A major project concerns the chromophore all-trans to 13-cis photoisomerization in retinal proteins, among them the light driven proton pump bacteriorhodopsin (BR), the light driven Cl--pump halorhodopsin (HR) and the photoreceptors sensory rhodopsin I and II (SRI and II). With transient IR-experiments on BR it was possible for the first time to time resolve the isomerization process directly by vibrational spectroscopy.
BR, HR and SR as well as further retinal proteins bind the same all-trans retinal chromophore however in slighty different binding pockets. This leads to different timing of the isomerization as well as different reaction pathways on the excited electronic state potential energy surfaces. Crucial for this diversity, and thus this kind of reaction control, are the properties of the chromophore binding pocket. Another approach for disentangling the various elemantary processes accompanying the chromophore isomerization is to employ retinal analogues.