Mass spectrometric studies on alkali metal cationized amino acids and peptides

In the present project, the theoretical calculated Li+, Na+ and K+ affinities of six amides were validated experimentally by the mass spectrometric kinetic method, and were used as reference compounds to determine the K+ affinities of 20 common α-amino acids and their O-methyl/ethyl ester derivatives by kinetic methods. Due to the presence of entropic effects (different K+ binding structures) in the dissociation of K+ bound heterodimers among different amino acids with functionalized side-chains, the K+ affinities of the 20 amino acids have to be determined separately in three separate relative affinity ladders, and the experimental K+ affinities of the 20 amino acids and their O-methyl/ethyl esters were determined.; Our experimental determined K+ affinities of Gly, Ala, Val, Leu, Ile, Cys, Pro, Ser, Thr, Phe, Gly-OMe, Ala-OMe, Val-OMe and Leu-OMe are excellent in agreement with high-level theoretical calculation results carried by Dr. F. M. Siu and Ms. C. H. S. Wong of Prof. Tsang's research group, with absolute deviations in the range of 0.1–7.3 kJ mol−1 . Such excellent agreement between our experimental and theoretical results lends confidence to the binding structures obtained in our theoretical studies.; The O/S/Ring heteroatom site in the side chain of serine, threonine, cysteine, phenylalanine, tyrosine, tryptophan are involved in tridentate binding to K+ to yield the most stable binding mode in charge-solvated (CS) conformations. Zwitterionic (ZW) binding modes are usually less stable primarily because of the ‘charge-separation’ factor (energy required to pull a positive charge and negative charge apart against their mutual attraction). However, the most stable K+ binding modes for the basic amino acids proline, histidine and arginine are in the zwitterionic conformation because the basicity factor favoring formation of ZW conformations more than compensate the unfavourable ‘charge separation’ factor. The relative stability of the charge-solvated and zwitterionic K+ binding modes of other amino acids were discussed.; Besides amino acids, we have also extended our kinetic method measurements to determine the K+ affinities of selected dipeptides consisting of aliphatic amino acids and amino acids with functionalized side chains (e.g. proline, serine, phenylalanine and tyrosine). The experimental K+ binding affinities of dipeptides were found to be within a relatively narrow range of 139.0–157.8 kJ mol−1, suggesting the binding modes in these dipeptides might be very similar. (Abstract shortened by UMI.)...