Single chain elongation

There exists hydrogen bonding governed elasticity in many water-soluble polymers, such as PEG, and PVA [11-13]. The single chain elongation of PEG is like the... 

The holo-ACP, in conjunction with an acyltransferase (AT) and a ketosynthase (KS), forms the minimal set of enzymatic domains reqnired for a single chain elongation reaction. This set of enzymatic domains is often termed a module. Polyketide biosynthesis is initiated by the loading of a starter unit, derived from an acyl-CoA, onto the PPant arm of the ACP [12, 13]. In fatty add biosynthesis the starter unit is typically limited to acetyl, whereas PKSs can utilise a variety of primer units including acetyl-, propionyl-, butyryl and other variants [10]. The role of the AT domain is to catalyse the transfer of a malonyl-derived extender unit from a CoA-thioester onto the thiol of the ACP phophopantetheinyl moiety (Fig. 1.3). 

Most of the pigments of flowers arise from a single polyketide precursor. Phenylalanine is converted to trans-cinnamic acid (Eq. 14-45) and then to cinnamoyl-CoA. The latter acts as the starter piece for chain elongation via malonyl-CoA (step a in the accompanying scheme). The resulting (3-polyketone derivative can cyclize in two ways. The aldol condensation (step b) leads to stilbenecar-boxylic acid and to such compounds as pinosylvin of pine trees. The Claisen condensation (step c) produces chalcones, flavonones, and flavones. These, in turn, can be converted to the yellow fla-vonol pigments and to the red, purple, and blue anthocyanidins.3 c... 

Figure 7.1. Molecular structure of RNA. The single-stranded RNA molecule consists of ribo-nucleoside residues linked to each other via phosphodiester bonds. The four nitrogenous bases in RNA are shown with their linkage at the Q position of ribose. The RNA chain elongates from the 5 to the 3 direction as the new nucleotide residues are added at the 3 -OH end of the chain during RNA synthesis in a cell. (Adapted from Textbook of Biochemistry with Clinical Correlations, T. M. Devlin, ed., Wiley, New York, 1982.)...

Polyketides are made by the sequential activity of domains of large, multifunctional enzymes called polyketide synthases (PKSs) (Fig. 6a and b). Polyketides are formed by the condensation and modification of acyl units derived from acyl-CoA precursors. Domains are organized in modules and each module carries out the series of steps necessary for one cycle of polyketide chain elongation. A single protein can have more than one module, and several different proteins together can make up a PKS. The number of modules determines the size of the polyketide. A growing polyketide chain is tethered to the enzyme as a thiol ester and moves sequentially from the N- to the C-terminus of a module, lengthened by two carbon units per module. The first module in a PKS... 

The measure of cooperativity mentioned above focuses on a single H-bond, say the terminal one, and monitors the change in its properties as the chain elongates. Another measure of cooperativity might focus on a given chain length, say a pentamer,... 

Bemis, J.E., Akhremitehev, B.B., and Walker, G.C. (1999). Single polymer chain elongation by atomic force mieroseopy. 15, 2799-2805. 

Zhang, W., Zou, S., Wang, C., and Zhang, X. (2000). Single polymer chain elongation of poly(V-isopropylacrylamide) and poly(acrylamide) by atomic force microscopy. J. Phys. Chem. B, 104, 10258-10264. 

This section seeks to make a quantitative evaluation of the relation between the elastic force and elongation. The calculation requires determining the total entropy of the elastomer network as a function of strain. The procedure is divided into two stages first, the calculation of the entropy of a single chain, and second, the change in entropy of a network as a function of strain. 

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