A fluorescence study of the temperature-dependent polymerization of bovine beta-casein A1

The intrinsic fluorescence properties of bovine beta-casein A1 solutions have been studied under a variety of conditions. The protein shows both tyrosine and tryptophan fluorescence emission, the former being more pronounced at low temperature (5 degrees C) and in the presence of urea. Approximate relative quantum yields for the tyrosine and tryptophan residues were determined using free tyrosine and free tryptophan respectively as standards. The tryptophan emission intensity of beta-casein shows an increase with temperature indicating a temperature-dependent transition of the protein. Two-state analysis of the emission-intensity--temperature data yielded positive enthalpy and entropy values for the transition over a range of protein concentrations (0.018--0.18%). A marked protein concentration effect was apparent which indicated that the transition was a consequence of polymerization rather than a prerequisite for polymerization. The size of the polymers appeared to be concentration-dependent. Decreased ionic strength and increased pH both caused a reduction in the increase in emission intensity when the temperature was increased. At higher pH both the enthalpy and the entropy for the transition were reduced. Experimental precision was insufficient to allow the effect of decreased ionic strength on these parameters to be determined. Many of the properties of the temperature-dependent transition can be explained if the transition is considered to be one of micelle formation.