Evidence for two binding sites of lanthanoid shift reagents at the carbonyl groups of camphor and canthaxanthin

Abstract

By observing the shifts and broadening of ¹H and ¹³C n.m.r. signals of camphor induced by lanthanoid shift reagents, it has been shown that the metal binds at two sites on the carbonyl group. A similar situation has been demonstrated for the binding of shift reagents to the carbonyl groups in the ionone rings of the carotenoid canthaxanthin. With the aid of a novel computer program (LISCA), employing a least-squares fitting routine, the conformations of the carotenoid have been investigated. The ring-flip forms are equally populated in solution at room temperature. Our results point to, but cannot distinguish between, either an ionone ring–polyene side chain angle of ca. 90° or a 50 : 50 equilibrium mixture of the forms s-trans and 40° away from s-cis.