Scientific American - Supercoiling

Date:
1979-1980
Reference:
PP/CRI/H/6/11
Catalogue details

Licence: In copyright

Credit: <i>Scientific American</i> - Supercoiling. Source: Wellcome Collection.

    12/201
    F ig. 3. Optical rotatory dispersion of alternating synthetic polynucleotides (25°C, 10 msi- sodium phosphate, pH 7-2); ( ), 0-1 M-NaCl; ( ), 4-0 M-NaCl0 4 . (a) Poly(dG-dC) • poly(dG-dC) ; (b) poly(G-C) • poly (G-C) ; (c) poly(dl-dC) • poly(dl-dC) ; (d) poly (dA-dT) •poly(dA-dT). or without 10 3 m-EDTA , with phosphate buffer or citrate buffer up to 50 mM in the pH range from 6-8 to 7*5, this property of the nucleic acid was unchanged. An apparent equilibrium constant iT RL or degree of transition 9 can be defined by = [L-form]/[R-form] = 0/(1 — 6) = (a — a R )/(a L — a), (1) where a is the specific rotation or extinction coefficient in the transition region and a L , a B the constants for the L- and the R-form, respectively. (i) Salt dependence Varying the nature of the electrolyte produces the same transition but at a different salt concentration. Figure 5 shows the degree of transition 6 as a function of the molar concentration of NaCl, NaC10 4 and MgCl 2 as measured by the change of optical rota tion and/or absorbance. The midpoints of the transition (6 — 0-5) for d(G-C) s are 2*56 m -NaCl, 1-67 m -NaC10 4 and 0-66 m -MgCl 2 . Thus the concentration of MgCl 2 needed to induce the transition is only about a quarter of the corresponding sodium chloride concentration. (ii) Temperature dependence Varying the temperature of the polymer solution at 2-5 m -NaCl between 20 and 50°C does not influence the ORD spectrum to a larger extent than can be accounted
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