GOLD(III) AND PALLADIUM(II) COMPLEXES OF GLYCYLGLYCYL-L-HISTIDINE - CRYSTAL-STRUCTURES OF [AU-III(GLY-GLY-L-HIS-H-2)]CL-CENTER-DOT-H2O AND [PD-II(GLY-GLY-L-HIS-H-2)]CENTER-DOT-1.5H(2)O AND HIS-EPSILON-NH DEPROTONATION

Proton NMR studies show that [AuCl4](-) reacts slowly with glycylglycy l-L-histidine (Gly-Gly-L-His) (t1/2=9.3 h at 310 K and pH 2) in D2O a t pH (meter reading) values as low as 1.5 to form the stable complex [Au-III(Gly-Gly-L-His-H-2)]Cl . H2O 1 via one intermediate. Complex 1 is shown by X-ray crystallography to be square-planar with gold bound to the terminal NH2[Au-N 2.049(10) Angstrom], two deprotonated amide n itrogens [Au-N- 1.941(9), 2.006(10) Angstrom] and His delta N [Au-N 2. 038(9) Angstrom] giving one six-membered and two five-membered chelate rings. At pH 7 the reaction of [AuCl4](-) with Gly-Gly-L-His follows a different course, apparently involving the formation of Au-III cros s-linked polymers. The anion [PdCl4](2-) reacts rapidly with Gly-Gly-L -His also at-pH 2, and forms a similar square-planar complex [Pd-II(G ly-Gly-L-His-H-2)]. 1.5H(2)O 2 involving the terminal NH2 [Pd-N 2.058( 7) Angstrom], two deprotonated amide nitrogens [Pd-N- 1.943(7), 1.983( 6) Angstrom] and His delta N [Pd-N 2.016(6) Angstrom]. By potentiometr y, pK(a) values of 2.58 (CO2H), 8.63 (His epsilon NH, 'pyrrole nitroge n') and 11.50 (co-ordinated NH2) for 1 and 11.30 (His epsilon NH) for 2 were determined and confirmed by H-1 NMR spectroscopy.