Thermal stability and reversibility of secondary conformation of alpha-crystallin membrane during repeated heating processes.

Reflectance FT-IR/DSC microspectroscopy was first used to study the structural conformation of alpha-crystallin membranes in the heating-cooling-reheating cycle. The thermotropic transition and the changes in secondary structure of alpha-crystallin membrane during heating and reheating processes were investigated. A thermal transition ranging between 50 and 70 degrees C with a midpoint at 60 degrees C for the alpha-crystallin membrane was easily obtained from the three-dimensional plots of the reflectance FT-IR spectra as a function of temperature. The secondary structural components of the alpha-crystallin membrane were modified step-by-step with the increase of temperature from 25 to 120 degrees C, but restored to original values after cooling to 25 degrees C. During the heating process, the compositions of the alpha-helix, random coil and beta-sheet structure decreased with temperature, but the content of the beta-turn structure increased, however, all of them were restored after cooling. The absence of significant alteration in the secondary structures for the alpha-crystallin membrane before and after the first-heating process strongly suggests the high thermal stability and reversibility of alpha-crystallin. Interestingly, the thermal behavior of the first-heated alpha-crystallin membrane during the reheating process exhibited a unique thermal behavior with two transitional temperatures at 35-50 and 55-70 degrees C. The reflectance FT-IR/DSC microscopic data indicated that alpha-crystallin in the membrane state had higher thermal stability and reversibility.