Figure 3.

²H NMR relaxation of retinal sheds new light on activation mechanism of rhodopsin. (a) Summary of analysis of solid-state ²H NMR measurements. Order parameters of rapidly spinning methyl groups are designated by $S_{{\mathrm{C}}_3}$; the pre-exponential factor is k₀ for 3-fold axial jumps or D₀ for continuous diffusion; and E_a indicates the activation energy. (The diffusion model assumes either D_⊥=0 (right) or η_D ≡ D_‖/D_⊥=1 (left) except for the C5-Me in Meta I, where η_D≠1.) (b–d) Proposed activation mechanism for rhodopsin in membranes based on X-ray¹⁹, FTIR¹¹, and ²H NMR data¹² (see text). Isomerization of retinal displaces the E2 loop towards the extracellular (e) side with fluctuations of TM helices H5 and H6 exposing transducin (G_t) recognition sites on the opposing cytoplasmic (c) surface. Figure produced (PDB code 1U19)³ using PyMOL [http://pymol.sourceforge.net/]