On the interaction of Cu2+ with the heavy dipeptide Gly-Trp

Distinct species are observed upon complexing of glycil-triptophan with Cu2+. The spectroscopic characterization of these complexes formed in different pH was made using visible light absorption (350-1100 nm) and electron paramagnetic resonance at room and liquid-nitrogen temperatures, with the samples in aqueous solution at the ratio of 10L:1M. Three species were identified in the following pH ranges: 4.0-6.0, 6.5-11.0, and above 12.00. The spectroscopic data and pK values of the Gly-Trp deprotonatable groups (in the presence of the metal) suggest that the complexes are CuL2(pH approximately 5.0), CuL(H2O). The complex above pH = 12.00 showed the bulky effect of the tryptophan side chain on the stereochemistry of the complex. The square planar symmetry is destroyed and a distorted tetahedral symmetry is achieved: the hyperfine parameter Az is reduced towards the value that occurs in blue proteins and the lowering of axial symmetry can be viewed by an increase in [gx-gy]. The tridentate complex CuL(H2O) was crystallized and single crystal measurements gave the molecular gyromagnetic tensor, but spin-spin interaction between neighbor ions masked the copper hyperfine interaction.