Carbonyl-17O n.m.r. of amino acid and peptide carboxamide and methyl ester derivatives§

Abstract

Specific carbonyl enrichment with 17O of amino acid OMe esters by up to 103 times over natural abundance was affected by treated [17O]-.alpha.-COOH amino acids with SOCl2 in MeOH. Carbonyl-[17O]-Gly-NH2, Gly-NHCH3 and Gly-N(CH3)2 were obtained from [17O]-Gly-OMe by (methyl)aminolysis with NH3, CH3NH2 and (CH3)2NH gases, respectively. Peptide [17O]-carboxamides were prepared by (methyl)aminolysis of Z-Pro-Leu-[17O]-Gly-OMe [Z = benzyloxycarbonyl] followed by catalytic hydrogenation to remove the Z group. 17O chemical shifts of amino acid and peptide carboxamides in H2O, MeOH, CH3CN and DMSO [dimethyl sulfoxide] were 260-324 ppm. downfield relative to H2O, depending on .alpha.-NH2 ionization, substitution on both amino groups and solvent H bonding, primarily to amide oxygen. Introduction of an amide-N methyl group usually caused upfield shifts (.apprx. - 10 ppm.), attributed to elimination of one NH bond, while a second N methylation had the opposite effect. Amino acid and peptide OMe ester carbonyl-[17O] resonances appeared 326-359 ppm. downfield relative to H2 reflecting on side chain interactions, state of .alpha.-NH2 ionization and H-bonding with the solvent. Effective rotational correlation times for glycine and peptide carboxamide-[17O], calculated from T2 relaxation data, were of similar magnitude to values derived from solution properties and depended on the MW and solvent viscosity.