Gas-phase H/D exchange reactions of polyamine complexes: (M+H)(+), (M plusalkali metal(+)), and (M+2H)(2+)

Gas-phase hydrogen/deuterium exchange reactions between noncovalent polyami ne complexes and D2O, CH3OD, or ND3 are undertaken in a quadrupole ion tray mass spectrometer. Structural features of the protonated polyamines can be differentiated by the rates and overall extent of exchange, specifically t he presence of propylene units and/or a cyclic structure noticeably decreas es exchange compared to the exchange observed for acyclic polyamines with o nly ethylene bridges between amino groups. Significant differences are obse rved for singly protonated vs, doubly protonated complexes, where the doubl y protonated complexes undergo more efficient exchange at a higher rate tha n the analogous singly protonated complexes. Molecular modeling calculation s suggest that more diffuse conformations may exist for the higher charge s tates, thus facilitating H/D exchange. In addition, H/D exchange reactions between the alkali metal cationized complexes and ND3 are nearly quenched, compared to the significant exchange seen for singly protonated complexes. A conformational change or the loss of a low energy reaction pathway may ex plain the limited exchange reactions seen when a bulky cation replaces a pr oton in the complex. (J Am Soc Mass Spectrom 2000, 11, 711-721) (C) 2000 Am erican Society for Mass Spectrometry.