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

The variation in the helix‐coil transition temperature, T m N , with oligomer length, N , for the system has been examined. The results for N = 4‐13, measured in 0.2 M Na+, have been analyzed in terms of the expression of Blake (1972): where c m is the free oligomer concentration at T m N , and V r f is the thermodynamic free volume available to a helical base‐triplet residue. The correlation coefficient for the fit to expression (II) of data obtained over a 50° temperature range is 0.997 when Δ H r = −12.6 kcal/mole of base‐triplets (independent of oligomer length (N ⪖ 4) or temperature), the value previously obtained from both calorimetry of (A) ∞ ·2(U) ∞ and (A) 4 concentration dependence of T m . It is found that V r f = 8.0 × 10 −4 1/mole (± 30%) or 1.33 Å 3 per helical base‐triplet, and is constant with temperature. A maximum value for V r f of 21.0 × 10 −4 1/M (± 1.3%), equivalent to 3.54 Å 3 per helical basetriplet is obtained by the same treatment of the helix‐coil transition data for the three‐stranded helix formed by adenosine ( N = 1) and 2(U) ∞ obtained by Davies and Davidson (1971).