J. A. Pardoen

Vibrational analysis of the all-trans retinal protonated Schiff base.

We have obtained Raman spectra of a series of all-trans retinal protonated Schiff-base isotopic derivatives. 13C-substitutions were made at the 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 positions while deuteration was performed at position 15. Based on the isotopic shifts, the observed C--C stretching vibrations in the 1,100-1,400 cm-1 fingerprint region are assigned. Normal mode calculations using a modified Urey-Bradley force field have been refined to reproduce the observed frequencies and isotopic shifts. Comparison with fingerprint assignments of all-trans retinal and its unprotonated Schiff base shows that the major effect of Schiff-base formation is a shift of the C14--C15 stretch from 1,111 cm-1 in the aldehyde to approximately 1,163 cm-1 in the Shiff base. This shift is attributed to the increased C14--C15 bond order that results from the reduced electronegativity of the Schiff-base nitrogen compared with the aldehyde oxygen. Protonation of the Schiff base increases pi-electron delocalization, causing a 6 to 16 cm-1 frequency increase of the normal modes involving the C8--C9, C10--C11, C12--C13, and C14--C15 stretches. Comparison of the protonated Schiff base Raman spectrum with that of light-adapted bacteriorhodopsin (BR568) shows that incorporation of the all-trans protonated Schiff base into bacterio-opsin produces an additional approximately 10 cm-1 increase of each C--C stretching frequency as a result of protein-induced pi-electron delocalization. Importantly, the frequency ordering and spacing of the C--C stretches in BR568 is the same as that found in the protonated Schiff base.

Absolute resonance Raman intensities demonstrate that the spectral broadening induced by the β‐ionone ring in retinal is homogeneous

The sensitivity of absolute resonance Raman cross sections to the excited state homogeneous width is used to examine whether the electronic spectral breadth induced by the β‐ionone ring of retinal is primarily a homogeneous or an inhomogeneous effect. Resonance Raman intensities have been measured for both all‐trans retinal and its 5,6‐dihydro analog. The dihydro analog has a structured absorption spectrum because the broadening effect of the C6–C7 bond is lost when the π system does not extend into the ionone ring. While the two molecules have similar relative Raman intensities, the absolute cross sections are about a factor of two lower in all‐trans retinal, indicating that the ‘‘effective homogeneous linewidth’’ is significantly larger in all‐trans retinal than in the dihydro analog. Calculations carried out to model both the absorption spectra and the resonance Raman intensities require an effective homogeneous linewidth that is more than twice as large for all‐trans retinal (∼1200 cm−1) as for the 5,...

Evidence from FTIR-Measurements for a Separation of the Protonated Schiff Base from the Counterion into a Less Polar Environment

The chromoprotein Bacteriorhodopsin transducer light-energy into electrochemical energy by a light-driven protontransfer across the membrane 1). In order to elucidate the proton pump mechanism in molecular detail, we investigated the photocycle with low-temperature FTIR Difference-Spectroscopy