Amide rotation temperature effects

Chemical shift differences of O-methylmandelates of four different methylcarbinols have been shown15a to be rather small, varying most in 3,3-dimethyl-2-butanol (4 Hz at 110 °C 13 Hz at — 90 °C). Pirkle and Simmons36 have studied the effect of temperature on ATEA derivatives and found that the configuration about the rotationally hindered amide bond is Z and is quite stable to temperature. 

Double-helical DNA in solution can undergo strand separation or dena-turation as a consequence of extremes of pH, heat, or exposure to chemicals such as urea or amides. Decrease in viscosity, increase in absorbance at 260 nm (hyperchromic effect), decrease in buoyant density, or negative optical rotation indicates denaturation of DNA. The denaturation process disrupts only noncovalent interactions between the two strands of DNA. Since G-C base pairs are held together by three hydrogen bonds in contrast to two for an A-T base pair, A-T rich DNA is easily denatured compared to G-C rich DNA (Figure 6.3). Electron microscopy can detect these A-T-rich regions in a DNA molecule since they form bubblelike structures. Hence the temperature of melting (Tm) of DNA increases in a linear fashion with... 

The synthesis of chiral poly(depsipeptides), polymers with alternating amide and ester bonds, by lipase-catalyzed ring opening of 3-isopropyl morpholino-2,5-dione (19) was shown by Hocker and coworkers (Scheme 11.5) [26], Various lipases were tested for the bulk polymerization of these heterocyclic monomers at temperatures of 100 °C or above. PPL and lipase type III from a pseudomonas species were shown to be effective catalysts. The isolated polymers showed Mn values of 3.5-17.5 kgmol-1. The influence of reaction temperature, the amount of enzyme and the presence of water in the reaction medium were shown to be important factors on the high molecular weight fraction and were investigated in detail [26b]. Comparison of optical rotation values for polymers prepared by... 

The effect of amide structure on the rotation barrier was the object of more precise work in the study of Rogers and Woodbury (1962). Several methods for analysing the rate processes from steady-state spectra were combined. The energy barriers and frequency factors are summarized in Table 5. There is some variation in the coalescence temperature Tc and the energy barriers Ea. Agreement with Gutowsky and Holm (1956) for the molecule AW-dimethylacetamide is good, but... 

Because amines are subject to hydrogen bonding, the shift depends on concentration, solvent, and temperature. Amide, pyrrole, and indole NH groups absorb from — 6 8.5 to 5.0 the effect on the absorption position of concentration, solvent, and temperature is generally smaller than jin the case of amines. The nonequivalence of the protons on the nitrogen atom of a primary amide and of the methyl groups of A Af-dimethylamides is caused by slow rotation around the C—N bond because... 

Enantiodifferentiating Z-E photoisomerization of cyclooctene (20Z) sensitized by chiral polyalkyl benzenepolycarboxylates has been extensively studied some of the results are summarized in Table 16.3. The crucial enantiodifferentiating step is the rotational relaxation of prochiral 20Z to enantiomeric perpendicular singlets within the exciplex of the chiral sensitizer with 20Z. In order to elucidate the detailed mechanism and to obtain higher enantiomeric excess (ee), the steric and electronic effects of the sensitizer and its chiral substituent as well as the involvement of the intra/intermolecular triplex have been studied extensively. Thus, the sensitizations with bulky 8-phenylmenthyl and 8-cyclohexylmenthyl 1,2,4,5-benzenetetracarboxylates afford good ees of up to 50% even at room temperature. The ee is improved at lower temperatures to reach 64% at -89°C. In contrast, chiral triplet sensitizers afford much lower ees, which are not appreciably improved even at low temperatures (at least in this system). The enantiodifferentiating photoisomerization can also be effected by chiral aromatic amides, phos-phoryl esters, - and phosphoramides. ... 

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