Nmr studies of a free and protected tetrapeptide glycyl‐ L ‐prolylglycylglycine in β‐turn‐supporting environment

Nmr studies of the protected and free tetrapeptide Gly‐Pro‐Gly‐Gly were carried out in β‐turn‐supporting solvents, that is, in CDCl3 for Z‐Gly‐Pro‐Gly‐Gly‐OMe and in Me2SO‐d6 for H‐Gly‐Pro‐Gly‐Gly‐OH. Comparisons with specifically α‐deuterated analogs gave complete assignments of the NH and methylene regions. Analysis of chemical shifts, coupling constants, and the temperature dependence of chemical shifts show that the peptide adopts a type II β‐turn conformation. This turn is stabilized for the protected tetrapeptide by two hydrogen bonds between (i) CO (Gly1) and NH(Gly4), and (ii) urethane function NH and methyl ester CO.