Tandem mass spectrometric evidence for the involvement of a lysine basic side chain in the coordination of Zn(II) ion within a zinc-bound lysine ternary complex

Abstract

We present the tandem mass spectrometry applications carried out to elucidate the coordination structure of Zn(II) bound lysine ternary complexes, (Zn+Lys+Lys-H)+, which is a good model system to represent a simple (metallo)enzyme-substrate complex (ES). In particular, experimental efforts were focused on revealing the involvement of a lysine side chain ε-amino group in the coordination of Zn2+ divalent ions. MS/MS fragmentation pattern showed that all the oxygen species within a complex fell off in the form of H2O in contrast to those of other ternary complexes containing amino acids with simple side chains (4-coordinate geometries, Figure 1a), suggesting that the lysine complexes have different coordination structures from the others. The participation of a lysine basic side chain in the coordination of Zn(II) was experimentally evidenced in MS/MS for Eε-Acetyl-L-Lys Zn(II) complexes with acetyl protection groups as well as in MS/MS for the ternary complexes with one NH3 loss, (Zn+Lys+Lys-NH3-H) +. Detailed structures were predicted using ab initio calculations on (Zn+Lys+Lys-H)+ isomers with 4-, 5-, and 6-coordinate structures. A zwitterionic 4-coordinate complex (Figure 7d) and a 5-coordinate structure with distorted bipyramidal geometry (Figure 7b) are found to be most plausible in terms of energy stability and compatibility with the experimental observations, respectively.