How the Counterion Affects Ground- and Excited-State Properties of the Rhodopsin Chromophore

Abstract

High-level DFT and ab-initio CASSCF/CASPT2 methodologies have been applied to study the ground and excited states of the retinal chromophore as a function of counterions of different complexities. On the basis of the recent 2.8 A X-ray structure of rhodopsin, the complete retinal chromophore was optimized in the presence of anionic Glu113, Thr94, and a water molecule (Wat2b), which together form a complex counterion of the protonated Schiff base. In addition, complexes were studied in which components of the counterion (Thr94 and/or Wat2b) were removed and also the free chromophore without any counterion, both in the protonated and deprotonated forms; for the CASSCF/CASPT2 calculations a reduced model chromophore with five conjugated double bonds was employed. In the presence of any counterion, bond alternation increases strongly in the region close to the Schiff base nitrogen, resulting in a structure similar to the protonated Schiff base. There is a conspicuous reduction in bond alternation in the three...