Thermal stability of UV-irradiated collagen in bovine lens capsules and in bovine cornea.

The thermal stability of UVB irradiated collagen in bovine lens capsules and in bovine cornea has been investigated by differential scanning calorimetry (DSC). During UVB irradiation the lens capsules and cornea were immersed in water to keep the collagen in a fully hydrated condition at all times. UV irradiation induced changes in collagen which caused both stabilization and destabilization of the collagen structure. The helix-coil transition for non-irradiated collagen in cornea occurred near 66 degrees C, instead for the irradiated one for 3h it occurred at 69 degrees C. After irradiating for longer times (20-96h) the helix-coil transition peak occurred at much lower temperatures. The peak was very broad and suggested that collagen was reduced by UV to different polypeptides of different molecular weight and different lower thermal stabilities. The irradiation of lens capsules with UVB light in vitro resulted in changes in the thermal properties of type-IV collagen consistent with increased cross-linking. DSC of lens capsules showed two major peaks at melting temperatures at 54 degrees C Tm1 and 78 degrees C Tm2, which can be attributed to the denaturation of the triple helix and 7S domains, respectively. UVB irradiation of lens capsules in vitro for 6 h caused an increase in Tm1 from 54 to 57 degrees C. The higher temperature required to denature the type-IV collagen after irradiation in vitro suggested an increase of intermolecular cross-linking.