Transport of the Cu(II) bound with histidine-containing tripeptides to cysteine. Coordination mode and exchangeability of Cu(II) in the complexes

The transport of Cu(II) complexed with histidine-containing tripeptides (Cu(H-iL), L = HisGlyGly (i = 2), GlyHisGly (i = 1), and GlyGlyHis (i = 2)) to cysteine was examined by a stopped-flow spectrophotometric method. The S --> Cu(II) charge transfer (LMCT) bands at 335 nm. and 390 nm were used as probes for tracing the reaction. Primarily formed was the ternary Cu(H-lL)(Cys(-)) complex. The rate of the Cu(H-1L)(Cys(-)) formation depended on the affinity of Cu(II) for the donor atoms at the fourth binding site of Cu(H-2L). Cu(H-1L)(Cys(-)) subsequently reacted with free Cys(-) to yield a binary Complex, Cu(CYS-)(2). The rate of Cu(H-1L)(Cys(-)) formation was generally faster than that of conversion from Cu(H-1L)(Cys(-)) to Cu(CYS-)(2). An exception was found in the reaction with Cu(H(-2)GlyGlyHis), where the relation k(1+) < k(2+) existed. The ternary complex, Cu(H(-1)HisGlyGly)(Cys(-)), was too labile to be detect by the conventional stopped-flow methods. Probably, Cu(H(-1)HisGlyGly)(Cys(-)) upon forming changed spontaneously to Cu(HisGlyGly)(Cys(-)), in which the N-terminal His residue coordinated to the Cu(II) via the amino and imidazole nitrogens, and rapidly changed to Cu(Cys(-))(2).