Assembly and stabilization of multi-amino acid zwitterions by the Zn(II) ion: a computational exploration

Density functional calculations were employed to stabilize multi-amino acid zwitterions by metal ions as well as to understand the stabilizing mechanism. This work distinguishes from the previous metalations and stabilizations of a single amino acid zwitterion, where the metal ions are coordination-unsaturated. It was found that up to three Gly zwitterions can be stabilized by the Zn(2+) ion at the same time. The addition of the fourth Gly zwitterion to the Zn(Gly)(3) system causes the spontaneous transformation of one zwitterion to the neutral isomer. The single mutations of Gly in Zn(Gly)(4) with Arg or Asp render the Zn(2+)-combined four amino acid zwitterions geometrically stable. The double mutation of Gly with Arg further stabilizes the structure with four amino acid zwitterions, with its energy being lower than the one containing one neutral and three zwitterionic isomers. The systematic studies on a series of M-combined amino acid systems (M = Zn(2+), Zn(+), Li(+), Na(+)) indicated that enough charge transfers from the metal ions are essential to stabilize the amino acid zwitterions. There exist critical values of charge transfers, and for Zn(2+)-combined amino acid systems, an estimation of 0.20 |e| is required to stabilize each Gly zwitterion. Further analysis revealed that the charge transfers rather than the formations of direct M-O bonds play a more important role in the stabilizations of amino acid zwitterions.