Influence of d orbital occupation on the binding of metal ions to adenine.

Threshold collision-induced dissociation of M(+)(adenine) with xenon is studied using guided ion beam mass spectrometry. M(+) includes all 10 first-row transition metal ions: Sc(+), Ti(+), V(+), Cr(+), Mn(+), Fe(+), Co(+), Ni(+), Cu(+), and Zn(+). For the systems involving the late metal ions, Cr(+) through Cu(+), the primary product corresponds to endothermic loss of the intact adenine molecule, whereas for Zn(+), this process occurs but to form Zn + adenine(+). For the complexes to the early metal ions, Sc(+), Ti(+), and V(+), intact ligand loss competes with endothermic elimination of purine and of HCN to form MNH(+) and M(+)(C(4)H(4)N(4)), respectively, as the primary ionic products. For Sc(+), loss of ammonia is also a prominent process at low energies. Several minor channels corresponding to formation of M(+)(C(x)H(x)N(x)), x = 1-3, are also observed for these three systems at elevated energies. The energy-dependent collision-induced dissociation cross sections for M(+)(adenine), where M(+) = V(+) through Zn(+), are modeled to yield thresholds that are directly related to 0 and 298 K bond dissociation energies for M(+)-adenine after accounting for the effects of multiple ion-molecule collisions, kinetic and internal energy distributions of the reactants, and dissociation lifetimes. The measured bond energies are compared to those previously studied for simple nitrogen donor ligands, NH(3) and pyrimidine, and to results for alkali metal cations bound to adenine. Trends in these results and theoretical calculations on Cu(+)(adenine) suggest distinct differences in the binding site propensities of adenine to the alkali vs transition metal ions, a consequence of s-dsigma hybridization on the latter.