蛋白質呈色反應—蛋白黃反應實驗
【原理】
在蛋白質分子中，具有芳香環的氨基酸(如酪氨酸，色氨酸等)殘基上的苯環經硝酸作用，可生成黃色的硝基化合物，在堿性條件下生成物可轉變為桔黃色的硝醌衍生物：
【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:
多數蛋白質分子含有帶苯環的氨基酸，所以都會發生黃色反應。苯丙氨酸不易硝化，需加少量濃硫酸后才能夠發生黃色反應。
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.
【 試劑 】
1.蛋白質溶液（與雙縮脲反應相同）
2.濃硝酸
3.20％NaOH溶液
4.0.1%石炭酸溶液
【實驗操作】
1.取1%石炭酸溶液約1ml放在 試管 內，加濃硝酸5滴,用微火小心加熱，觀察結果。
2.取干燥潔凈 試管 1支，加蛋白質樣液1ml和濃硝酸5滴，出現沉淀，加熱，不必至沸騰，則沉淀變成黃色，待 試管 冷卻后，向兩管各加20％NaOH溶液使成堿性，觀察顏色變化，記錄結果并解釋現象。
【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.
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