Sidechain motions

Poly(n-butyl acrylate). A study of the relaxation properties of PBA was initiated for several reasons. There are two backbone carbons with directly bonded protons thus the effect of the side chain on backbone motion might be determined. Also, the CH carbon should more directly reflect the distribution of correlation times necessary to begin analysis of alkyl sidechain motion. Finally, the lack of the additional chain-CH3 groups significantly loosens motional constraints in PBA. The effect of this on the overall dynamics of PBA was of interest. 

The scenario would envisage that the crystal strain increases in the phenylurethane series such that a blue shift, which is essentially that of the LT to HT transition, would result. The loss of detailed structure occurring on the thermochromic transition to the HT spectrum would result from an extreme increase in disorder which could be mainly attributed to dynamic processes, such as sidechain motions, which are intensified at higher temperatures. A theoretical approach not unlike this has been recently advanced by Schweizer (24). Both weak and strong disorder regimes are considered in that treatment. 

The fluctuations of atoms and the motions of sidechains in proteins have been examined experimentally and theoretically. In this section we characterize these two types of motions in terms of their amplitudes, time scales, and other properties by describing a series of theoretical studies related to them. Some results on the functional roles of specific atomic and sidechain motions are also presented. Comparisons with experiment are provided, where available a more detailed analysis of the experimental measurements is given in Chapt. XI. 

The motions of sidechains in proteins play an important role in their dynamics. The time scales involved range from picoseconds for local oscillations in a single potential well to milliseconds or longer for some barrier crossings, such as the 180° rotations (ring flips ) of aromatic sidechains. This range of motions makes it necessary to use a variety of theoretical approaches in the analysis of sidechain dynamics they include molecular dynamics, activated dynamics, and stochastic dynamics (see Chapt. IV.). There are a number of well-characterized examples where sidechain motions have been shown to play a specific role in protein function. 

Dielectric Measurements. Recent dielectric experiments at abovezero temperatures have detected a number of molecular motions that might occur at low temperatures (23, 24, 25). In particular, sidechain motions and the motions of counter-ions could contribute to the water relaxation processes. Frequencies in the range of 104 to 108 have been suggested for such effects, but, at present, there is no direct line of evidence connecting these motions to the low frequency nmr results. 

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