Gas-phase hydrogen deuterium exchange behavior of lysine and its homologs

The gas-phase hydrogen/deuterium exchange behavior for protonated lysine and its three shorter homologs, ornithine (Orn), 2,4-diamino butanoic acid (Daba), and 2,3-diamino propanoic acid (Dapa) were studied in a modified LCQ-DECA ion trap mass spectrometer. The HDX behavior of protonated lysine has been studied previously by several groups. It undergoes five equivalent exchanges, corresponding to the hydrogen atoms attached to the protonated lysine side chain and amine terminus nitrogen, and one slower exchange corresponding to the COOH hydrogen. The shorter homologs also exchange all six labile hydrogen atoms but at different rates. Relative to protonated lysine, DapaH+ exchanges two times faster, DabaH+ exchanges slightly slower, and OrnH+ exchanges nearly 30 times more slowly. High-level hybrid density functional theory calculations attribute this difference to the increased barrier for the relay-type exchange mechanism in OrnH+, relative to the other homologs.

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► We determined HDX rate coefficients for lysine its three lower homologs.
► All four homologs exchange all six labile hydrogens for deuterium using D2O.
► The data are consistent with relay and flip–flop mechanisms.
► DapaH+ exchanges the fastest, LysH+ and DabaH+ are slower, and OrnH+ is 30–50 times slower.
► The exchange rates correlate with calculated HDX barriers at the B3LYP/6-31+G(d,p) level.