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Fourier transform infrared spectroscopy was used to measure the structural changes in lipid hydrocarbon chain and interface regions of membranes that were obtained from the cortex and the nucleus of the normal human lens. Temperature-dependent studies, ie, phase transitions, were performed to evaluate the differences in the thermodynamic properties of the nuclear and cortical lipids. The structure of the fatty acyl chain region showed a higher degree of order for the nuclear lipid membranes as compared with the cortical ones. At physiologic temperature, the acyl chains of lipid from the cortical region of a 51-yr-old lens showed a degree of disorder of 63 +/- 0.6% compared with 23 +/- 1% for the nuclear region. The gel-to-liquid crystalline-phase transition temperatures were 27.2 +/- 0.3 and 39.2 +/- 1 degree C for the cortical and nuclear lipids, respectively. From the phase transition data, the enthalpy (strength of lipid-lipid interactions), entropy (randomness of the bilayer), and cooperativity (influence of adjacent lipid molecules) were calculated to be 2.6, 1.8, and 2 times greater, respectively, for the nuclear lipid transition compared with the cortical lipid transition. These differences show stronger lipid interactions and higher order in the nuclear membranes as compared with those in the cortex. Energetic differences between the cortical and nuclear membranes may arise from differences in the level of hydration or in the packing at the interface region. This last possibility is supported by changes in the contour of the carbonyl band near 1743 cm-1.