Density features for tryptophan residues
There are density bulges close to the positions of all five of the bulky tryptophan residues in the ground-state structure. Since these residues have associated density features in the map, it is possible to rule out large rotations of helices 1, 3, and 4. There is a density bulge in the middle of helix 6, on the side facing the β-ionone ring, which lies close to the ground-state position of Trp265. This density feature significantly deviates from the position of Trp265 in the ground-state structures, suggesting the possibility of movement of Trp265. The kink in the helix in this region, and hydrogen-bonds from Cys264, Tyr268 and Pro291 to a water molecule (10,13), might allow a localized rearrangement of this part of the helix, without moving the distal ends. We attribute this movement to meta I formation.