Salt Bridge Structure of Microhydrated Arginine Kinetically Trapped in the Gas Phase by Evaporative Cooling

Aminoacids and peptides generally exhibit zwitterionic forms withsalt bridge (SB) structures in solution but charge-solvated (CS) motifsin the gas phase. Here, we report a study of non-covalent complexesof the protonated amino acid arginine, ArgH(+)(H2O)( n ) (n = 1-5),produced in the gas phase from an aqueous solution with a controllednumber of retained water molecules. These complexes were probed bycold ion spectroscopy and treated by quantum chemistry. The spectroscopicchanges induced upon gradual dehydration of arginine were assignedby structural calculations to the transition from SB to CS geometries.SB conformers appear to be present for the complexes with as few as3 retained water molecules, although energetically CS structures shouldbecome prevailing already for ArgH(+) with 7-8 watermolecules. We attribute the revealed kinetic trapping of argininein native-like zwitterionic forms to evaporative cooling of the hydratedcomplexes to as low as below 200 K.