Compact coil configuration

Simple coacervation is dependent on the structure of the macromolecules. Khalil et al. [5] and Nixon et al. [6] have shown the importance of the gelatin molecules being in a compact coil structure for simple coacervation to take place. Gelatin molecules attain this structure at pH values close to their isoelectric pH (pi) values and at low ionic strength. The compact coil configuration results from the intermolecular attractive forces which occur at the pi value... 

Aside from chemical composition and chain length the properties of macromo-lecular substances are substantially determined by the conformation and configuration of the individual macromolecules. Isolated macromolecules do not take up a precisely defined three-dimensional shape they rather assume a statistically most probable form which approximates to the state of maximum possible entropy. This is neither a compact sphere nor an extended rigid chain, but rather a more or less loose statistical coil (Fig. 1.7). 

The hydrocarbon chains of the fatty acids may be completely saturated (saturated fat) or may contain one or more double bonds. The geometric configuration of the double bond in fats and oils is normally cis. If the chain includes more than one double bond, the fat is called polyunsaturated. The presence of a double bond puts a kink in the regular zigzag arrangement characteristic of saturated carbons. Because of this kink in the chains, the molecules cannot form a neat, compact lattice and tend to coil, so unsaturated triglycerides often melt below room temperature and are thus classified as oils. 

Supercoiling markedly alters the overall form of DNA. A supercoile DNA molecule i more compact than a relaxed DNA molecule of the same length. Hence, supercoiled DNA moves faster than relaxed DNA when analyzed by centrifugation or electrophoresis. Unwinding will cause super-coiling in circular DNA molecules, whether covalently closed or constrained in closed configurations by other means. 

While the ability of podophyllotoxin to inhibit cell division was established in the mid-1940s, the mechanism of action of etoposide and teniposide was not elucidated until the mid-1980s. Like the vinca alkaloids, podophyllotoxin leads to a breakdown of microtubules and prevents polymerization to produce new ones. Etoposide and teniposide have no affect on microtubule assembly, even at doses 20 times higher than the amount of podophyllotoxin required to achieve this. They do, however, cause breaks in DNA strands, and this seems to be due to inhibition of an enzyme called topoisomerase II. In order to understand the normal functions of this enzyme, we have to consider the way in which DNA is packed into the cell nucleus. If all of the DNA of our chromosomes (we have 46 chromosomes in the nucleus of almost all of our cells) was laid end to end, it would stretch to two metres in length. Normally, the two DNA strands are wound around one another in a double helical configuration, and this is further super-coiled to provided a very compact strcuture. However, in order to reproduce itself, the DNA must become partially unravelled, and the main role of the enzyme topoisomerase II is to cause breaks in the DNA strands to allow a degree of reorganization to occur. 

In this context, a chain in its Brownian configuration is sometimes called coil and in its more compact configuration it is called globule. The transformation corresponding to the fact that X0 becomes smaller than one is called coil-globule transition, people also say that the chain collapses. 

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