Phase separation and hexagonal HII phase formation by gramicidins A, B and C in dioleoylphosphatidylcholine model membranes. A study on the role of the tryptophan residues.

The role of the tryptophan-residues in gramicidin-induced HII phase formation was investigated in dioleoylphosphatidylcholine (DOPC) model membranes. 31P-NMR and small angle X-ray diffraction measurements showed, that gramicidin A and C (in which tryptophan-11 is replaced by tyrosine) induce a similar extent of HII phase formation, whereas for gramicidin B and synthetic analogs in which one tryptophan, either at position 9 or 11 is replaced by phenylalanine, a dramatic decrease of the HII phase inducing activity can be observed. Modification of all four tryptophans by means of formylation of the indole NH group leads to a complete block of HII phase formation. Sucrose density centrifugation experiments on the various peptide/lipid samples showed a quantitative incorporation of the peptide into the lipid. For all samples in a 1/10 molar ratio of peptide to lipid distinct bands were found, indicative of a phase separation. For the gramicidin A'/DOPC mixture these bands were analyzed and the macroscopic organization was determined by 31P-NMR and small-angle X-ray diffraction. The results demonstrate that a quantitative phase separation had occurred between a lamellar phase with a gramicidin/lipid ratio of 1/15 and a hexagonal HII phase, which is highly enriched in gramicidin. A study on the hydration properties of tryptophan-N-formylated gramicidin in mixtures with DOPC showed that this analog has a similar dehydrating effect on the lipid headgroup as the unmodified gramicidin. In addition both the hydration study and sucrose density centrifugation experiments showed that, like gramicidin also its analogs have a tendency to aggregate, but with differences in aggregation behaviour which seemed related to their HII phase inducing activity. It is proposed that the main driving force for HII phase formation is the tendency of gramicidin molecules to self-associate and organize into tubular structures such as found in the HII phase and that whether gramicidin (analogs) form these or other types of aggregates depends on their tertiary structure, which is determined by intra- as well as intermolecular aromatic-aromatic stacking interactions.