小中大Proteins display a characteristic ultraviolet (UV) absorption spectrum around 280 nm predominately from the aromatic amino acids tyrosine and tryptophan. If the primary sequence contains no or few of these amino acids,then this method will give erroneous results. Quartz crystal cuvettes are routinely used for measurement as plastic materials can leach plasticizers, and are not UV transparent. Similarly, buffer components with strong UV absorbance such as some detergents especially Triton X-100 should be avoided (Table 8.1) and ‘‘blank’’ samples should be measured using the sample buffer solution but with no protein present. UV absorbance is routinely used to give an estimate of protein concentration but if the molar extinction coefficient of the protein is known then the Beer–Lambert law can be used to accurately quantitate amount of protein by UV absorbance, assuming the protein is pure and contains no UV absorbing nonprotein components such as bound nucleotide cofactors, heme, or iron–sulfur centers.
在280nm附近蛋白质所表现出来的紫外吸收特性主要是由芳香性氨基酸,色氨酸和酪氨酸,所贡献。如果一级序列中没有或含很少此类氨基酸,本方法将得到错误的结果。通常用石英杯来进行测定,而塑料制品会释放塑化剂之类的物质,有紫外吸收不可用。同理,缓冲液组分中含有强紫外吸收物质的,如一些表面活性剂,特别是Triton X-100,应该避免使用。空白校正样应该用不含待测蛋白的样品缓冲液制备。紫外吸收值通常用来估计蛋白浓度,但是如果满足下列几个条件,就可以用Beer–Lambert定律准确定量蛋白浓度:样品蛋白的摩尔消光系数已知;纯蛋白;不含结合核苷酸辅基、亚铁血红素或铁-硫中心等有紫外吸收的非蛋白组分。
在此先更正自己的一个错误认识。在(一)中我以为280nm吸收的氨基酸还包括苯丙氨酸(Phe),在本节中看到还没有Phe,不禁去仔细地查看了一下《生物化学》教材。应该说本节的描述没有错。准确地说,近紫外区(220-300nm)吸收的氨基酸有色氨酸、酪氨酸和苯丙氨酸,但苯丙氨酸的最大吸收波长为257nm,而且其摩尔消光系数较小,对280nm紫外吸收的贡献小,可以忽略。
1、苯丙氨酸最大吸收波长257nm,此处的摩尔消光系数为200;
2、酪氨酸最大吸收波长275nm,此处的摩尔消光系数为1400;
3、色氨酸最大吸收波长280nm,此处的摩尔消光系数为5600;
用狗搜了一张图片如下: