Interaction of Enantiomers of Lysyl-7-Azatryptophyl-Lysine with Acidic Phospholipid Vesicles: A Fluorescence Study

A tripeptide containing a single 7-azatryptophan (7AW) residue was synthesized with the use of racemic 7AW and purified by high-performance liquid chromatography (HPLC). The peptide NH 2 -lysyl-7-azatryptophyl-lysine (K7AWK) is analogous to the peptide lysine-tryptophyl-lysine (KWK) whose interactions with acidic lipids are well characterized. It has not been possible to achieve the separation of 7AW D and L enantiomers under HPLC conditions that normally are used to separate enantiomers of amino acids. HPLC of the peptide, however, indicated that there were two main components of nearly equal concentration in the partially purified synthetic peptides. Thin-layer chromatography, paper electrophoresis, amino acid analysis, and steady-state absorption and fluorescence spectra showed that the two HPLC peaks corresponded to peptides with the same composition but with different enantiomers of 7AW. Time-resolved fluorescence measurements of the purified enantiomers indicated that there were differences in the excited-state decay parameters. The nature of the interaction of the different enantiomers of the basic peptide with vesicles of the acidic lipid palmitoyl-oleoyl phosphatidylserine (POPS) at pH 5.0 has been elucidated with the use of steady-state fluorescence measurements. Interaction of the peptide with vesicles in the liquid crystalline phase suggested that proton transfer from the POPS to the 7AW occurs, resulting in enhanced emission at 450 nm. These results are compared to those from the Trp containing peptide KWK, and the unique advantages of 7AW as a spectroscopic probe are described.