ESR studies of low water content 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in oriented multilayers. 2. Evidence for magnetic-field-induced reorientation of the polar headgroups

In this continuation of ESR studies on a variety of spin probes incorporated into low-water-content 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, results with the headgroup-region probes, PD-Tempone and VO²⁺, are reported for the biaxial phase. The fluidity of the weakly ordered PD-Tempone probe is quite high for higher water content (14 wt %) but decreases for lower water content (9 wt %) (i.e., R_⊥, the diffusion coefficient, decreases from 10⁹ to 10⁸ s^-1 and shows properties probably related to slowly relaxing local structure in the latter case). The VO²⁺ ions are, however, immobilized on the ESR time scale and weakly oriented. Both probes show distributions in local director, which, however, become better aligned as H→, the magnetic field, is tilted into the bilayer plane. Also both probes then have a preferential alignment perpendicular to the projection of H→ in this plane. These results are taken as definite evidence for cooperative headgroup alignment by the magnetic field. While positive (vs. negative) diamagnetic susceptibility is expected they could not be distinguished by these experiments. Anomalous results from a stearamide probe in the Lα(1) phase, viz. an unusually large observed splitting of 19.05 G with plate samples but typical splitting of 16.1 G with tube samples, is taken as evidence for significant differences in microscopic ordering properties of the DPPC samples resulting from different anchoring constraints imposed by the shapes of the holders. The nature of the biaxiality observed in the biaxial phase by other techniques is considered in the light of these ESR results.