Influence of electronic, steric and stacking interactions in ternary Ni(n) and Cu(II) complexes containing 2,2 ',2"-terpyridine and aseries of amino acids

Abstract

Formation constants for ternary complexes MLA, where M=Cu(II) or Ni(II), L=2,2',2"-terpyridine (terpy) and A=alanine (ala), phenylalanine (phe), tyrosine (tyr), tryptophan (trypt), threonine (thr), methionine (met) or histidine (hist) have been determined <em>p</em>H-metrically in aqueous solutions at 35.0°C and μ = 0.2 <em>M</em> (KNO₃). The ability of the various amino acids to bind the binary (M(II)-terpy)) complex relative to the aquo metal ion has been quantitatively assessed in terms of the parameter <img src='http://www.niscair.res.in/jinfo/smaller.gif' border=0> log <em>K. </em> With respect to the metal ion, Cu(II) ternary complexes have been found to be substantially unstable relative to the corresponding Ni(II) complexes. In ternary Ni(II) complexes amino acids without an aromatic side chain form stable complexes due to Ni(II) → terpy π-interaction, whereas amino acids with an aromatic side chain form still more stable complexes due to similar n-interactions and also metal ion mediated stacking interactions. The intramolecular equilibrium between the stacked and unstacked isomers and the percentage of the stacked isomer have been quantified in terms of the parameters <em>K</em>₁ and (% MLA)_st, respectively.