No abstract

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

Primary photochemical events in the unusually thermostable proton pumping rhodopsin of Thermus thermophilus bacterium (TR) are reported for the first time. Internal conversion in this protein is shown to be significantly faster than in bacteriorhodopsin (BR), making it the most rapidly isomerizing microbial proton pump known. Internal conversion (IC) dynamics of TR and BR were recorded from room temperature to the verge of thermal denaturation at 70 °C and found to be totally independent of temperature in this range. This included the well documented multiexponential nature of IC in BR, suggesting that assignment of this to ground state structural inhomogeneity needs revision. TR photodynamics were also compared with that of the phylogenetically more similar proton pump Gloeobacter rhodopsin (GR). Despite this similarity GR has poor thermal stability, and the excited state decays significantly more slowly and exhibits very prominent stretched exponential behavior. Coherent torsional wave-packets induced by impulsive photoexcitation of TR and GR show marked resemblance to each other in frequency and amplitude and differ strikingly from similar signatures in pump-probe data of BR and other microbial retinal proteins. Possible correlations between IC rates and thermal stability and the promise of using torsional coherence signatures for understanding chromophore protein binding in microbial retinal proteins are discussed.

Controlling the nonlinear optical (NLO) response properties at the molecular level is a key to develop strong NLO active materials for technological applications. In this paper, we report quantum chemical investigation of NLO response properties of select arylsubstituted Boron-Dipyrromethene (BODIPY) dyes, a class of intramolecular charge transfer (ICT) probes. Density functional theory (DFT) with long-range corrected CAM-B3LYP functional and cc-pVTZ, 6-31G(d,p) and 6-31+G(d,p) basis sets are employed to compute the electronic structures and NLO response of the aforesaid molecules. Calculations at the second order Møller-Plesset perturbation (MP2) level of theory are performed for comparison. The results suggest that the charge transfer process in these molecules is mostly unidirectional and the total first hyperpolarizability (β total ) values of these molecules are dominantly dictated by the response in the direction of charge transfer. Alteration of conjugation strength through donor/acceptor substitution as well as twisting of the phenyl ring obtained through incorporation of methyl groups affect the NLO response of thesemolecules. The vector components of first hyperpolarizability (β vec ) of the probe molecules are also studied to analyze the angle between the vector components of β vec and the dipole moment vector. The results presented here are expected to shed light on the origin of NLO response of several aryl-substituted BODIPY dyes and provide means to optimizing it for future technological applications.___________________________________________________________________

In microbial rhodopsins (also called retinal proteins), the retinal chromophore is used for harvesting light. A carotenoid molecule has been reported to complement the retinal as light harvesting antenna in bacterial retinal proteins, although examples are scarce. In this paper, we present the formation of a novel antenna complex between thermophilic rhodopsin (TR) and the carotenoid salinixanthin (Sal). The complex formation and its structure were studied using UV–visible absorption as well as circular dichroism (CD) spectroscopies. Our studies indicate that the complex is formed in both the trimeric and monomeric forms of TR. CD spectroscopy suggests that excitonic coupling takes place between retinal and Sal. The binding of Sal with artificial TR pigments derived from synthetic retinal analogues further supports the contribution of the retinal chromophore to the CD spectrum. These studies further support the possibility of interaction between the 4-keto ring of the Sal and the retinal in TR–Sal complexes. Temperature-dependent CD spectra indicate that the positive band (ca. 482 nm) of the bisignate CD spectra of the studied complexes originates from the contribution of excitonic coupling and induced chirality of Sal in the protein binding site. The presence of a relatively smaller glycine residue in the vicinity of the retinal chromophore in TR is proposed to be crucial for binding with Sal. The results are expected to shed light on the mechanism of retinal–carotenoid interactions in other biological systems.

No abstract

Studies of microbial rhodopsins revealed that hydrolysis of the retinal protonated Schiff base is the rate-determining step of the thermal denaturation process.

We present here the effects of geometrically constrained environments on the proton transfer reaction of 4-methyl 2,6-diformyl phenol (MFOH) both in the ground and excited states by employing steady-state and time-resolved fluorescence spectroscopy having picosecond and femtosecond resolutions. The nanometer-sized water pools formed in the ternary microemulsion of n-heptane-aerosol OT-water promote reprotonation of the probe. As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited state. The emission intensity has a complex behavior as the water content is changed in the system. The lower fluidity of confined water within the reverse micelle with respect to the normal bulk water alters the related dynamics of the H-bonded network. These observations are rationalized on the basis of altered ionic water activity in the confined surroundings, i.e., on dielectric constant, ionic mobility, pH, and the favorable orientation of dipoles in the medium. Our observations might be helpful to infer about the characteristics of nanoreactors, which often mimic many biological hydrophilic pockets.