Linkage rotations about

In a protein, the amino-acid residues are linked by -OC-NH- units, which are called peptide linkages. Rotation about the C-N bond is not possible, therefore the amide group is flat. The flexibility of protein chains comes from rotation about the asymmetric carbon atom in the amino-acid residue. 

The aryl C—O—C linkage has a lower rotation barrier, lower excluded volume, and decreased van der Waals interaction forces compared to the C—C bond. Therefore, the backbone containing C—O—C linkage is highly flexible. In addition, the low barrier to rotation about the aromatic ether bond provides a mechanism for energy dispersion which is believed to be the principal reason for the toughness or impact resistance observed for these materials.15 17... 

In analyzing the conformational properties of carbohydrates, it has often been the practice to consider individual monomer rings to be rigid units, with the only molecular flexibility lying in torsional rotations about the linkage bonds. 

Laouenan, P. et al., NMR structural investigations and conformational analysis of condensed tannins. A continuing challenge due to restricted rotation about the interflavanyl linkage, Analusis, 25, M29, 1997. 

Figure 25-12 Ball-and-stick model of a peptide unit showing the coplanarity of the CNCC atoms of the amide linkage, here in the trans configuration, and the possibility of rotation about the C-C and N-C bonds.

Treatment of 348 with Na[EIBEt3] has also been reported, affording initially the Pt(II) formyl complex anion [Tp PtMe C(0)H ] (355 ), as determined by 1EI and 13C NMR spectroscopic data that again reveal a chiral metal center. At 193 K two formyl environments (2 1 ratio) are apparent, which coalesce at 201 K. These are attributed to restricted rotation about the Pt—C linkage, and AG for conversion of the minor to major isomer was calculated at 8.8 kcal mol-1.120 Protonation of 355 affords exclusively the Pt(IV) formyl complex Tp PtMe C(0)H H (356), with no evidence for protonation of either pyrazole or the formyl, or... 

In 12, the donor and acceptor moieties are chiral, and, as noted by the authors, the molecule therefore exists as a pair of diastereomers which are separable by chromatography. However, rotation about the linkages between the porphyrin macrocycle and the attached aryl rings must be very slow on the time scale of electron transfer. Thus, non-interconverting diastereomers should be present, with slightly different separations and orientations between the aniline donor and the quinone acceptor. This distribution would be expected to influence the decay kinetics of D+-P-QT if charge recombination is via direct electron transfer. If a... 

The Q-P-P-Q tetrad 43 has been synthesized by Sessler and coworkers [69, 98]. As expected, it demonstrates strong quenching of the porphyrin fluorescence, and this presumably occurs by electron transfer. It is probable that selective metallation of such a tetrad would produce a species which would behave as a triad in a manner analogous to 17 and 18, but it is not obvious that such a molecule would have any significant advantages over these systems. As is the case with a number of the other complex systems, 43 may in principle demonstrate stereoisomerism when rotations about the linkage bonds are slow on the time scales of electron and energy transfer. 

Pyridines attached to another aryl or hetaryl ring also introduce the possibility of restricted rotation about the biaryl linkage. Typically, this requires three substituents at the or/ o-positions on the biaryl as in the case of the naphthyl derivatives 48, where the stereochemistry is determined by NMR spectroscopy <2001J(P1)1785>. Other methods of determining conformations, such as the comparison of experimental and computed circular dichroism spectra, have been applied to the related Yaoundamine alkaloids such as the derivative 49 <1997T2817>. 

The arrangement of the chains in the molecules, when the heads have packed closely, is a matter of speculation at present. There can, however, be little doubt that the chains do not remain vertical, but tilt over until the space above the heads is filled as completely as possible. The ordinary stereochemical theory of Organic Chemistry requires that the long chains should be flexible, with a joint at every carbon atom produced by the power of free rotation about the carbon-to-carbon linkage and evidence as to the flexibility of the chains will appear at various points in this... 

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