In rhodopsins, cis-trans photoisomerization of the retinal chromophore occurs in the specific protein environment even at low temperature, where molecular motions are significantly prohibited. In this study, we found that illumination of the rhodopsin chromophore in clay interlayers at 77 K results in formation of a bathochromic product, which is reverted to the original state by light in a manner similar to what is observed in the rhodopsins. Infrared spectral changes suggest that the chromophore is protonated in clay, and that trans-cis isomerization occurs at 77 K. The clay interlayer thus works to mimic the primary photochemical reaction in rhodopsin. (C) 2002 Elsevier Science B.V. All rights reserved.