Attached Single-Stranded Polymer
In the remainder of the book, except for Chapters 5 and 6 and Section 24, we shall treat linear polymers formally as if they consist of a single strand only, as in Fig. 2-1(a). To be more precise: we shall assume that there is only a single subunit attachment or departure site at a polymer end or if there are several such sites, that they are all equivalent. In effect, then, there is a single overall on rate constant for a polymer end and a single off rate constant, and these rate constants are constant (see below). This model would be exact for the structure in Fig. 2-1(a) and it would also be exact in Figs. 2-l(b) and 2-1(c) if the intersubunit interactions in the polymer are so strong that there is always only one significant addition site (see the arrows in the figure) and only one significant departure site despite the fact that there is more than one strand. Figure 2-1(b) illustrates a 1-start, 2-strand helical structure (as in actin) and Fig. 2-1 (c) shows a 1-start, 3-strand helical structure (flattened). In the limiting case just mentioned, both structures would behave kinetically like a single helix (i.e., in effect, a single strand). This would be true of any 1-start tubular helical polymer in the strong-interaction limit.
KeywordsBulk Polymer Free Polymer Subunit Exchange Grand Partition Function Kinetic Diagram
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