Pearce K N
Eur J Biochem. 1975 Oct 1;58(1):23-9. doi: 10.1111/j.1432-1033.1975.tb02344.x.
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.
在多种条件下研究了牛β-酪蛋白A1溶液的固有荧光特性。该蛋白质表现出酪氨酸和色氨酸的荧光发射，前者在低温（5摄氏度）和存在尿素的情况下更为明显。分别以游离酪氨酸和游离色氨酸为标准，测定了酪氨酸和色氨酸残基的近似相对量子产率。β-酪蛋白的色氨酸发射强度随温度升高而增加，表明该蛋白质存在温度依赖性转变。对发射强度-温度数据进行双态分析，得出在一系列蛋白质浓度（0.018%-0.18%）范围内转变的正焓值和熵值。明显存在显著的蛋白质浓度效应，这表明该转变是聚合的结果而非聚合的先决条件。聚合物的大小似乎与浓度有关。离子强度降低和pH值升高均导致温度升高时发射强度增加幅度减小。在较高pH值下，转变的焓和熵均降低。实验精度不足以确定离子强度降低对这些参数的影响。如果将该转变视为胶束形成之一，则可以解释温度依赖性转变的许多特性。
