for peptides. How peptides interact with the SDS micelle Binding affinities of tryptophan dipeptides, Trp-X (X Γ
A, F, G, and how their structures change in the media have been I, K, L, S, T, V, W, and Y), X-Trp (X Γ
A, F, G, K, L, P, R, V, discussed (5-7). Results indicating that some of the rela-W, and Y), and X-Trp-NH 2 (X Γ
A, F, G, L, Q, S, V, and W) types tively small peptides, especially their aromatic residues, are to sodium dodecyl sulfate (SDS) micelles were studied using the localized at the interfaces of SDS micelles 8) and also conventional method of ultraviolet absorption spectrophotometry.
of membrane bilayers (1, 3, 9, 10) have been accumulated.
The free energies of transfer for the dipeptides from the micellar to
One of the distinct structural characteristics of the interface the aqueous phases were obtained from the values of the distribution of the SDS micelle, compared with the membrane bilayer, coefficients estimated on the basis of the ratio of absorbances at the is its simplicity. Fundamental studies using a simple system, peaks around 280 nm to those at the valleys around 245 nm. The free energies depend on the positive and negative charges and also such as small peptides and SDS micelles, are necessary for on the hydrophobicity of the amino acid residues of the dipeptides. a detailed elucidation of the interactions between peptides It was observed that the hydrophobic amino acid residues stabilized and membrane interfaces consisting of a variety of constitthe complexes corresponding to the free energies of transfer for amino uents.
acid residues of peptides from octanol to water and that small hy-Previously, we have investigated the interaction between drophobic residues, such as Ala, did not contribute to the affinities the SDS micelle and indole, which is a side chain compound of the dipeptides for the SDS micelles. The large values for the of tryptophan derivatives, using ultraviolet (UV) absorption effective dielectric constant, D eff , suggest that the indole rings of the and fluorescence photometries (11). The indole in the SDS dipeptides exist in and/or near the interfaces of the SDS micelles. micelle (11) and also in the membrane bilayer (12) may be These results suggest that the dipeptide backbone in the dipeptidelocalized near or in the micelle-water interface, irrespective SDS micelle complex exists in the hydrophilic area of the micelle of the serious contribution of hydrophobic interaction to the and that a part of the hydrophobic side chain is immersed in the hydrophobic area of the micelle. It was also observed for charged binding affinities. In this study, we have investigated the dipeptides that the free energies depended on their amino acid sebinding affinities and sites of various tryptophan dipeptides, quences.