17O, 14N and 15N n.m.r. studies of the CO2+ complexes of cyclo(Pro17O-Gly15N) and cyclo(Gly17O-Pro) in aqueous solution

The complexation of cyclo(Pro¹⁷O-Gly¹⁵N) and cyclo(Gly¹⁷O-Pro) with CO²⁺ ions has been studied by ¹⁷O, ¹⁴N and ¹⁵N n.m.r. spectroscopy in aqueous solution. ¹⁷O, ¹⁴ N and ¹⁵N transverse relaxation times and chemical shifts were measured as a function of temperature. The ¹⁷O n.m.r. studies unequivocally demonstrate that the cobaltous ion binds to the peptide oxygen of both compounds. The hyperfine coupling constant and the peptide residence times were found to be A =–0.165 MHz and – 0.145 MHz, μ= 16, and 92μsec for cyclo(Pro¹⁷O-Gly¹⁵N) and cyclo(Gly¹⁷O-Pro), respectively. The ¹⁴N and ¹⁵N studies of labeled cyclo(Pro¹⁷O-Gly¹⁵N) do not indicate binding at either the Gly¹⁵N or the Pro¹⁴N site.