All‐trans, 11‐cis and 9‐cis isomers of the C17 aldehyde analogs of retinal bound with purple membrane apoprotein, probably through a Schiff base linkage at the normal retinal binding site. The complex formed from C17 aldehyde and purple membrane apoprotein was slowly decomposed by 10mM hydroxylamine. The C17 aldehyde competitively inhibited the regeneration of purple membrane from all‐trans‐retinal and purple membrane apoprotein. The differential ability of the different isomers to inhibit the regeneration suggests that purple membrane has a binding site for the side chain of retinal in addition to the Schiff base binding site.
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Tokunaga et al. (1981) studied this question.