The present work shows that meta I formation involves no large (rigid-body) movements or rotations of helices from their position in the ground-state. Instead, changes seem to be localised, probably involving movement of side-chains in kinked regions of helices close to the retinal-binding pocket, such as Trp265. The observed small changes could account for spectroscopic changes known to accompany meta I formation. The larger movement of helices that characterise meta II formation (
14) are not initiated at the early photointermediate stage, but occur during the transition from meta I to meta II.
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