Abstract Distinct species are observed upon complexing of glycil-triptophan with Cu 2+. The spectroscopic caracterization 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 CuL 2(pH ~ 5.0), CuL(H 2O). The complex above pH = 12.00 showed the bulky effect of the triptophan side chain on the stereochemistry of the complex. The square planar symmetry is destroyed and a distorted tetahedral symmetry is achieved: the hyperfine parameter A z 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(H 2O) was crystallized and single crystal measurements gave the molecular giromagnetic tensor, but spin-spin interaction between neighbor ions masked the copper hyperfine interaction.