Mechanical calorimetry of red cell membranes 1

Abstract

Mechano-calorimetry involves temperature dependent mechanical experiments to decompose reversible changes in membrane free energy density into internal energy and entropy density contributions. The measurements required are the membrane surface elastic properties (area compressibility and shear moduli) as a function of temperature plus the thermal area expansivity of the membrane. The results include the reversible heats of expansion for the membrane as well as the energetic contributions. Consequently, the approach directly assesses the thermodynamic state of the membrane, in situ; comparison with well defined chemical systems (i.e., specific vesicle combinations of lipids, proteins, etc.) provides insight into the chemical state of the composite or natural membrane “mixture”. For the human red cell membrane, the thermal area expansivity has been found to be 1.2 × 10⁻³C^0–1. The heat of expansion is determined to be 110–200 ergs/cm²; the heat of extension is 2 × 10⁻³ ergs/cm² for unit extension of the red cell membrane. The heat of expansion is a measure of the thermal repulsive forces acting in the membrane surface; the heat of expansion measured for the red cell membrane is of the order anticipated for a lipid bilayer idealized as twice the behavior of a monolayer at an oil-water interface. On the other hand, the heat of extension measured for the red cell membrane is five orders of magnitude smaller than the heat of expansion. Assuming that the red cell membrane shear rigidity and heat of extension is associated with “spectrin”, unit extension of the membrane increases the configurational entropy of spectrin by about 500 cal/mole; this is opposed by enthalpic increases of 600–700 cal/mole produced by extension.