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We did these experiments in the hope of developing a general procedure for separating dipeptides from higher oligopeptides. Dobbie & Kermack (1955a, b; cf. Dobbie, Kermack & Lees, 1955) have given evidence that tripeptides form different types of complex with cupric ions from those formed by dipeptides. This could be the basis for an ionophoretic group separation. Our preliminary experiments were encouraging and we have now made a detailed comparison of the ionophoretic behaviour of the cupric complexes of glycine, glycylglycine and diglycylglycine with that predicted from the equilibrium constants given by Kermack and colleagues. The agreement is good. Triglycylglycine (whose equilibrium constants are not available in the literature) behaved similarly to the tripeptide under our conditions, so the method has possibilities for group separations of naturally occurring mixtures. Kermack and colleagues used filter-paper ionophoresis for testing their theory of complex formation with glycylglycine, but not with diglycylglycine. Our results with the latter compound provide an independent line of evidence on the reaction of cupric ions with the tripeptide. Moreover, whereas their experiments were done with a limited amount of cupric ion migrating in a buffer containing the ligand at fixed concentration, we have studied the migration of a limited amount of ligand in a buffer solution saturated with cupric ion. We gave a preliminary account of this work to the New Zealand Institute of Chemistry (Carnegie

Filter-Paper ionophoresis of cupric complexes of neutral amino acids and oligopeptides