蛋白质呈色反应—蛋白黄反应实验

【原理】

在蛋白质分子中，具有芳香环的氨基酸(如酪氨酸，色氨酸等)残基上的苯环经硝酸作用，可生成黄色的硝基化合物，在碱性条件下生成物可转变为桔黄色的硝醌衍生物：

【Principle】

Benzene ring in residue of amino acid with aromatic ring in protein molecule (such as tyrosine and tryptophan) can react with nitric acid to give yellow nitro compound, which can change into saffron nitrylchinone derivative in basic condition. The reaction is as follows:

多数蛋白质分子含有带苯环的氨基酸，所以都会发生黄色反应。苯丙氨酸不易硝化，需加少量浓硫酸后才能够发生黄色反应。

Most proteins have amino acids with aromatic ring, so they have yellow reaction. Phenylalanine is not easy to nitrify, so it is necessary to add in a little vitriol oil.

【 试剂 】

1.蛋白质溶液（与双缩脲反应相同）
2.浓硝酸
3.20％NaOH溶液
4.0.1%石炭酸溶液

【实验操作】

1.取1%石炭酸溶液约1ml放在 试管 内，加浓硝酸5滴,用微火小心加热，观察结果。
2.取干燥洁净 试管 1支，加蛋白质样液1ml和浓硝酸5滴，出现沉淀，加热，不必至沸腾，则沉淀变成黄色，待 试管 冷却后，向两管各加20％NaOH溶液使成碱性，观察颜色变化，记录结果并解释现象。

【Reagents】

1. Protein solution (the same as biuret reaction)
2.Aquafortis.
3. 20% NaOH solution
4.1% carbolic acid solution

【Procedures】

1.Take a clean and dry test tube, add 1 ml 1% carbolic acid solution and 5 drops of a quafortis, then heat carefully with slow fire and observe the result.
2.Take a clean and dry test tube, add 1 ml protein sample solution and five drops of aquafortis in it, and heat in the boiling water bath for five minutes. Then add 20% NaOH solution in each of the tubes, mix up. Observe color varieties, note down the results and explain them.