Hydration of guanidinium depends on its local environment

Hydration of gaseous guanidinium (Gdm(+)) with up to 100 water molecules attached was investigated using infrared photodissociation spectroscopy in the hydrogen stretch region between 2900 and 3800 cm(–1). Comparisons to IR spectra of low-energy computed structures indicate that at small cluster size, water interacts strongly with Gdm(+) with three inner shell water molecules each accepting two hydrogen bonds from adjacent NH(2) groups in Gdm(+). Comparisons to results for tetramethylammonium (TMA(+)) and Na(+) enable structural information for larger clusters to be obtained. The similarity in the bonded OH region for Gdm(H(2)O)(20) (+) vs. Gdm(H(2)O)(100) (+) and the similarity in the bonded OH regions between Gdm(+) and TMA(+) but not Na(+) for clusters with <50 water molecules indicate that Gdm(+) does not significantly affect the hydrogen-bonding network of water molecules at large size. These results indicate that the hydration around Gdm(+) changes for clusters with more than about eight water molecules to one in which inner shell water molecules only accept a single H-bond from Gdm(+). More effective H-bonding drives this change in inner-shell water molecule binding to other water molecules. These results show that hydration of Gdm(+) depends on its local environment, and that Gdm(+) will interact with water even more strongly in an environment where water is partially excluded, such as the surface of a protein. This enhanced hydration in a limited solvation environment may provide new insights into the effectiveness of Gdm(+) as a protein denaturant.