Quantum effects in a simple ring with hydrogen bonds

Complexes containing multiple arginines are common in proteins. The arginines are typically salt-bridged or hydrogen-bonded, so that their charges do not repel. Here we present a quantum calculation of a ring in which the components of a salt bridge composed of a guanidinium, the arginine side chain, and a carboxylic acid are separated by water molecules. When one water molecule is displaced from the ring, atomic charges of the other water molecule, as well as other properties, are significantly affected. The exchange and correlation energy differences between optimized and displaced rings are larger than thermal energy at room temperature, and larger than the sum of other energy differences. This suggests that calculations on proteins and other systems where such a ring may occur must take quantum effects into account; charges on certain atoms shift as substituents are added to the system: another water molecule, an -OH, or -CN bonded to either moiety. Also, charge shifts accompany proton shifts from the acid to guanidinium to ionize the salt bridge. The consequences of moving one water out of the ring give evidence for electron delocalization. Bond order and atomic charges are determined using natural bond orbital calculations. The geometry of the complex changes with ionization as well as the -OH and -CN additions but not in a simple manner. These results help in understanding the role of groups of arginines in salt-bridged clusters in proteins.