Bicycle pedal mechanism

I, 4-di-o-tolyl-l,3-butadiene in glassy media at 77 K one-bond-twist and bicycle-pedal mechanisms. Angew. Chem. Int. Ed., 47, 1237-1240. 

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... 

Explain the following concepts and keywords photostationary state Rydberg state NEER principle bicycle-pedal mechanism phototransduction phototherapy photochemical deracemization enantiodifferentiating photosensitization photochromism di-Tt-methane rearrangement asymmetric photochemical synthesis single-crystal-to-single-crystal photochemistry photochemotherapy rule of five. 

More than 30 years ago Warshel proposed, on the basis of semiempirical simulations, an isomerization mechanism that could explain how this process can occur in the restricted space of the Rh binding pocket (Warshel 1976). Since two adjacent double bonds were found to isomerize simultaneously the mechanism reveal a so-called bicycle pedal motion. Due to the concerted rotation of two double bonds in opposite directions the overall conformational change is minimized and hence this mechanism was found to be space-saving. The empirical valence bond (EVB) method (Warshel and Levitt 1976) was used to compute the excited state potential energy surface of the chromophore during a trajectory calculation where the steric effects of the protein matrix were modeled by specific restraints on the retinal atoms. Since then, Warshel and his coworkers have improved the model employing better structural data and new computational developments (Warshel and Barboy 1982 Warshel and Chu 2001 Warshel et al. 1991). The main refinement of the bicycle pedal mechanism was that the simultaneous rotation of the adjacent double bonds is aborted at a twist of 40° and leads to the isomerization of only one bond (Warshel and Barboy 1982). 

Several years ago, we found that the isomerization of n-butylammonium ( Z,Z)-muconate produces the corresponding EE-isomer in a high yield in the crystalline state under photoirradiation [41]. This solid-state photoisomerization was re e e< t0 he a one-way reaction and no EZ-isomer was formed during the reaction, unsaturated compounds such as olefins, polyenes, and azo compounds generally undergo reversible one-bonded photoisomerization to form a mixture of omers. Previously, we pointed out the possibility that the isomerization of the but thni(i er Va ves the s°hd state f°ll°ws the bicycle-pedal reaction mechanism, et al [4 eta s °ffhe molecular dynamics ofthe reaction had not been clarified. Saltiel react an< fl have independently discussed volume-conserving... 

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... 

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. 

Warshel first postulated the bicycle-pedal (BP) mechanism as a volume-conserving mechanism for photoisomerization.In it, two alternating double bonds rotate concertedly with only the two CH units turning in and out of the plane of the molecule. 

A wheel and axle is a rotating lever (the wheel) that moves around the fulcrum (the axle). The wheel and axle utilizes the mechanical advantage of a second-class lever. The larger the wheel diameter, the greater the mechanical advantage that is gained (Figure 10-9). Examples of this second-class lever (wheel and axle) include a door knob and the pedal spindle on your bicycle. 

Have you ever wondered how pedaling moves your bicycle Have you ever thought about how the disc tray on your CD player opens and closes These processes and many more are made possible by people designing and controlling devices called mechanisms. A mechanism is a device that transmits movements so that the output movement is different from the input movement. Mechanisms can change the movement s speed, its direction, or its type of motion. Motion can be in the form of linear motion, rotary motion, or reciprocating motion. 

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