engleski

Kinetics of gas-phase hydrogen/deuterium exchange and gas-phase structure of protonated phenylalanine, proline, tyrosine and tryptophan

Site-specific rate constants for the gas-phase hydrogen/deuterium (H/D) exchange of four, three, five and five hydrogen atoms in protonated phenylalanine (Phe), proline (Pro), tyrosine (Tyr) and tryptophan (Trp), respectively, were determined from matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICRMS) experiments with D2O, D2S, and CH3OD as deuterating agents. No H/D exchange was observed with D2S. For exchange with both CD3OD and D2O, which is about ten times slower in the latter, results indicate for all compounds protonation of the -amino group in agreement with theoretical results. Also, with both reagents, all compounds exchange at the COOH site more than ten times faster than at the protonation site, with OH and NH sites of Tyr and Trp, respectively, exchanging slowest. The observation of H/D exchange despite the high differences in proton affinities between the amino acids and deuterating agent exceeding 200 kJ mol-1 is in agreement with lowering of the barrier for proton transfer through hydrogen bonding proposed by Lebrilla and coworkers.

hydrogen/deuterium exchange ; mass spectrometry

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