like cholic acid, deoxycholic acid, chenodeoxycholic acid and glycocholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found out to increase with concomitant red shift with gradual addition of different bile acids. Binding constant of the probe with different bile acids as well as critical micelle concentration was obtained from the variation of fluorescence intensity on increasing concentration of bile acids in the medium. The increase in fluorescence quantum yields, fluorescence decay times and substantial decrease in nonradiative decay rate constants in bile acids micellar environment points to the restricted motion of the fluorophore inside the micellar subdomains.

Photophysical properties and photochemistry of substituted cinnamates and cinnamic acids for UVB blocking: effect of hydroxy, nitro, and fluoro substitutions at ortho, meta, and para positions

Photophysical properties and photochemistry of various substituted cinnamates and cinnamic acids for ultraviolet B blocking were investigated experimentally and theoretically. This series includes monohydroxy, -nitro, and -fluoro derivatives. The absorption spectra were satisfactorily reproduced by the direct SAC-CI method with respect to the peak position and intensity. The transition character of the low-lying two ππ* and σπ* states for these 18 derivatives was analyzed. The para derivatives have a different transition character of the ππ* transitions compared with those of the ortho and meta derivatives. To elucidate the relaxation mechanism, the emission spectra were observed with oxygen quenching and the photostability was examined experimentally. The calculated radiative lifetimes indicate that the ortho- and meta-substituted derivatives have longer lifetimes for emission than the para derivatives. The potential energy curves of the first and second singlet excited states of the hydroxy derivatives as well as the vertical singlet and triplet transitions were examined to investigate the relaxation qualitatively. The ortho and meta derivatives have an energy barrier or flat surface in S1 resulting in fluorescence, whereas the para derivatives show nonradiative decay without an energy barrier. The para-hydroxy derivative was found to be an excellent UV absorber based on its broad absorption in the UVB/UVA regions, less emission, and higher photostability.

Fluorescent chemosensor based on sensitive Schiff base for selective detection of Zn2+

A Schiff-base fluorescent compound - N, N'-bis(salicylidene)-1,2 - phenylenediamine (LH2) was synthesized and evaluated as a chemoselective Zn(2+) sensor. Addition of Zn(2+) to ethanol solution of LH2 resulted in a red shift with a pronounced enhancement in the fluorescence intensity. Moreover, other common alkali, alkaline earth and transition metal ions failed to induce response or minimal spectral changes. Notably, this chemosensor could distinguish clearly Zn(2+) from Cd(2+). Fluorescence studies on free Schiff base ligand LH2 and LH2 - Zn(2+) complex reveal that the quantum yield strongly increases upon coordination. The stoichiometric ratio and association constant were evaluated using Benesi - Hildebrand relation giving 1:1 stoichiometry. This further corroborated 1:1 complex formation based on Job's plot analyses.

Fluorescence solvatochromism in lumichrome and excited-state tautomerization: a combined experimental and DFT study

Fluorescence solvatochromism of lumichrome (LC) was studied by steady-state and time-resolved fluorescence spectroscopy. The excited-state properties of LC do not show any correlation with solvent polarity, however, reasonably good correlation with solvent E(T)(30) parameter was observed. A quantitative estimation of contribution from different solvatochromic parameters, like solvent polarizability (π*), hydrogen bond donor (α), and hydrogen bond acceptor (β) ability of the solvent, was made using linear free energy relationship on the basis of Kamlet-Taft equation. The analysis reveals that hydrogen bond donating ability (acidity) of the solvent is the most important parameter that characterizes the excited-state behavior of lumichrome. Quantum mechanical calculations using density functional theory (DFT) were done to study the most stable structure and excited-state tautomerization process of LC toward the formation of isoalloxazines. Charge localization in the excited state and formation of hydrogen-bonded cluster through solvent hydrogen bond donation on the N10 atom of alloxazine moiety were predicted to be the key step toward this water-catalyzed tautomerization process.

Interaction of cinnamic acid derivatives with serum albumins: a fluorescence spectroscopic study

Cinnamic acid (CA) derivatives are known to possess broad therapeutic applications including anti-tumor activity. The present study was designed to determine the underlying mechanism and thermodynamic parameters for the binding of two CA based intramolecular charge transfer (ICT) fluorescent probes, namely, 4-(dimethylamino) cinnamic acid (DMACA) and trans-ethyl p-(dimethylamino) cinnamate (EDAC), with albumins by fluorescence spectroscopy. Stern-Volmer analysis of the tryptophan fluorescence quenching data in presence of the added ligand reveals fluorescence quenching constant (κ(q)), Stern-Volmer constant (K(SV)) and also the ligand-protein association constant (K(a)). The thermodynamic parameters like enthalpy (ΔH) and entropy (ΔS) change corresponding to the ligand binding process were also estimated. The results show that the ligands bind into the sub-domain IIA of the proteins in 1:1 stoichiometry with an apparent binding constant value in the range of 10(4) dm(3) mol(-1). In both the cases, the spontaneous ligand binding to the proteins occur through entropy driven mechanism, although the interaction of DMACA is relatively stronger in comparison with EDAC. The temperature dependence of the binding constant indicates the induced change in protein secondary structure.

Fluorescence of 5-arylvinyl-5'-methyl-2,2'-bipyridyl ligands and their zinc complexes

The photophysical properties of 5-arylvinyl-5'-methyl-2,2'-bipyridyls (AVMBs, 1-9, 11) and their zinc complexes were studied. Similar 2,2'-bipyridyl-based ligands have been applied as optical sensors for metal ions and sensitizers for solar energy conversion. The goal of this investigation is to reveal the factors that determine the emission band shift and fluorescence quantum yield change of the title ligand system upon zinc binding. The outcome of this study will not only advance the fundamental understanding of the coordination-driven photophysical processes embodied in the AVMB platform but facilitate the rational design of fluorescent probes for metal ions, particularly zinc. The AVMB ligands were synthesized using the Horner-Wadsworth-Emmons reaction. AVMBs containing electron-donating aryl groups show absorption and emission in the visible region, which can be assigned to charge-transfer transitions as supported by solvent-dependency and computational studies. The binding between AVMB ligands and zinc ion in acetonitrile was studied using isothermal titration calorimetry (ITC). A multicomponent equilibrium model is suggested that explains the multiple transitions evidenced in fluorescence titration isotherms. Coordination to zinc ion stabilizes the charge-transfer excited state of an AVMB ligand with an electron-donating aryl substituent, consequently results in bathochromic shifts in both absorption and emission. However, unlike the emission band shift, the fluorescence quantum yield change upon zinc complex formation does not have an intuitive correlation with the electronic nature of the aryl group. Lifetime measurements using the Time-Correlated Single Photon Counting method enabled the determination of nonradiative and radiative decay rate constants. Both rates of an AVMB ligand decrease upon zinc binding. The collective effect gives rise to the change in fluorescence quantum yield with the apparent lack of correlation with the electronic property of the aryl group.

Effect of solvent hydrogen bonding on the photophysical properties of intramolecular charge transfer probe trans-ethyl p-(dimethylamino) cinamate and its derivative

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino) cinamate (EDAC) and 4-(dimethylamino) cinnamic acid (DMACA) were studied by steady state absorption and emission, picosecond time-resolved fluorescence experiments in various pure and mixed solvent systems. The large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (nu(0,0)) for EDAC shows very good linear correlation with static solvent dielectric property; however, fluorescence emission maximum, stokes shift and fluorescence quantum yield show significant deviation from linearity in polar protic solvents, indicating a large contribution of solvent hydrogen bonding on the excited state relaxation mechanism. A quantitative estimation of contribution from different solvatochromic parameters was made using linear free energy relationship based on Kamlet-Taft equation.