Quantitative analysis of the structure–hydrophobicity relationship for di- and tripeptides based on voltammetric measurements with an oil/water interface

The transfer of 18 di- and 27 tripeptides with un-ionizable amino acid side chains at a nitrobenzene/water (NB/W) interface was studied by cyclic voltammetry. The reversible half-wave potential (E(r)(1/2)), i.e., the midpoint potential could be accurately determined at pH 2 for both the facilitated and non-facilitated transfers, respectively, in the presence and absence of dibenzo-18-crown-6 (DB18C6) in NB. A multiple linear regression analysis was then performed for the E(r)(1/2) using the 'corrected' Dubois steric parameter for amino acid side chain substitutents. The result shows that the hydrophobicity of the peptides is governed not only by the intrinsic hydrophobicity of the peptide backbone and side chains, but also by the steric effects of side chain substituents. For the non-facilitated transfer of peptides, the steric effect of a bulky side chain is more significant at the N-terminus than at the C-terminus (and central for tripeptides). The more bulky the side chain at the N-terminus, the less hydrophobic the peptide becomes due to inhibition of the solvation of a terminal -NH(3)(+) group by organic solvents. For the facilitated transfer by DB18C6, however, the steric effect of a bulky side chain is the most significant at the central position of a tripeptide. A MOPAC calculation of optimized structures of DB18C6-peptide complexes has also shown that there is a notable steric hindrance between the central side chain and the benzene rings of DB18C6, which would reduce the 'apparent' hydrophobicity or transferability of the tripeptide.