Insights into the post-translational methylation of arginine from studies of guanidinium-water nanodroplets

Highly solvated methylated guanidinium ions ([Me(n)Guan(H2O)(m)](+); n=0-4 and 6; m=1-> 50), which model the various post-translationally methylated states of arginine, were generated in the electrospray ionization (ESI) source of an unmodified commercial tandem mass spectrometer. The dehydration processes of highly solvated [Me,Guan(H2O)(21)](+) ions were monitored by using energy-dependent ESI MS. These data, together with supporting calculations, provide a detailed picture of the interplay of methylation and hydration, and both the calculations and the experimental evidence suggest a specific structural basis for the observed effects. The ramifications for biochemical binding events that are controlled by the post-translational methylation of arginine to give dimethylarginines are discussed.