Ultraviolet spectra of dehydropeptides by double beam measurement

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.