Rhodopsin has been selectively spin-labeled near the cytoplasmic termi
ni of helices C and G. Photoactivation with a light flash induces an e
lectron paramagnetic resonance spectral change in the millisecond time
domain, coincident with the appearance of the active metarhodopsin II
intermediate. The spectral change is consistent with a small movement
near the cytoplasmic termination of the C helix and reverses upon for
mation of the MIII state. These results provide an important link betw
een the optical changes associated with the retinal chromophore and pr
otein conformational states.