Theory of the Effects of Concentration and Chain Length on Helixmdash;Coil Equilibria in Twohyphen;Stranded Nucleic Acids

摘要

The formalism for treating a dissociating twohyphen;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 helixmdash;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 Richmdash;Tinoco predictions for stackhyphen;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.