Models 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. 

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],... 

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. 

This motion is very similar to the hula-twist motion, a model of the cis-trans isomerization of retinylidene chromophore in rhodopsin as shown in Fig. 4.24 [16-19]. Such a motion is believed to be very suitable for isomerization in a highly restricted environment, for example, in enzymatic reactions in a Uving cell or... 

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