Table 4. Number of water molecules mediating interaction between residues with opposite or like charges across the interface.

		Interface Type
		Inter_H	Inter_C
(a) Residues are of the opposite charges	sc-sc	68	54
	mc-mc	71	39
	mc-sc	134	99
(b) Residues are of the same charge	sc-sc	69	41
	mc-mc	77	37
	mc-sc	82	68

sc and mc correspond to the side chain and the main chain, respectively. All the possible pairs of hydrogen bond interactions are considered (so that a given water molecule can contribute to more than one category of interactions). As such the total number of cases under Inter_H and Inter_C (501 and 338) exceeds the number (334 and 217) given in Table 3.