Dissociation pressures of some gas hydrates have been evaluated using the Lennard-Jones 12-6 28-7 and Kihara potentials in the Lennard-Jones-Devonshire cell model. The Lennard-Jones 28-7 pot~ntial 'gives the least satisfactory results. The Lennard-Jones 12-6 potential works satisfactorily for the monatomic gases and ClL but poorly for the rodlike molecules C2H6, C02, N2, 02, C2fu. This failure may be due to (i) d~~.tortio~s of t~e hydrate la~tice, (ii) neglect of.m?lecul~r shape and size in determining the cavity potential (m) bamer to mternal rotatIOn of the molecule m Its cavity. A crude model for the lattice shows that it is not distorted. The Kihara potential predicts better dissociation pressures for the hydrates of the rodlike moleules. U,nlike the previously used Lennard-Jones 12-6 potential, it depends on the size and shape of the mteractmg molecules. The absence of lattice distortions, improved dissociation pressures through the use of the Kihara potential and the restriction of the motion of the solute molecule to around the center of a cavity makes a large barrier to rotation unlikely. A small barrier may still be present.

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Summary The DNA double helix stable in water is denatured by solvents with widely varing properties. To understand quantitatively the role of water in keeping the helix together, the solvent contributions to the unitary free energy difference between random single strands and the helix are calculated for water, MeOH, glycol, formamide, glycerol, ethanol, n‐propanol, and n‐butanol. The property of solvent crucial in denaturation is found to be mainly the enthalpy part of surface tension. The number of molecules solvating the A, T, G, C bases, hence their energy is drastically reduced in going from random coils to the helix. ‘Hydrophobic bonding’ thus results to a large extent from the large surface enthalpy of water. The same interactions may cause the ‘stacking’ of adjacent bases in a single strand of DNA in water. Such stacking may be what makes thymine photodimerization under the action of uv favorable in some thymine hydrate crystals and in aqueous single strand DNA.

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