Helix formation by d(TA) oligomers

摘要: Experiments with d(TA)N§ hairpin helices are reported which contradict the simple Ising (or nearest-neighbor interactions) model for DNA helix-coil transition at low or moderate counterion concentrations. The experiments show that electrostatic interactions extend beyond nearest neighbors must be taken into account. basic observations as follows. First dependence of Tm(N) on log M (where is monovalent molarity) falls off sharply decreasing N in range 8 ≤ 22, so set melting curves compressed a narrow temperature M. At = 0.0028, d(TA)20 has higher Tm than poly d(AT) and d(TA)9 only slightly lower. Second, have relatively small effect breadths these curves: all oligomers sharpen concentration reduced. We used method Schildkraut & Lifson (1965) to study problem by priori computation. Their based summation discrete pairwise between charges phosphate groups. As theory Record (1967), quantity zp2D sole adjustable parameter, where zp effective charge per group D dielectric constant. Two results found: (1) can represented values single value zp2D, (2) it necessary count both helix adjoining non-helical segments. There approximation effects may account while retaining much formalism standard theory. In this stability constant s replaced an s∗(N), function 2N, number nucleotides oligomer, although independent length partly helical molecule. predicted straightforward manner using they agree experimental curves. However, further work needed define limitations approximation.