Circular dichroic properties of the tyrosine residues in tetrazole analogues of opioid peptides

CD studies on tetrazole analogues of opioid peptides show that peptides sharing the same N ‐terminal sequence, H‐TyrΨ[CN 4 ]Gly‐, give very large Cotton effects of the Tyr side chain in the near‐UV region. CD spectra of five such peptides: H‐TyrΨ[CN 4 ]Gly‐Gly‐Phe‐Leu‐OH ( I ), H‐TyrΨ[CN 4 ]Gly‐Phe‐Pro‐Gly‐Pro‐Ile‐NH 2 ( II ), H‐TyrΨ[CN 4 ]Gly‐Phe‐Pro‐NH 2 ( III ), H‐TyrΨ[CN 4 ]Gly‐Phe‐Gly‐Tyr‐Pro‐Ser‐NH 2 ( IV ), and H‐TyrΨ[CN 4 ]Gly‐Phe‐Asp‐Val‐Val‐Gly‐NH 2 ( V ), and two others for comparison: H‐Tyr‐GlyΨ[CN 4 ]Gly‐Phe‐Leu‐OH ( VI ) and H‐TyrΨ[CN 4 ]Ala‐Phe‐Gly‐Tyr‐Pro‐Ser‐NH 2 ( VII ), were measured in methanol, 2,2,2‐trifluoroethanol, and water at different pH values. The spectra show that the conformations of the Tyr 1 residue in peptides I – V are very similar in all solvents used but differ distinctly from those observed for VI and VII . Strong Tyr bands in the aromatic region result probably from the rigid structure of the common N ‐terminal part of peptides I – V . These bands are weaker for IV , which maybe due to the presence of a second Tyr residue in that peptide, giving an opposite contribution to the CD spectrum as that arising from Tyr1. It seems that the rigid structure of the N ‐terminal part of I – V results from the interaction of the Tyr 1 side chain and the tetrazole ring. The CD bands of the Tyr residues of VI and VII are much smaller than those of I – V in all solvents, except VII in trifluoroethanol (TFE) where Tyr bands comparable in intensity to those of I – V are observed. This spectral property may derive from the same sign contribution of both Tyr residues of VII to the CD spectrum. Copyright © 2005 European Peptide Society and John Wiley & Sons, Ltd.