The peptide-chain size dependence of positive dissociation energy effect in metallized and protonated polydentate oligoglycine peptides

摘要

The dependence of the positive dissociation energy(PDE)on the size of both protonated and various metal cationized(M~(+/2+)=Na~+,K~+,Mg,and Ca~(2+))oligoglycines G_nRM(G=glycine;n=l-5,and including 6 for G_nHK)in the gas phase has been first determined at the B3LYP level with different basis sets.Results show that these PDEs become negative ones when n>=5 for G_nHNa,n>=6 for G_nHK,n>=2 for G_nHMg,and n>=3 for G_nHCa.Moreover,the PDE effect decreases gradually along the increase of oligoglycine size for all these PDE systems.More charge transfer to the ambient ligand and large electrostatic effect of the two divalent cation(Mg~(2+)and Ca~(2+))involved systems make them lose the PDE effect more rapidly,and hold more compact and oblate shapes than the monovalent ones.Differently,polydentate G_nHK prefers a spherical to oblate shape along with the increase of oligoglycine size.