Photoreactions of retinochrome at very low temperatures

Retinochrome is one of the retinal proteins found in the retina of cephalopods. It catalyses the light isomerization of retinal from the all‐transto the 11‐cisform. On cooling to 25 K, the absorption peak of retinochrome (λmax490 nm) was broadened with a shoulder, showing the spectrum steepened on the long wavelength side. On irradiation with yellow‐green light (550 nm), retinochrome produced an intermediate with λmaxat a shorter wavelength, around 465 nm, and a lower extinction coefficient than lumiretinochrome. It changed to lunuretinochrome (λmax475 nm) in the dark on warming to liquid nitrogen temperature. We shall call this new intermediate prelunuretinochrome.