Ultraviolet resonance Raman evidence for the absence of tyrosinate in octopus rhodopsin and the participation of Trp residues in the transition to acid metarhodopsin

The ultraviolet (244 nm) resonance Raman spectra of octopus rhodopsin and its photoproduct, acid metarhodopsin, do not give any evidence for a tyrosinate. This finding excludes the possibility that Tyr-112 serves as the counter anion to the protonated Schiff base as does Glu-113 in bovine rhodopsin. Upon photoconversion from rhodopsin to acid metarhodopsin, Trp and Tyr Raman bands decrease in intensity and concomitantly a Trp band shifts in frequency. The changes of Trp Raman bands are ascribed to changes in hydrophobic interactions and conformation, suggesting a possible role of Trp in the photoconversion process of octopus rhodopsin.