Polynucleotides. XI. Thermodynamics of (A)N·2(U)∞ from the dependence of TmN on oligomer length

✍ Scribed by R. D. Blake; Jacques R. Fresco

• Publisher: Wiley (John Wiley & Sons)
• Year: 1973
• Tongue: English
• Weight: 874 KB
• Volume: 12
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1973.360120407

✦ Synopsis

The variation in the helix-coil transition temperature, TmN, with oligomer length, N , for the system has been examined. The results for N = 4-13, measured in 0.2M Na+, have been analyzed in terms of the expression of Blake (1972):

where cm is the free oligomer concentration a t TmN, and Vrf is the thermodynamic free volume available to a helical base-triplet residue. The correlation coefficient for the fit to expression (11) of data obtained over a 50" temperature range is 0.997 when AH, = -12.6 kcal/mole of base-triplets (independent of oligomer length (N 7 4) or temperature), the value previously obtained from both calorimetry of (A),.2(U), and (A)4 concentration dependence of T,. l/mole (.t 30%) or 1.33 Aja per helical base-triplet, and is constant with temperature. A maximum value for Vrr of 21.0 X 1/M (f 1.3%)) equivalent to 3.54 A3 per helical basetriplet is obtained by the same treatment of the helix-coil transition data for the threestranded helix formed by adenosine ( N = 1) and 2(U), obtained by Davies and Davidson (1971). It is found that VVf = 8 . 0 X ~~ ~~ ~ ~ * The observed fraction of the maximum hypochromicity a t 22OC, Id, was fitted to the empirical smoothing function: j N h = (c, -C N ) / ( C ,ern) = 1 -0.18exp [-0.02(Nl)] -0.82exp [-0.48(Nl)] ** jNh = (N -1)/N. *** From an observed ratio €257/625'J = 0.9845 f 0.0008. a References cited in the previous report.