The resonance Raman spectrum of the 11-cis retinal protonated Schiff base chromophore in rhodopsin exhibits low-frequency normal modes at 93, 131, 246, 260, 320, 446, and 568 cm-1. Their relatively strong Raman activities reveal that the photoexcited chromophore undergoes rapid nuclear motion along torsional coordinates that may be involved in the 200-fs isomerization about the C11C12 bond. Resonance Raman spectra of rhodopsins regenerated with isotopically labeled retinal derivatives and demethyl retinal analogues were obtained in order to determine the vibrational character of these low-frequency modes and to assign the C11C12 torsional mode. 13C substitutions of atoms in the C12−C13 or C13C14 bond cause the 568-cm-1 mode to shift by ∼8 cm-1, and deuteration of the C11C12 bond downshifts the 568- and 260-cm-1 modes by ∼35 and 5 cm-1, respectively. The magnitudes of these shifts are consistent with those calculated for modes containing significant C11C12 torsional character. Thus, we assign the 568-cm-1 ...