Effect of selected hydrogen-bonding solvents on the absorption maxima of N-retinylidene-n-butylammonium salts

The opsin shift and mechanism of spectral tuning in rhodopsin

Molecular dynamics simulations and combined quantum mechanical and molecular mechanical calculations have been performed to investigate the mechanism of the opsin shift and spectral tuning in rhodopsin. A red shift of -980 cm(-1) was estimated in the transfer of the chromophore from methanol solution environment to the protonated Schiff base (PSB)-binding site of the opsin. The conformational change from a 6-s-cis-all-trans configuration in solution to the 6-s-cis-11-cis conformer contributes additional -200 cm(-1), and the remaining effects were attributed to dispersion interactions with the aromatic residues in the binding site. An opsin shift of 2100 cm(-1) was obtained, in reasonable accord with experiment (2730 cm(-1)). Dynamics simulations revealed that the 6-s-cis bond can occupy two main conformations for the β-ionone ring, resulting in a weighted average dihedral angle of about -50°, which may be compared with the experimental estimate of -28° from solid-state NMR and Raman data. We investigated a series of four single mutations, including E113D, A292S, T118A, and A269T, which are located near the PSB, along the polyene chain of retinal and close to the ionone ring. The computational results on absorption energy shift provided insights into the mechanism of spectral tuning, which involves all means of electronic structural effects, including the stabilization or destabilization of either the ground or the electronically excited state of the retinal PSB.

Vibrational polarization and opsin shift of retinal schiff bases: theoretical study

The changes in the electronic excitation energy arising from molecular structural displacement induced by external electric field (so-called vibrational polarization) are examined theoretically for the protonated and neutral 11-cis retinal Schiff bases. It is shown that the magnitude of the field-induced structural displacement is significantly large for the protonated species, so that the change in the electronic excitation energy arising from this structural displacement is of the same order of magnitude as that arising from the direct effect of electric field on the electronic wave function. These two effects contribute additively to the field-induced spectral shift. The intensity-carrying mode (ICM) theory is employed to extract a single vibrational mode (called primary infrared ICM) that is most important for the field-induced structural displacement. A simple one-dimensional model is constructed, and the extent to which we can interpret the field-induced spectral shift by such a model is examined. In the case of the neutral species, only a small change in the electronic excitation energy is induced by external electric field, mainly because the vibrational polarizability of this species is small. The meaning of these results in the spectral tuning of visual pigments is discussed.

Combined QM/MM study of the opsin shift in bacteriorhodopsin

Combined quantum mechanical and molecular mechanical (QM/MM) calculations and molecular dynamics simulations of bacteriorhodopsin (bR) in the membrane matrix have been carried out to determine the factors that make significant contributions to the opsin shift. We found that both solvation and interactions with the protein significantly shifts the absorption maximum of the retinal protonated Schiff base, but the effects are much more pronounced in polar solvents such as methanol, acetonitrile, and water than in the protein environment. The differential solvatochromic shifts of PSB in methanol and in bR leads to a bathochromic shift of about 1800 cm(-1). Because the combined QM/MM configuration interaction calculation is essentially a point charge model, this contribution is attributed to the extended point-charge model of Honig and Nakanishi. The incorporation of retinal in bR is accompanied by a change in retinal conformation from the 6-s-cis form in solution to the 6-s-trans configuration in bR. The extension of the pi-conjugated system further increases the red-shift by 2400 cm(-1). The remaining factors are due to the change in dispersion interactions. Using an estimate of about 1000 cm(-1) in the dispersion contribution by Houjou et al., we obtained a theoretical opsin shift of 5200 cm(-1) in bR, which is in excellent agreement with the experimental value of 5100 cm(-1). Structural analysis of the PSB binding site revealed the specific interactions that make contributions to the observed opsin shift. The combined QM/MM method used in the present study provides an opportunity to accurately model the photoisomerization and proton transfer reactions in bR.

Spectral tuning in bacteriorhodopsin in the absence of counterion and coplanarization effects

The basis for wavelength regulation in bacteriorhodopsin (BR) and retinylidene proteins in general has been studied for decades but is still only partially understood. Here we report the preparation and spectroscopic characterization of BR analogs aimed at investigating the existence of spectral tuning mechanisms other than the two widely accepted mechanisms, weakened counterion interactions and ring/chain coplanarization. We synthesized two novel retinal analogs containing a saturated 13-14 bond, which interrupts the interaction of the protein counterions with the chromophore conjugation system. Furthermore, one of the analogs has a planar polyene system so that the contribution to the red shift of BR by retinal ring/chain coplanarization is also absent. We incorporated these analogs into bacterioopsin and discovered a sizable amount of red shift, which can be accounted for by interactions between the polar or polarizable groups of the protein and the retinal polyene chain. Our results suggest that the wavelength regulation in BR is achieved by synergistic chromophore/protein interactions including ring/chain coplanarization, excited state stabilization by polar or polarizable protein side chains located along the polyene chain, and weakened counterion interactions near the Schiff base positive charge.

Spectroscopic observation of solvent interaction with selected retinal Schiff bases

Iodide salts of several N-retinylidenedialkylamines were prepared and their UV-VIS spectra recorded. Their lambda max values increased as the number of hydrogen atoms on the carbons alpha to nitrogen increased. In separate experiments, iodide salts of N-retinylidene-n-butylammonium (1) and N-retinylidene-n-butylmethylammonium (2) were prepared, and their excitation energies (delta E) were measured in selected solvents of varying dielectric constant (epsilon). Data from each compound gave a straight line which converged at epsilon = 0. On the other hand, when delta E values of the iodide and bromide of 2 were plotted vs solvent epsilon, parallel rather than convergent lines were obtained. When lambda max values of 1 and 2 were measured in a greater number of solvents, the solvents fell into four main groups. The first group, regular solvents, are rigid, have fixed dipoles, neither donate nor accept H-bonds, and the delta E of 1 and 2 decreases linearly as solvent epsilon increases. This line for 2 is taken as a standard against which other solvents are judged. A second class of solvents that are good H-bond donors like CHCl3, whose dipole moment coincides with the C-H bond axis, is located in an area below the standard line. A third group, nucleophilic solvents like tetrahydrofuran, whose dipole moment is coincident with a strongly nucleophilic oxygen atom are good H-bond acceptors and are found above the standard line. Solvents with unpredictable spectroscopic behavior are classed as anomalous.