Determination of retinal Schiff base configuration in bacteriorhodopsin

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Resonance Raman spectra of the BR568, BR548, K625, and L550 intermediates of the bacteriorhodopsin photocycle have been obtained in 1H2O and 2H2O by using native purple membrane as well as purple membrane regenerated with 14,15-13C2 and 12,14-2H2 isotopic derivatives of retinal. These derivatives were selected to determine the contribution of the C14—C15 stretch to the normal modes in the 1100- to 1400-cm-1 fingerprint region and to characterize the coupling of the C14—C15 stretch with the NH rock. Normal mode calculations demonstrate that when the retinal Schiff base is in the C[unk]N cis configuration the C14—C15 stretch and the NH rock are strongly coupled, resulting in a large (≈50-cm-1) upshift of the C14—C15 stretch upon deuteration of the Schiff base nitrogen. In the C[unk]N trans geometry these vibrations are weakly coupled and only a slight (<5-cm-1) upshift of the C14—C15 stretch is predicted upon N-deuteration. In BR568, the insensitivity of the 1201-cm-1 C14—C15 stretch to N-deuteration demonstrates that its retinal C[unk]N configuration is trans. The C14—C15 stretch in BR548, however, shifts up from 1167 cm-1 in 1H2O to 1208 cm-1 in 2H2O, indicating that BR548 contains a C[unk]N cis chromophore. Thus, the conversion of BR568 to BR548 (dark adaptation) involves isomerization about the C[unk]N bond in addition to isomerization about the C13[unk]C14 bond. The insensitivity of the native, [14,15-13C2]-, and [12,14-2H2]K625 and L550 spectra to N-deuteration argues that these intermediates have a C[unk]N trans configuration. Thus, the primary photochemical step in bacteriorhodopsin (BR568 → K625) involves isomerization about the C13[unk]C14 bond alone. The significance of these results for the mechanism of proton-pumping by bacteriorhodopsin is discussed.

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