Infrared studies of octopus rhodopsin. Existence of a long-lived intermediate and the states of the carboxylic group of Asp-81 in rhodopsin and its photoproducts.

The infrared absorption spectra of octopus rhodopsin and its photoproducts have been observed at 282K and 210K under irradiation of blue and orange light in a neutral condition. The acid metarhodopsin-minus-rhodopsin and lumirhodopsin-minus-rhodopsin difference spectra have been obtained. A new intermediate (called transient acid metarhodopsin) with a lifetime of about 5 s has been found to exist prior to acid metarhodopsin. The present results, together with the data obtained previously, give information on the state of the carboxylic group in the side chain of Asp-81, which is the only acidic amino-acid residue in the part of opsin buried inside the membrane. This carboxylic group is protonated throughout the transformation series, but its state changes on going from transient acid metarhodopsin to acid metarhodopsin. It is probable that these two photoproducts are different from each other only in the opsin moiety.