Thermal transitions in the structure of tubulin. Environments of aromatic aminoacids.

The environment of aromatic aminoacids in the thermal transition of brain tubulin has been studied by several spectroscopic techniques (Fourth Derivative, Difference Absorption, Fluorescence and Circular Dichroism), in order to study its denaturation. An irreversible, temperature-induced, structural transition was found at around 48 degrees C. In order to establish the relative degree of hydrophobicity of tubulin aromatic residues, before and after the thermal transition, difference and fourth derivative absorption spectra at different temperatures were compared with spectra of tyrosine and tryptophan model compounds in different media. It was found that at high temperatures, tubulin acquires a partially denatured stable state, with a significant amount of residual structure still preserved. This state is characterized by a general increase of the exposure of tyrosine residues to the medium, while the environment of tryptophans becomes more hydrophobic.