Fig. 4.

Interactions that stabilize the dimeric structure. a Position of F298 in relation to the critical residue R201. These residues appear to be engaged in a cation–π stacking interaction with a distance of 3.6 Å between ηN1 and ζC, a distance of 3.5 Å between ηN2 and εC1, and a distance of 3.6 Å between εN and δC2. However, the R201 side chain is not fully resolved in the electron density (Fig. 2). The position of helix 15, which contains N316 is also visible. b Position of N316 relative to the crossover helix (helix 14), showing how N316 appears to stabilize the turn from helix 15 to the final strand of the C-terminal β-sheet (strand 13). N316 hydrogen-bonds with the backbone of residues 323 and 324. The interaction distances in angstroms are as follows: N316 δO to L323 N (2.8), N316 δO to V324 N (2.8), and N316 δN to V324 O (2.9). c Interactions between the N-terminal domain and the adjacent C-terminal domain. R129 forms a salt-bridge with E331 (3.5 Å between ηN1 and εO2). E330 forms a salt-bridge with R132 (3.4 Å between εO1 and ηN2) and a hydrogen-bond with W128 (3.0 Å between εO1 and εN)