Photoisomerization path

M. Garavelli, T. Vreven, P. Celani, F. Bernardi, M. A. Robb, and M. Olivucci,/. Am. Chem. Soc., 120, 1285 (1998). Photoisomerization Path for a Realistic Retinal Chromophore Model The Nonatetraeniminium Cation. 

Accordingly, while the calculations on ZsZsZa BV, ZsZsEa BV and ZaZsZa P4>B discussed above were concerned with both FC relaxation and photoisomerization paths, the calculations on the other stereoisomers/bilins hitherto not discussed— ZaZsEa BV, ZsZsZa PCB, ZsZsEa PCB, ZaZsEa PCB, ZaZsEa P4>B and ZsZsEa P B—focused exclusively on the FC relaxation. For all these systems, the calculations showed that the FC relaxation is strongly dominated by stretching and torsion at the 10-11 bond. This indicates that all bilin chromophores utilized by phytochromes are intrinsically most prone to isomerize at 10-11, irrespective of which configuration (Pr or Pfr) they adopt. Importantly, it should be stressed that this conclusion is not to be taken as support for an alternative mechanism where the primary event of phytochromes is achieved by a Z -> photoisomerization at 10-11 instead of 15-16, but should be viewed as evidence that the protein must substantially alter the photochemical reactivity of the bilin chromophores for the primary event to occur at 15-16. 

Fig. 10.11 Retinal photoisomerization paths in the AT and 13C forms of ASR and energies E) of the relevant states during the photocycle of ASR given relative to the AT form. The rotary motion of retinal during the photocycle is indicated by red arrows...

Migani, A., Rohh, M. A., Olivucci, M., Relationship between Photoisomerization Path and Intersection Space in a Retinal Chromophore Model, J. Am. Chem. Soc. 2003,125,2804 2808. 

Fig. 2.2 Possible mechanisms of cis-trans (Z-E) isomerization of olefins and related compounds via heterolytic cleavage of the C=C bond photoisomerization (path A).

Cembran A, Bemardi F, Olivucci M, Garavelli M (2005) The retinal chromophore/chloride ion pair stracture of the photoisomerization path and interplay of charge transfer and covalent states. Proc Natl Acad Sci 102 6255... 

Migani A, Robb M, Olivucci M. Relationship between photoisomerization path and intersection space in a retinal chromophore model. J Am Chem Soc. 2003 25 2804-2808. 

---