Cis/trans conformational change

The cis/trans conformational change of the rotamers of chloroacetaldehyde, C1CH2C(H)=0, is another case in point the c/ s-form has a higher dipole moment and is stabilized by the more polar solvents. Its mole fraction is 45% in cyclohexane, 61% in dichloromethane, 72% in acetone and 84% in dimethylsulfoxide (Reichardt 1988). 

As discussed elsewhere in this volume, Kawabata and co-workers showed that a light-induced cis-trans conformational change in azobenzene could affect the electrical conductivity of an LB film [68]. 

The 3-methyl- and 3-phenylperhydropyrido[l,2-c][l,3]oxazines all adopt the trans-fused conformation (274—277).135 The two 4-methyl derivatives 278 and 279 also adopt the trans conformation (Jgem — 7.8 Hz in both isomers) but 279 shows an enhanced Aae (0.87 ppm) (cf. 0.59 ppm in 278) which cannot be due to changes in cis trans conformational equilibria.279... 

Figure 12.2 cis-trans conformational conversion upon formation the Au-Ln arrays by incorporating gold(I) acetylide diphosphine complexes with Ln(hfac)3 units [122], (Redrawn from H.B. Xu et al, Conformation changes and luminescent properties of Au-Ln (Ln = Nd, Eu, Er, Yb) arrays with 5-ethynyl-2,2 -bipyridine, Inorganic Chemistry, 47, 10744, 2008.)... 

High temperature searches of conformational space (see Quenched Dynamics on page 78), can produce unwanted conformational changes, such as cis-trans peptide flips, ring inversions, and other changes that you cannot reverse easily by geometry optimization. You can use restraints to prevent these changes. 

One interesting phenomenon was the effect of the boron substituent on enantioselectivity. The stereochemistry of the reaction of a-substituted a,/ -unsatu-rated aldehydes was completely independent of the steric features of the boron substituents, probably because of a preference for the s-trans conformation in the transition state in all cases. On the other hand, the stereochemistry of the reaction of cyclopentadiene with a-unsubstituted a,/ -unsaturated aldehydes was dramatically reversed on altering the structure of the boron substituents, because the stable conformation changed from s-cis to s-trans, resulting in production of the opposite enantiomer. It should be noted that selective cycloadditions of a-unsubsti-tuted a,/ -unsaturated aldehydes are rarer than those of a-substituted a,/ -unsatu-... 

The similarity of the Rieske protein in the P6s crystal form to the structure of the spinach RFS is also obvious from the conformation of residue Pro 175 in the Pro loop As in the structure of the soluble ISF, the trans conformation is observed for Pro 175 of the Rieske protein in the P6s22 crystal form, while the cis conformation, as in the RFS, is observed in the P65 crystal form (42). The conformational change occurring at Pro 175 as well as a bend occurring at Gly 137 lead to a rotation by approximately 6° of the cluster binding subdomain between the two crystal forms. 

In principle, RTK autophosphorylation could occur in cis (within a receptor monomer) or in trans (between two receptors in a dimer). In the first case, ligand binding would cause a change in receptor conformation that would facilitate c/ s-autophosphorylation of tyrosine residues located within or outside the PTK domain. In the second case, no conformational change must occur upon dimerization. The simple proximity effect would provide sufficient opportunity for trans-phosphorylation of tyrosines in the cytoplasmic domain by a second RTK. 

Comparing the potentials across each row, we can test the idea of additivity of ortAo-substituent effects for 2-fluoro, 6-chloro, and then 2-fluoro-6-chloro substitution. The definition of a = 0 changes across each row to permit easy visual addition of the potentials in the physically appropriate manner to assess the degree of additivity of ort/io-chlorine and ort/io-fluorine substituent effects. For o-fluorotoluene and 2-fluoro-6-chlorotoluene, a = 0 denotes the (minimum-energy) pseudo-trans conformation for o-chlorotoluene, a=0 denotes the pseudo-cis conformation. The notion of additivity has considerable merit in all three electronic states. 

Because of the difference in energy of these two groups, temperature changes alter the population of the two states. Ab initio calculations indicated [57,58] that the high-frequency peak is due to the trans conformation and the lower frequency peak to the cis conformation. Analysis of the relative band area gives the population of the two states as a function of temperature and hence allows... 

The active compound within the bacillary layer is retinal. To simplify the photo-physics within the rods and cones hugely, absorption of a photon initiates a series of conformational changes that lead ultimately to photo-isomerization of retinal from the 11-cis isomer to the 11-trans isomer see Figure 9.20. The uncoiling of the molecule following photo-excitation triggers a neural impulse, which is detected and deconvoluted by the brain. The photochemical reaction is breakage and, after rotation, re-formation of the C=C bond. 

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