Conformational similarities in the beta-ionone ring region of the rhodopsin chromophore in its ground state and after photoactivation to the metarhodopsin-I intermediate. by Spooner, P, Sharples, J, Goodall, S, Seedorf, H, Verhoeven, M, Lugtenburg, J, Bovee-Geurts, P, DeGrip, W, Watts, A