Comparing the polarities of the amino acids: side-chain distribution coefficients between the vapor phase, cyclohexane, 1-octanol, and neutral aqueous solution

To obtain an indication of the tendencies of amino acids to leave water and enter a truly nonpolar condensed phase, distribution coefficients between dilute solution in water and dilute solution in wet cy- clohexane have been determined for each of the common amino acid side chains at pH 7; they are found to be closely related to the inside-outside distributions of the side chains observed in globular proteins. There was no evidence that excess water enters cyclohexane in association with these solutes. Cyclohexane-to-water distribution coefficients can be combined with vapor-to-water distribution coefficients reported earlier to yield vapor-to-cyclohexane distribution coefficients. Vapor-to-cyclohexane distribution coefficients provide an experimental index of susceptibility to attraction by dispersion forces, and the corresponding free energies are found to be linearly related to side-chain surface areas. Observations using different solvents and variously substituted side chains suggest that alcohols such as 1-octanol exert a specific attraction on the side chain of tryptophan. When less polar phases are used as a reference, leucine, isoleucine, valine, phenylalanine, and methionine are found to be more hydrophobic than tryptophan.

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