Structural and spectroscopic studies of diamminecobalt(III) complexes of tripeptides

The crystal structure of [Co(NH3)2(L-ala-gly-gly)]-2H20 has been determined by X-ray structure analysis. The crystals are triclinic with the space group P1 (C12; No. 1), with Z =, and with a = 7.2913 (3) ‘, b = 8.7373 (15) A, c = 10.7212 (8) A, α = 85.930 (9), β = 77.169 (6), and γ = 84.253 (9). The structure has been refined by least-squares methods to an R value of 0.0627 (2056 reflections). The N-terminal chelate ring adopts a nonplanar structure with the methyl group in an equatorial orientation. The absorption and circular dichroism spectra of this complex and analogous complexes with other N- and C-terminal substituted peptides are reported. The effects of the different substituents are compared with those for similar divalent metal complexes and are discussed in terms of different contributions to the chirality of the complexes including the side-group rotamer populations and the above nonplanarity of the chelate ring system.