Random coil arrangement

Melting Temperature. The double helix of polynucleotides described above becomes thermodynamically unstable at particular temperatures (with specified conditions of solute concentration, pH, etc.) and is transformed into the open random-coil arrangement. This transformation is rather sharp, and can be measured by the concurrent changes in a number of physical properties of the nucleic acid, such as the optical absorption coefficient. The midpoint of the transition region is called the melting point. 

Random coil arrangement (Section 24.8) A type of protein secondary structure that is flexible, changing, and statistically random in its conformations. 

Amorphous thermoplastic polymers have polymer chains in a random coil arrangement without any degree of local order, whereas a semi-crystalline polymer would have some degree of order of the polymer chains. The chains are entangled and because they are not fixed, can slip past one another, whereas a thermoset resin, when subjected to local stress breaks, in a brittle maimer. The ability of the thermoplast to dissipate energy by chain slippage confers the property of toughness to the composite. 

Polymer chain Random coil arrangement Degree of local order. Can be annealed to control the level of crystallinity. If too high, polymer is brittle... 

In practical rubbers, a few cross-links are added to prevent slippage of the molecular chains and permanent deformation under tension (or flow). With such polymers, when the stress is released, the normal thermal motion of the atoms causes them to return to a random-coiled arrangement. If the forces between the chains are strong and the chains fit easily into a regular geometric pattern, the material is a typical fiber. In cases where the forces are moderate and the tendency to form a regular lattice is also moderate, the result is a typical plastic. 

Proteins are the most abundant species in cells and tissues and hence their absorptions dominate the spectra of cells and tissues (see the section above on proteins). The spectra of proteins vary with the second order structures of proteins, that is, spiral form (a-helix), an extended chain O-form) and a random coil arrangement, the protein s state of hydration and the ionic strength of the solvent. The spectra of metabolic and structural proteins found in cells have similar features. The only proteins to exhibit distinctly different features are found in connective tissue, such as collagen. 

The CD spectra of poly(D-lysine) reflect a markedly low degree of complexation. The prevalent part of poly(D-lysine) remains in unchanged random coil arrangement. The extent of complexation may be judged from comparison with the CD spectrum of poly(D-lysine) at pH = 11.3 (Fig. 11.1). Complexation efficiency of poly(L-lysine) with pectate is almost 100% while with poly(D-lysine)... 

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