Theory of the Effects of Concentration and Chain Length on Helix—Coil Equilibria in Two-Stranded Nucleic Acids

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Abstract
The formalism for treating a dissociating two‐stranded helix is developed from the system partition function. The behavior of finite chains is examined according to the model in which only one helical region is allowed in a molecule, and staggering of the paired strands is not allowed. Numerical results for chain lengths in the range of 1 to 300 residues show that the helix—coil transition curve is markedly broadened and shifted as the chain length decreases, and that a decrease in concentration results in a significant decrease in degree of bonding for chain lengths less than about 100. The ultraviolet absorbance transition curves are calculated from the Rich—Tinoco predictions for stack‐length dependence of hypochromicity, and are found to be markedly different from the transition curves in terms of degree of bonding for chain lengths less than about 100.

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