Possible origins of mutational effects of H587N and the double mutant K603Q/N667K, derived from the snapshots of various mutant simulations at 44 ns. The JH2 C helix is shown in orange and the JH1 C-helix is shown in pink. The H587 side chain forms hydrogen bonds with the Y590 backbone and the S591 backbone. In N587, the hydrogen bond network is modified. The S591 backbone now forms a hydrogen bond with the N587 backbone. The changes in hydrogen bond pattern result in a rotation of the JH2 C-helix (indicated by the yellow) as well as changes of the pattern of the hydrogen bond network between E890, R588, and N589. In K603Q/N667K, a control mutant, the JH1/JH2 interface seems to be maintained but the hydrogen bond network is significantly different from that of the wild-type.