Hula twist

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 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. 

In such a situation, reliable theoretical studies on the absorption spectra would provide useful information on the relationship between the structure and the absorption spectrum. As shown in Figure 4-3, three models, Al, A2, and B, were examined for the photo-isomerization. The Models Al and A2 were based on the Resonance Raman study by Kneip et al [59], For Model A2, we also referred to a study by Lippitsch et al. [60] in which a rotation around a single bond (C14-C15) was also suggested (Hula Twist). Model B was based on the Resonance Raman study by Andel III and co-workers [56],... 

Liu, R.S.H. (2002) Introduction to the symposium-in-print photoisomerization pathways, torsional relaxation and the hula twist. Photochem. Photobiol., 76, 580-583. 

However, a positive proof for hula-twist could not be obtained by X-ray diffraction, as the product phase became amorphous. The external shape of the crystal did not change at the microscopic level, but AFM indicated some loss of acetone on (100) by efflorescence, forming a protective cover which can be correlated with the crystal packing. Further molecular migrations on other faces were not detected with the molecular sensitivity of AFM [6], The crystal stayed clear transparent, but a topotactic conversion is excluded if a crystalline phase becomes an amorphous product phase. 

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. 

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... 

Fig. 2.3 Three possible pathways for the photo-CTI of polyenes (A) one-bond flip (OBF) (B) hula twist (HT) (C) bicyclic pedal (BP).

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. 

Fig. 5.4 The r (N1-C1-C2-C3) and q> (Cl-C2-C3-C4) dihedral angles of the green fluorescent protein chromophore. In the protein R, is Gly67 and R2 is Ser65, and in HBDI, an often used model compound, = R2 = CH3. In r one-bond flips (r-OBF) the dihedral rotation occurs around the r torsional angle, in a (p-OBF it is around the (p dihedral angle, in a hula twist (HT) the (p and r dihedral angles concertedly rotate.

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. 

The radiationless decay has been investigated by ultrafast polarization spectros-copy [44] and time-resolved fluorescence [45,46]. The results confirm that the radiationless decay occurs by an ultrafast internal conversion, due to intramolecular motion about the bridging bond of the chromophore in the excited state, that the isomerization is nearly barrierless, and that there is only a very weak dependence on medium viscosity, thereby implying that the isomerization occurs by a volume-conserving motion such as a hula twist [47]. 

The two lowest energy minimaareatr = 27°and

headed arrows show the low-energy hula-twist pathway from the planar chromophore to the perpendicularly twisted chromophore. 

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