The differential distribution of Cu+ between separate a and 0 domains of metallothionein (the isolated peptide fragments) and the rate of transfer of Cu+ between the two domains using copper-thiolate specific emission spectroscopy are reported. Kinetic data show the rate of transfer of Cu+ from the Cu(6)alpha to the Cd(3)beta domain is 2 x 10(-1) s(-1) while the transfer from Cu(6)beta to the Cd(4)alpha domain is much slower at 8 x 10(-3) s(-1), indicating the greater binding affinity of Cu+ for the NIT P domain. We report that the emission intensity Of Cu(6)beta is 0.45 the emission intensity of Cu(6)alpha-MT.lambda(max) is shown to be a probe of the environment of the Cu+. A series of copper-containing domain intermediates to the formation of the filled Cu6S9-beta and Cu6S11-alpha-clusters are identified. A mechanism is proposed for the formation of Cu-12(betaalpha)-MT that involves metal exchange reactions of Cu+ ions from the alpha to the beta domain with initial formation of a Cu(4)beta-cluster. (C) 2004 Elsevier Inc. All rights reserved.