| Abstract: | The interaction of 2-(p-toluidinyl) naphthalene-6-sulfonate (TNS) with casein micelles (CM) was studied by fluorescence spectroscopy. Fluorescence emission spectra of the complex showed blue shift and intensity enhancement of TNS fluorescence, suggesting the insertion of the marker in low polarity regions of CM. An energy transfer process between the proteins and the marker was detected, showing that most of the TNS binding sites were in the proximity of CM fluorescent residues. TNS inhibited the aggregation step of CM enzymic coagulation, producing probably a decrease of size and amount of aggregates formed. This effect could not be related only to changes in CM net charge, but also possibly to the occupancy of surface hydrophobic regions by the marker. About a 20% decrease in the TNS fluorescence intensity was observed during the proteolytic step of coagulation which could be attributed to release of the marker from its binding sites located in the CM external layer. A bound TNS release was also observed during the initial time of the aggregation step, probably by removal of the bound marker from contact regions between aggregating particles upon their collisions. A further increase, related to the aggregation step, could indicate the uptake of the marker by new hydrophobic sites created in the complex structure of the clusters. The results pointed to the participation of surface hydrophobic regions of renneted CM in their aggregation process. |
