1. Convenient methods for the preparation of tritiated 11-cis-retinol, 11-cis-retinal and rhodopsin are described. Irradiation of labelled rhodopsin in the presence of sodium borohydride resulted in the irreversible binding of the retinyl moiety to the active site. Degradative studies established that the retinyl moiety in this reduced derivative of rhodopsin was attached to the ∈-amino group of lysine. In connexion with this investigation the synthesis of a number of N-retinylidene- and N-retinyl-amino acids was achieved. Derivatives of lysine with the retinyl moiety attached either to the α-amino group or to the ∈-amino group were also synthesized and characterized. 2. It is suggested that the involvement of a charge-transfer interaction between the retinylidene chromophore and a suitable group −X or −X·H on the opsin best explains the spectroscopic properties of rhodopsin and other visual proteins.
