On irradiation at liquid helium temperatures, rhodopsin is converted into hypsorhodopsin which decays to bathorhodopsin above 23 K. The absorption spectrum of cattle hypsorhodopsin (λmax = 435 nm) is found to include a new sideband around 540 nm. This sideband may be due to π* ← π transition to 1A− g like state, which is made partially allowed by distortion of polyene chain of the retinylidene chromophore.
