Octan-1-ol–water partition coefficients of zwitterionic α-amino acids. Determination by centrifugal partition chromatography and factorization into steric/hydrophobic and polar components
The distribution coefficients of free α-amino acids have been measured around their isoelectric value by centrifugal partition chromatography (CPC), allowing the determination of the partition coefficient (log P) of the amino acids in their zwitterionic form. Good correlations with published distribution coefficients of amino acid derivatives allowed log P values to be calculated for the five amino acids (Arg, Asn, Asp, Glu, Lys) whose high polarity prevented direct measurements by CPC. The log P values were factorized into a steric component (molecular volume V, mainly accounting for inductive and hydrophobic forces) and a polarity factor (Λ, accounting for ion–dipole and dipole–dipole interactions and hydrogen-bonds) which correlates with other amino acid parameters expressing mainly polar interactions. Using two examples from the literature, we show that multiple linear regression analysis based on steric and polar parameters is able to afford a quantitative interpretation of factors influencing protein conformational stability.