Characterization of structural conformers of κ-casein utilizing fluorescence spectroscopy.

The intrinsically disordered proteins (IDPs) belong to an important class of proteins due to their higher structural flexibility and diverse functions. IDPs lack stable three-dimensional structure and exist as structural ensemble in solution. Furthermore, IDPs have been found to be associated with various neurodegenerative diseases like Alzheimer's, Parkinson's, diabetes and spinocerebellar ataxia. Several spectroscopic techniques are being employed to predict the structure of IDPs as the X-ray crystallography is immensely challenging due to their structural dynamism. κ-Casein, an important milk protein containing net negative charge, belongs to the class of IDPs. κ-Casein has been found to assume different structural conformations at various pH and surfactant concentrations. In this work, we have utilized quenching efficiency of ionic liquid (1-butyl-3-methylimidazolium bromide) and energy transfer efficiency between tryptophan and 1,6-Diphenyl-1,3,5-hexatriene (DPH) to evaluate the conformation of κ-casein at various pH and cetyltrimethylammonium bromide (CTAB) concentrations. Our results validated the applicability of above-mentioned methods to determine the polarity of tryptophan environment in κ-casein at different conditions, where κ-casein assumes diverse structural conformations. The present work opens up the possibility to employ quenching properties of ionic liquid in conjunction with energy transfer efficiency between tryptophan and DPH for the elucidation of protein conformation.