Modeling metal cation-phosphate interactions in nucleic acids: activated dissociation of Mg+, Al+, Cu+, and Zn+ complexes of triethyl phosphate.

Threshold collision-induced dissociation techniques are employed to determine the activation energies (AEs) and bond dissociation energies (BDEs) of metal cation-triethyl phosphate complexes, M(+)(TEP), where M(+) = Mg(+), Al(+), Cu(+), and Zn(+). Activated dissociation resulting in loss of ethene, C(2)H(4), corresponds to the primary and lowest energy pathway for all four systems examined. Sequential loss of additional C(2)H(4) molecules and loss of the intact TEP ligand is also observed at elevated energies. Theoretical calculations at the B3LYP/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of neutral TEP and the M(+)(TEP) complexes, transition states, intermediates, and products of the activated dissociation of these complexes. Theoretical AEs and BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) level using the B3LYP/6-31G* optimized geometries. The agreement between the calculated and measured AEs for elimination of C(2)H(4) is excellent for all four systems. In contrast, less satisfactory agreement between theory and experiment is found for the M(+)-TEP BDEs and may indicate limitations in the competitive model used to analyze these high energy dissociation pathways. The influence of the valence orbital occupation of the metal cation on the binding and activation propensities for elimination of ethene from TEP is examined. The binding of metal cations to TEP is compared to that of the nucleobases to assess the binding preferences of metal cations to nucleic acids.