A systematic study of Pd(II)—dipeptide chemistry followed by CD, NMR, and electronic absorption spectroscopies

Natural CD, proton NMR, and electronic absorption spectroscopies were employed in order to determine the nature of the Pd(II) coordination sphere and to learn of changes of configuration which the coordinated dipeptide molecule might undergo, as the pH is changed from 1.3 to 13, but especially from 7 to 13, of solutions containing Pd(II)-dipeptide complexes. One of the most important observations is the different behavior between the previously studied tripeptide complex, Pd(GlyGly-L-Ala) 1-, and the carboxylate terminal leucine or valine dipeptide complexes, Pd(II)-ValVal, -ValLeu, -LeuVal, -LeuLeu, -GlyVal, and -GlyLeu, i.e., there is no inversion of the signs of CD rotational strengths in the d-d excitation bands of the above dipeptide complexes of this study, as the pH of their solutions is changed stepwise from 7 to 13, whereas similar changes of pH caused CD sign inversions for the tripeptide complex. The origin of the conservation of the CD activity of these particular dipeptide complexes can be understood in terms of the hexadecant sector model of Martin et al., in conjunction with using dimensionally scaled peptide molecular models, viz., the alkyl groups, R′, at the α-position are large so as to be constrained within the original sector (as at pH ~7) when the carboxy terminal group leaves the coordination sphere (as at pH ~13). In other respects the above Pd(II)-dipeptide complexes and the others of this study behave in the manner of the Pd(II)-tripeptide complexes of earlier study, e.g., at low pH of ca. 1.3 the protonation of the dipeptides is favored over their coordination to Pd(II), so that the electronic spectra in the region of d-d excitations have the appearance of the simpler ion, [PdCl 4] 2-, on using HCl for acidification.