Hydration of ion-biomolecule complexes: ab initio calculations and gas-phase vibrational spectroscopy of K+ (indole)m(H2O)n

In recent years neutral indole(H2O)n clusters have been used to model the hydration of biomolecules containing an indole moiety. Both experimental and theoretical studies of the binary indole...OH2 system show NH...OH sigma hydrogen-bonding. By introducing a cation to the indole...OH2 model, cation...pi and ion...dipole electrostatic interactions are placed in direct competition with conventional indole...OH2 hydrogen-bonding. The effects, arising from a monovalent potassium cation on (indole)m(H2O)n clusters, were investigated using infrared photodissociation spectroscopy in the OH and NH stretching regions. In K+ (indole)1(H2O)(n < or = 4) and K+ (indole)2(H2O)(m < or = 3) clusters, the electrostatic ion...ligand interaction inhibits the formation of an indole NH...OH2 sigma hydrogen-bond. However, indole...H2O pi hydrogen-bonding via the five-membered indole ring is observed with three or more ligands around the ion.