Small-angle X-ray investigation of the equilibria between copper(II) and glycyl-L-histidylglycine in water solution. A method for analysing polydispersed systems

Water solutions of copper(II) and glycyl-L-histidylglycine contain a large particle of molecular weight of approximately 9000 and also low-molecular-weight particles with an average molecular weight of 430. This is shown in a small-angle X-ray scattering study at 21° and neutral pH using a Kratky camera. The concentration of copper(II) was 34-72 mM and the molar ratio of peptide to copper was 1.39 to 1. This study further indicates that the particles contain about ten weight per cent water, and their shapes are an oblate ellipsoid of revolution with an axial ratio of 0.2. The scattering data were analysed with a method similar to that of Mittelbach & Porod [Kolloid Z. Z. Polym. (1965). 202, 40-49]. It was assumed that the particles in solution follow a discontinuous size distribution function dependent on two parameters: one associated with the complex formation constants, and the other with the relative weights of the units building up the complexes. That a very large particle exists is in qualitative agreement with a previous electromotive-force study which indicated that a series of large complexes is formed. The presence of such a large particle in the solution is also in agreement with a single-crystal study on the same system. The crystal structure consists of a three-dimensional network which contains 40% disordered water located in channels limited by rings of six dimers of copper and peptide.