Conformations and orientations of aromatic amino acid residues of tachyplesin I in phospholipid membranes

Tachyplesin I, an antibacterial and antiviral heptadecapeptide from the hemocyte debris of Tachypleus tridentatus, contains four aromatic amino acids (Trp², Phe⁴, Tyr⁸, and Tyr¹³) which have been shown to be crucial for activity. In order to investigate conformations and orientations of the aromatic amino acid residues of tachyplesin I in lipid bilayers, its analogs, [Phe⁸]- and/or [Phe¹³]-tachyplesin(s) I in which Tyr⁸ and Tyr¹³ are replaced by Phe, were synthesized. Circular dichroism spectral studies showed that three peptides are considerably different in conformation in aqueous solution at pH 8.0 whereas they take similar conformations in the presence of neutral egg yolk phosphatidylcholine (EYPC) liposomes. Energy transfer kinetics showed that the distances of Trp²-Tyr⁸ and Trp²-Tyr¹³ are 16 Å (max of 18.3 Å, min of 15.1 Å) and 18 Å (max of 20.2 Å, min of 16.6 Å) in buffer but are 12 Å (max of 15.2 Å, min of 8.6 Å) and 18 Å (max of 22.9 Å, min of 12.9 Å), respectively, in the presence of acidic EYPC/EYPG (3:1) liposomes. Decay kinetics for Trp² fluorescence indicated that Trp² takes at least three conformations in buffer and in acidic liposomes where fractions of three Trp² conformers vary by changing the medium from buffer to acidic liposomes. Although tachyplesin I is not in amphiphilic structure in buffer in spite of its rigid antiparallel β-conformation, its interaction with lipid bilayers appears to induce amphiphilic structure via minor alteration of peptide backbone and side chain orientations, resulting in shortening the distance of Trp²-Tyr⁸.