Platinum-195 NMR spectroscopy of nitrogen-15-labeled-peptide complexes

Abstract

The reactions of PtCl{sub 4}{sup 2{minus}} with oligoglycyl peptides in aqueous solution proceed by amine coordination followed by sequential deprotonation and coordination of available peptide nitrogens. Complexes of PtLCl{sub 3}{sup 2{minus}}, Pt(H{sub {minus}1}L)X{sub 2}{sup 2{minus}}, Pt(H{sub {minus}2}L)X{sup {minus}}, and Pt(H{sub {minus}3}G{sub 4}){sup 2{minus}} (where L = triglycine (G{sub 3}), triglycinamide (G{sub 3}a), or tetraglycine (G{sub 4}), X = Cl{sup {minus}} or OH{sup {minus}}, and H{sub {minus}n} designates the number of deprotonated-N(peptide) bonds) are characterized by use of {sup 195}Pt NMR spectroscopy. Several bis(peptide) complexes also are identified. Structural assignments are made on the basis of coupling constants with ({sup 15}N)peptides. The magnitude of the J{sub Pt-N} coupling constants are Pt-N{sup {minus}}(peptide) > Pt-N(amine) {approx} Pt-NH{sub 3} (N{sup {minus}} denotes the deprotonated peptide nitrogen); the trans influence on Pt-N(amine) or Pt-N{sup {minus}}(peptide) is N{sup {minus}}(peptide) >> OH{sup {minus}}, RNH{sub 2}, NH{sub 3} > Cl{sup {minus}}. The chemical shifts for {sup 195}Pt are determined primarily by the elements coordinated to platinum(II) and secondarily by their chemical form. Monodentate amine coordination by the oligopeptide or the formation of the first chelate ring with amine and N{sup {minus}}(peptide) coordination gives an upfield shift of 110 ppm relative to the signal fore the corresponding Pt(NH{submore » 3})X{sub 3}{sup {minus}} or cis-Pt(NH{sub 3}){sub 2}X{sub 2} complex. Two linked consecutive five-membered rings from peptide chelation cause an offsetting downfield shift of 120 ppm, and three linked consecutive five-membered chelates introduce an additional downfield shift of 250 ppm (relative to the coordination of one or two more NH{sub 3} molecules). 38 refs., 10 figs., 5 tabs.« less