Ultraviolet spectra of dehydropeptides by double beam measurement

Abstract

The double beam measurement (d.b.m.) method, in which the u.v. spectrum of an unsaturated peptide is scanned using the saturated analogue as a reference, allowed the isolation of the u.v. absorption peaks of the dehydroamino-acid moiety in unsaturated dehydropeptides (DHP). Three types of DHP were measured as models: ΔAla peptides having no β-substituents, showed peaks at 200 and 240 nm; ΔLeu peptides with a β-alkyl substituent also showed two peaks at 200 and 220–230 nm; and ΔPhe peptides with a β-phenyl substituent had three peaks at 200, 220, and 280 nm. These distinct peaks were caused by steric effects about the double bond of the dehydroamino-acid moieties. The effect of configuration (Z or E) was also discussed.