Comparison of two thioxopeptide bond photoswitches in insect kinin

The photoisomerization abilities of secondary thioxopeptide bond (CS–NH) and thioxo prolyl bond (CS–N) incorporated into the C-terminal pentapeptide of insect kinin were compared. H-Phe-Phe-ψ[CS–NH]-d-Ala-Trp-Gly-NH2 and H-Phe-Tyr-ψ[CS–N]-Pro-Trp-Gly-NH2 were studied by UV–vis absorption. The isomerization energy barriers of the two segments, Ac-Phe-ψ[CS–NH]-d-Ala-NH2 and Ac-Tyr-ψ[CS–N]-Pro-NH2 picked from the two peptides, were calculated using ab initio method. The cis isomer of CS–N is more stable than that of CS–NH due to higher energy barrier, so the former is more suitable in peptide structure–activity relationship studies.