Hula-Twist mechanism

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

The hula twist mechanism (HT, Fig. 2.3B), first validated with carotenoids, is not consistent with the time-scale of photoisomerization of chromoproteins since CTI of the retinal chromophore, which is inserted deep inside the protein, necessitates a major reorganization of the peptide molecular framework. Therefore, a new volume-conserving mechanism, called bicyclic pedal (BP, Fig. 2.3C), was proposed. In fact, all these mechanisms are still a topic of discussion since chromoprotein photo-intermediates highlighted by recent studies do not confirm this hypothesis. In particular, several photo-products of the retinal Schiff base in the... 

Reactions of 1,4-Diketones. - The hula-twist mechanism is suggested as the only possible mechanism for the ,Z-isomerism of tru s-1,2-dibenzoylethene in the crystalline phase. The irradiation in sunlight of the yellow bi-indenylidenedione (152) brings about a colour change to red. The red compound has been shown to be the biradical (153). The red colour can be dispersed thermally when the radical (153) reverts to the parent (152). The photoinduced electron transfer within (154) has been studied. ... 

A review has highlighted the photoisomerism of dienes in confined conditions. The results obtained are apparently consistent with the so-called hula-twist mechanism. Liu has reviewed the recent literature dealing with cis,transisomers involving the hula-twist mechanism. The H-vinyl conical intersection, the hula-twist process, of buta-1,3-diene have been explored. ... 

Liu and co-workers have shown that irradiation of the 2,2 -dimethyl-stilbene isomers in glassy 2-methylpentane brings about isomerization by a hula-twist mechanism. A hula-twist mechanism to afford the trans isomer is also involved in the isomerization of a complex (1 1) of c/.y-3,3 -bis(di-phenylhydroxymethyl)stilbene and acetone. " The conformational dynamics... 

If the assignments in Scheme 7 are correct, formation of the Tachy products involves rotations at adjacent double and single bonds, providing the first experimental verification of an outcome consistent with Liu s hula-twist mechanism for ds— trans photoisomerization. They have stimulated the recent resurrection of this mechanism with spedal attention to the key role of the volume-conserving requirements of the rigid medium. ... 

Its practical importance aside, photochemistry in the vitamin D field continues to claim its place at the forefront of the science. Its central role in the renewed interest in the hula-twist mechanism for trans-cis photoisomerization will no doubt prompt much research to test the validity and generality of that concept. Through it all, it provides a plethora of observations confirming Havinga s NEER principle. 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

Three different photoisomerization mechanisms of singlet excited polyenes have been proposed (1) one-bond twist, a typical process observed in fluid solutions but also in glassy media,525 553 565 (2) the bicycle-pedal mechanism involving simultaneous rotation about two original double bonds, assumed to occur in a constraining environment,566-568 and (3) a volume-conserving two-bond hula twist 531 569 510 (Scheme 6.7). In some cases, the existence of the last mechanism has been ruled... 

Liu, R. S. H., Hammond, G. S., Hula Twist a Photochemical Reaction Mechanism Involving Simultaneous Configurational and Conformational Isomerization. In Horspool, W. M., Lenci, P. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, PL, 2004, Chapter 26, pp. 1 11. 

Due to constraints of space, I could not introduce many important theoretical studies here. Various important models have been proposed on the primary isomerization mechanism in rhodopsins, including the bicycle pedal model [101], sudden polarization [102], and the hula-twist model [103]. The finding of a conical intersection between the excited and ground states is also an important contribution [104]. Since the atomic structures of visual and archaeal rhodopsins are now available, theoretical investigations will become more important in the future. The combination of three methods - diffraction, spectroscopy, and theory - will lead to a real understanding of the isomerization mechanism in rhodopsins. 

A model for the light/dark behavior of GFP has been proposed [40]. It is based on quantum mechanical calculations of the energy barriers for the cp and z one-bond flips (OBF) and the (p/z hula twists (HTs) that were calculated in the ground and first singlet excited states for a small nonpeptide model compound. Figure 5.5 shows the calculated energy profiles. 

A volume-conserving mechanism such as hula-twist (H-T) that requires a concomitant twisting of the double bond and the adjacent single bond to accomphsh the double-bond ds-trans isomerization, different from the usual one-bond rotation mechanism. 

For ds-trans isomerization in highly condensed media at very low temperature <77 K, liu and Hammond postulated mechanisms called one-bond flip and hula-twist mecdianism [35-38]. According to the H-T mechanism, isomerization takes place not by the one-bond rotation around the double bond but by the concomitant twist of the double bond and the adjacent single bond to accomplish the double-bond isomerization (Figure 4.10). These mechanisms were assumed to reduce free volume... 

Hula-Twist A Photochemical Reaction Mechanism Involving Simultaneous Configurational and Conformational Isomerization... 

Liu, RS.H. and Hammond, G.S., The case of medium-dependent dual mechanisms for photoisomerization one-bond-fhp and hula-twist, Proc. Natl. Acad. Sci. USA, 97, 11153, 2000. 

Wilsey, S. and Houk, K., HA inyl conical intersections for dienes a mechanism for the photochemical hula-twist, Photochem. PhotobioL, 76, 616, 2002. 

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