Mechanism and thermodynamics of multilevel structural transitions in liquid crystals under external actions

The temperature dependence of the viscosity η and electric conductivity σ of high-, low- and very-low-density blood serum lipoproteins and apolipoprotein A-I is studied. The study reveals an anomalous range of T c at (35–38) ± 0.5 °C. The width of the transition is 2 °C. The viscous flow enthalpy, the activation energy Δ H, the transition enthalpy Δ H tr and the temperature coefficients Δη / Δ T and Δ σ/ Δ T on either side of T c are calculated. For all lipoprotein classes, the observed anomalous changes in viscosity and electric conductivity are due to structural phase transitions in the physiological temperature range. They are observed in both lipid and protein components of lipoproteins. Because Δ H tr in high-density lipoproteins is low, it is thought that phospholipids feature an oriented A ↔ C smectic transition referring to the second-order transition. The structural transition in apolipoprotein A-I, because of its very low enthalpy, can also be referred to the second-order transition. This transition results from the change in symmetry in response to a change in the secondary protein structure (coil ↔ β-structure transition). In very-low-density lipoproteins, the transition is likely to be of the smectic → cholesteric type. Similar structural transitions were detected in erythrocyte membranes of participants of a Soviet-Canadian transarctic ski expedition as the result of extreme conditions. The findings suggest that liquid crystalline biostructures (lipoproteins, cell membranes) can assume various thermo-dynamic states that affect their properties. This fact makes liquid crystalline biostructures similar in behavior to solid crystals in external force fields.