Complex formation of cyclo(L-Phe-L-Pro)4 with noradrenaline hydrochloride

The 13C-NMR spectrum of cyclo(L-Phe-L-Pro)4 (1) and DL-noradrenaline hydrochloride (DL-NA.HCl) in a mixture of CDCl3 and CD3OD suggested the formation of a complex, which was demonstrated to be 1:1 from examination of the titration curves. The complex retained the C2-symmetric conformation of 1 containing two cis-peptide bonds, and was linked through hydrogen bonds between the carbonyl groups of the Phe1 and Pro2 residues, and the ammonium moiety of NA.HCl.