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Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
Why not use a polymer with a flexible backbone? This should in principle be feasible, and some attempts along these lines have actually been undertaken.16,24 We feel, however, that performing the dendrimerization with a rodlike backbone might have two advantages that should pay off as far as the achievement of a high degree of coverage with fragments is concerned. The first is related to the approachability of functional groups. Flexible polymers form random coils. Consequently, many of the functional groups are “buried” in the inner part and are not as exposed to chemical modification as those on the rod-like polymers. The second advantage is related to entropy. If flexible polymers are grafted with bulky substituents, the backbone conformation goes from a random coil to a stretched, more or less linear one. This is costly in entropy and therefore contraproductive to the achievement of a high degree of coverage. In the case of the polypropellanes this price has already been paid during the synthesis of the rod. At present, it is too early to say which of the approaches is the best.
Amino Acids, Peptides, and Proteins
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
A random coil is a type of secondary structure that is “random” and does not conform to a distinct structure. The peptide chains arrange in a random manner, held together by hydrogen bonding. Are most proteins composed of one of the above-mentioned secondary structures or mixtures of several?
Polymer Diffusion
Published in Anil Kumar, Rakesh K. Gupta, Fundamentals of Polymer Engineering, 2018
The theoretical prediction of the diffusion coefficient of spheres moving through a low molecular weight liquid is a problem that was examined by Einstein more than one hundred years ago [44]. This situation is of interest to the polymer scientist because isolated polymer molecules in solution act as random coils.
Enhancement of protein flocculant properties through carboxyl group methylation and the relationship with protein structural changes
Published in Journal of Dispersion Science and Technology, 2021
Rafael A. Garcia, Phoebe X. Qi, Matthew Essandoh, Lorelie P. Bumanlag
Notable changes in secondary structure included an increase in unstructured random coil content (Table 3). Typical synthetic polymer flocculants take on a random coil configuration because of electrostatic repulsion of like charges along the polymer backbone.[47,48] Presumably this configuration results in relatively low steric interference for colloidal substances that might interact with the charged sites, compared to the interference that would be present if the polymer were folded in a stable configuration. So, treatments that increase the random coil component of protein configuration might be expected to make charged sites more accessible, and consequently improve flocculant activity. Indeed, there seems to be some correspondence between the magnitude of increase in random coil and the magnitude of increase in KCE.
Exploration of ligand-induced protein conformational alteration, aggregate formation, and its inhibition: A biophysical insight
Published in Preparative Biochemistry and Biotechnology, 2018
Saima Nusrat, Rizwan Hasan Khan
Protein folding is a biophysical process where protein folds into its specific three-dimensional functional state from its random coil or other structural conformation.[10] The sequence of amino acid residues provides the knowledge regarding its native conformation,[11] and the right sequence is crucial for the functional activity of proteins. The inaccurate folding of native protein causes loss in the functional activity and misfolding, leading to the formation of toxic complexes,[5] as observed in case of aggregation-linked diseases.[12,13] The deposition and accumulation of misfolded proteins occur in the form of amorphous or amyloid fibrils in different body parts of human beings.[1,14,15]
Improvement of mechanical properties of elastic materials by chemical methods
Published in Science and Technology of Advanced Materials, 2020
Yukikazu Takeoka, Sizhe Liu, Fumio Asai
A general linear polymer chain has a diameter of approximately 0.1 nm, and when the molecular weight is 100,000, the fully extended length is approximately 1000 nm. For example, polyethylene, which is the simplest structure, has a structure in which carbon is connected by covalent bonds in the main chain, and the stable angle of C-C-C is approximately 109.5°. At room temperature, this chain is constantly and drastically changing its conformation due to 1) stretching motions between bond points, 2) bending motions of bond angles, and 3) rotational motion at bond points. However, considering the potential energy associated with the rotation of the C-C bond, it can be in the gauche state or the trans state, so that the entire polymer chain exists in a random coil state in which the string is rolled. Judging from the fact that the conformation is changed by thermal motion, it will be possible to easily stretch with a relatively weak force by grasping both ends of this polymer chain and stretching it in one direction. At this time, energy elasticity occurs in phenomena such as 1) and 2) that change the distance between atoms, but these phenomena can be ignored, and the C-C bond rotation in 3) is the most important. The chain is thereby extended. Nevertheless, the polymer chains try to return to their original random coil state, just as when the spring is stretched. This is due to the entropic elasticity of the stretched chains. When the distance x between the ends of the polymer chain follows a Gaussian distribution, the polymer chain with such characteristics is called a Gaussian chain. In its natural state, the polymer chain is a Gaussian random coil, and entropy decreases when trying to extend it from this state (Figure 2(a)). Therefore, entropy elasticity is generated by the propensity to return to the original state. In other words, the elasticity of a rubber made of a polymer chain and of a spring made of a metal is based on different principles. It is known that the tension f generated by the entropy elasticity of a single chain is expressed by the following equation in terms of statistical mechanics.