N.m.r. and e.p.r. investigation of the interaction of copper(II) and glycyl-l-histidyl-l-lysine, a growth-modulating tripeptide from plasma

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Abstract
Interaction of Cu(II) and Gly-His-Lys, a growth-modulating tripeptide from plasma, was investigated by 13C- and 1H-NMR and EPR spectroscopy. The NMR line-broadening was interpreted in terms of major and minor species formed as a function of pH. The NMR line-broadening is due to the presence of minor species in rapid exchange and not due to the major species in solution, which has a large .tau.M. The technique of 13C- and 1H-NMR line broadening, caused by paramagnetic Cu(II) ion, should be undertaken with caution, since the method may not be useful for obtaining structural information on the major species. The EPR spectra over a wide pH range are almost entirely due to similarly co-ordinating species. Starting at pH 5.5, the narrowest absorption near 340 mT shows superhyperfine structure, which comes out sharply in the pH region 6.0-9.6. The spectra in this pH range showed the 7 lines of nitrogen superhyperfine splitting, indicating clearly the co-ordination of 3 nitrogen atoms to Cu(II). The EPR parameters in the medium pH range, A.dblvert. = 19.5 mT and g.dblvert. 2.21, fit well with the contention that Cu(II) is ligated to Gly-His-Lys through 1 oxygen atom and 3 nitrogen atoms in a square-planar configuration.

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