Back-folded isomer

Using conformational searching/quench dynamics and 7) relaxation measurements, each back-folded isomer was determined to be smaller than its extended counterpart. Thus, the effective distance of electron transfer was not reflected in the hydrodynamic radius of the molecule. Rather, the back-folded isomers were argued to be less mobile with the iron-sulfur core buried more deeply within them. The extended isomers were more mobile with the iron-sulfur core more able to come closer to the molecular surface. By this reasoning, the back-folded isomers had a larger effective electron transfer distance than the extended isomers. 

Other modifications include a formamide residue (191), a ring fragmentation product of thymine, which exists as either a cis or a trans conformer. Both isomers are rotated out of the helix, and the bases on either side of (191) occupy the space vacated by it. A pyrene abasic site base pair in DNA duplexes adopts the usual B-form duplex, with the pyrene residue within the duplex stacking on adjacent nucleobases. The abasic site folds back over the opposite strand to shelter the hydrophobic base from exposure to water.The photoresponsive azobenzene analogues (133, R- and 5-forms) have been incorporated into DNA for NMR analysis. Both isomers intercalate between neighbouring base pairs, and the 5-isomer exhibits more disturbance in its duplex structure which is refiected in lower Tms compared to the R-isomer. ... 

The best studied oxidative addition is that of H2 to the 16e square planar d species, IrCl(CO)(PPh3)2, known as Vaska s complex [97]. This gives anl8e d octahedral Ir(III) dihydride complex (Eq. 2.31). In a concerted addition of this sort, two mutually trans ligands in the starting Ir(I) complex fold back with the result that the cis dihydride isomer is formed, at least initially. Subsequent rearrangement may also occur. 

The second series of related materials was based on the (3,4-didodecyloxyphenyl)diaminodi-oxime derivative (73). Vicinal dioximes are capable of coordinating through N,N or N,0 sites of the oxime groups. Upon the complexation, the ligands fold back, and the transition metal complexes are V,V-coordinated with a square-planar structure. Both isomeric forms were selectively isolated for the nickel complexes, but only the anii-isomer for the palladium complex. The two uwti-complexes complexes exhibited a Colh phase ((73) M = Ni Cr 78 Colh H7 I M = Pd Cr 80 Colh 131 I). Most astonishing, however, was the observation of a Colh phase for the isomeric amphi-mckel complex ( , Z-isomer), between 66 °C and 145 °C. Not only was the existence of mesomorphism surprising and unexpected, but the mesophase stability also increased with respect to the anti-complex. 

X-ray crystallographic studies on a tetradentate N2S2-macrocyclic complex of palladium, [Pd(tetraL)]Cl2 2H20, reveal the existence of two five-coordinate isomers, (4) and (5), one with a Pd-Cl distance of 3.20 A, and the other with a Pd-Cl distance of 3.68 This supports previous solution studies, which indicate that as Cl approaches [Pd(tetraL)], the square folds back about the N-Pd-N axis to form a trigonal bipyramid. ... 

However, before moving on to LSCE materials, we will discuss briefly the isomerisation of azophenols in both isotropic and liquid-crystalline environments. Specifically, 4-(5-hexenyloxy)-4 -hydroxyazobenzene (AZO-OH) has been chosen as a representative example since it will be further used as a comonomer in LSCEs. The cis isomer of AZO-OH exhibits relaxation times of 311 and405 ms in ethanol and acetonitrile at 298 K, respectively (Fig. 18.8b). Otherwise, its isomerisation rate is more than 1000-fold slower when it is dissolved in toluene at the same temperature (t = 23 min). In both ethanol and acetonitrile, the thermal back reaction of AZO-OH proceeds through a solvent-assisted tautomerisation to yield a hydrazone-like intermediate. Subsequently, a rotation around the N-N bond of the hydrazone-like intermediate can occur regenerating thereby the thermodynamically stable trans isomer. Such an intermediate cannot be promoted when the azo dye is dissolved in toluene. 

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