A structural study of bovine lens alpha-crystallin and its subunits by absorption and linear dichroism spectroscopy

The structure of the bovine alpha-crystallin aggregate and its reaggregated isolated subunits has been studied by measurement of their absorption and linear dichroism spectra over the range 250-350 nm. Also, changes in structure with respect to time have been monitored in this way. From the absorption spectra it appears that the aromatic residues in subunit aggregates are in the same chemical environment as those in native protein. The light scattering due to the size of the protein molecules increases when the proteins are kept in solution, this effect being much stronger for the subunits. The linear dichroism spectra point to strong structural ordering in alpha-crystallin, the absorption transition dipoles of the aromatic residues being preferentially aligned along the long axis of the molecules. Moreover, a considerable deviation from a spherical or tetrahedrally symmetric structure of alpha-crystallin is inferred. The subunit aggregates show less ordering and might be more spherical. When kept in solution, their structural order seems to be decreased. The linear dichroism spectra show absorption at 325 nm, which is not detectable in the normal absorption spectra.