The co-planar BpyAla bipyridine rings are wedged into a shallow pocket formed by the turn between G85 and P89 at the edge of the apolar tunnel. This conformation is stabilised by van der Waals interactions between C1 of BpyAla and both G85 H (3.35 Å) and D88 H (3.36 Å), C2 of BpyAla and H of P89 (2.87 Å: 3.36 Å to the C) and C3 of BpyAla and H of P89 (2.74 Å: 3.26 Å to the C) (Figure ). In addition, the side-chains of D88 and K115 (one turn down on -helix 5) lie on either face of the bipyridine and may stabilise the BpyAla conformation by aromatic- electrostatic interactions.