Resonance Raman evidence for an all-trans to 13-cis isomerization in the proton-pumping cycle of bacteriorhodopsin

Using a dual-beam flow technique, we have obtained resonance Raman spectra of the M412 photointermediates of both native purple membrane (15H M412) and purple membrane regenerated with 15-deuterioretinal (15D M412). For comparison, we have also obtained Raman spectra of the n-butylamine Schiff bases of the 13-cis and all-trans isomers of 15H and 15D retinal. The 15D model compound spectra, when compared to the 15H spectra, show isotopically induced spectral changes that are markedly different for the two isomers. There is a very close agreement between the frequency and intensity changes which occur upon deuteration of M412 and those which occur upon deuteration of the 13-cis model compound, but not even a qualitative correspondence exists when M412 and the all-trans model compound are similarly compared. These data demonstrate that the chromophore of M412 is an unprotonated Schiff base of 13-cis-retinal rather than all-trans-retinal. An analogous spectral comparison of 15H and 15D light-adapted bacteriorhodopsin (bRLA) with the 15H and 15D protonated Schiff bases of 13-cis- and all-trans-retinal demonstrates that bRLA contains an all-trans chromophore, in agreement with previous extraction experiments. Thus, a trans leads to cis isomerization occurs in the proton-pumping photocycle of Halobacterium halobium.