Electrostatic and conformational effects on the electronic structures of distortional isomers of a mixed-valence binuclear Cu complex.

The electronic structure of the binuclear copper complex [Cu(2)(L)](3+) [L = N(CH(2)CH(2)N(H)CH(2)CH(2)N(H)CH(2)CH(2))(3)N] has been investigated by resonance Raman and electroabsorption spectroscopy. Crystallographic Cu(2) distances of 2.364(1) and 2.415(1) A determined for the nitrate and acetate salts, respectively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the solid state and in solution by the ligand environment through coupling to ligand torsional modes that are, in turn, stabilized by hydrogen bonding. Electroabsorption data on the three major visible and near-infrared electronic transitions of Cu(2)L, lambda(max) (epsilon(max)) = 1000 nm ( approximately 1200 M(-1) cm(-1)), 748 nm (5600 M(-1) cm(-1)), and 622 nm (3350 M(-1) cm(-1)), reveal a difference dipole moment between the ground and excited states (Deltamu(A)) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is more polarizable than the excited state. A general model to explain this behavior in terms of the proximity of accessible transitions involving copper d electrons is proposed to explain the larger polarizability of the ground state. Raman excitation profiles (REPs) provide evidence for multiple conformational states of [Cu(2)(L)](3+). Separate REPs were obtained for each of the components of the two major Raman bands for nu(1) (a Cu-Cu stretching mode) and nu(2) (a Cu-Cu-N(eq) bending mode). The Raman data along with quantum chemical ZINDO/S CI calculations provide evidence for isomeric forms of Cu(2)L with strong coupling between the conformation of L and the Cu-Cu bond length.

Tyrosine structural changes detected during the photoactivation of rhodopsin.

We present the first Fourier transform infrared (FTIR) analysis of an isotope-labeled eukaryotic membrane protein. A combination of isotope labeling and FTIR difference spectroscopy was used to investigate the possible involvement of tyrosines in the photoactivation of rhodopsin (Rho). Rho --> MII difference spectra were obtained at 10 degrees C for unlabeled recombinant Rho and isotope-labeled L-[ring-2H4]Tyr-Rho expressed in Spodoptera frugiperda cells grown on a stringent culture medium containing enriched L-[ring-2H4]Tyr and isolated using a His6 tag. A comparison of these difference spectra revealed reproducible changes in bands that correspond to tyrosine and tyrosinate vibrational modes. A similar pattern of tyrosine/tyrosinate bands has also been observed in the bR --> M transition in bacteriorhodopsin, although the sign of the bands is reversed. In bacteriorhodopsin, these bands were assigned to Tyr-185, which along with Pro-186 in the F-helix, may form a hinge that facilitates alpha-helix movement.

Effects of temperature on rhodopsin photointermediates from lumirhodopsin to metarhodopsin II.

Absorbance changes following the photolysis of mildly sonicated membrane suspensions of bovine rhodopsin are monitored using multichannel detection at 15, 20, 25, 30, and 35 degrees C. Difference spectra collected with microsecond time resolution are analyzed by singular value decomposition and multiexponential fitting. Several kinetic schemes are tested using methods that compare the observed rates and associated spectral amplitudes to the eigenvalues and eigenvectors of kinetic matrices. The time evolution of the spectra is more complex than can be accounted for by the traditional lumi-->metarhodopsin I<-->metarhodopsin II scheme. Above 25 degrees C, the formation of metarhodopsin II is achieved without a large transient accumulation of metarhodopsin I. Within the framework of first-order kinetics, the observations are explained by simple kinetic schemes that lead to the formation of a deprotonated Schiff's base species temporally distinct from metarhodopsin II directly upon the decay of lumirhodopsin.

Solvent and temperature effects on the excited singlet state absorption of diphenylbutadiene.

Nanosecond excited state absorption spectra of all-trans-1,4-diphenyl-1,3-butadiene (DPB) and a rigid s-cis DPB analog, 1,4-diphenyl-1,3-cyclopentadiene, were obtained in several hydrocarbon solvents at room temperature and low temperatures. Analysis of the excited state absorption spectra of these two molecules suggests the presence of excited state s-cis rotamers in DPB at room temperature.

Photolysis of rhodopsin results in deprotonation of its retinal Schiff's base prior to formation of metarhodopsin II.

Absorption changes following photolysis of bovine rhodopsin in mildly sonicated membrane suspensions are monitored at 25 degrees C. Difference spectra collected at 17 times between 1 microsecond and 75 ms following excitation are analyzed globally using singular value decomposition and non-linear least-squares fitting techniques. The results are not consistent with the simple scheme: Lumirhodopsin-->Metarhodopsin I<-->Metarhodopsin II, but indicate that an intermediate with a deprotonated Schiff's base is formed nearly simultaneously with metarhodopsin I upon the decay of Lumirhodopsin.