HIGH-FIELD MAGNETIC BIREFRINGENCE STUDY OF THE PHASE-BEHAVIOR OF CONCENTRATED-SOLUTIONS OF PHOSPHOLIPID TUBULES

High-resolution, high-field magnetic birefringence has been used to st udy the behavior of single-bilayer phospholipid tubules in concentrate d solutions and in the vicinity of the lipid chain-melting temperature T(m). Results are presented for five different initial lipid densitie s, rho=0.5, 1.0, 2.0, 4.0, and 6.0 mg/cm3-which yield tubule concentra tions in the range v=10(9)-10(10) cm-3-at fields to 15 T. On approach to T(m) from room temperature, the maximum induced refractive-index an isotropy DELTAn(max) evolves with concentration from a smoothly decrea sing to a sharply discontinous function; a sample with concentration c orresponding to rho=4.0 mg/cm3 shows evidence of a nearly critical pha se transition. Above T(m) the system appears isotropic (there is no in duced anisotropy), although measurements of the optical transmission I (T) indicate that at T approximately T(m) and for rho almost-equal-to 4.0 mg/cm3, the structure of the isotropic phase changes significantly . For T sufficiently above T(m), however, the isotropic phase has the same properties for all rho. The significance of these results is disc ussed qualitatively in terms of models proposed for tubule formation a nd phase behavior. Data are also presented for cooling samples in a hi gh field from T > T(m). Finally, preliminary measurements indicate tha t the magnetic susceptibility anisotropy DELTA(chi) in the lower conce ntration samples has a different temperature dependence than the induc ed refractive-index anisotropy for T < T(m).