Alpha carboxyl group of one amino acid reacts with alpha amino group of another amino acid to form a peptide bond or CO-NH bridge (Fig. 1). Proteins are made by polymerization of amino acids through peptide bonds.

Fig1. Peptide bond formation

Color Reactions of Amino Acids and Proteins

 1. Ninhydrin Reaction (Ruhemann, 1910)

All amino acids when heated with ninhydrin can form complexes; pink, purple or blue in color. The color complex is called Ruhemann's purple. Proline and hydroxy proline will give yellow color with ninhydrin. Amino acids with amide group (glutamine, asparagine) produce a brown color. The ninhydrin reaction may be adopted for qualitative as well as quantitative estimation of amino acids. It is often used for detection of amino acids in chromatography. Proteins do not give a true color reaction; but N-terminal end amino group of protein will also react with ninhydrin, to produce a blue color.

2. Biuret Reaction: Cupric ions in an alkaline medium form a violet color with peptide bond nitrogen (Schiff, 1896). This needs a minimum of two peptide bonds, and so individual amino acids and di-peptides will not answer this test. This reaction can be used for quantitative estimation also. The name is derived from the compound biuret (NH2—CO—NH—CO—NH2), a condensation product of two urea molecules, which also gives a positive color test. Magnesium and ammonium sulphates interfere with this reaction.

 3. Xanthoproteic test: The ring systems in phenyl alanine, tyrosine and tryptophan undergo nitration on treatment with concentrated nitric acid when heated (Salkowski, 1888). The end product is yellow in color which is intensified in strong alkaline medium. This reaction causes the yellow stain in skin by nitric acid.

 4. Millon’s Test: The phenol group of phenylalanine and tyrosine containing proteins, when heated with mercuric sulphate in sulphuric acid and sodium nitrite (or, mercurous and mercuric nitrates in nitric acid) form red colored mercury phenolate (Millon, 1849). Chloride interferes with this reaction and so it is not suitable to test for tyrosine in urine samples. Both xanthoproteic and Millon's tests are negative for tapioca (casava) which is deficient in phenylalanine and tyrosine.

 5. Aldehyde tests for tryptophan: In the Hopkins-Cole test, tryptophan containing protein is mixed with glyoxylic acid, and the mixture is layered over concentrated sulphuric acid. A violet ring at the interface of liquids shows the presence of the indole ring. Formaldehyde and mercuric sulphate is used similarly in Acree-Rosenheim reaction to get a violet color. Para-dimethyl-amino-benzaldehyde and strong hydrochloric acid give dark blue color (Ehrlich's reaction). Gelatin with limited tryptophan content will not answer these tests.

6. Sakaguchi’s test for arginine: Free arginine or arginyl residues in proteins react with alpha-naphthol and alkaline hypobromite to give bright red color. This is due to the guanidinium group.

 7. Sulphur test for cysteine: When cysteine or cysteine containing proteins are boiled with strong alkali, organic sulphur splits and forms sodium sulphide, which on addition of lead acetate produces lead sulphide as a black precipitate. Methionine does not answer this test because sulphur in methionine is in the thio-ether linkage which is difficult to break. Albumin and keratin will answer sulphur test positively; but casein will give a negative test.

8. Nitroprusside reaction for SH groups: Proteins with free sulfhydryl groups give a reddish color with sodium nitroprusside, in ammoniacal solution. Many proteins give a negative, reaction in the native state, but when denatured, reaction will be positive, showing the emergence of free SH groups.

 9. Pauly's test for histidine or tyrosine: Diazo benzene sulfonic acid reacts with imidazole group of Histidine to form a cherry-red colored diazotised product under alkaline conditions. The same reagent will give an orange red colored product with phenol group of Tyrosine.

Color reactions of amino acids are shown as summary in Table 1 The amino acid Seleno-cysteine is described in Box1.

Table1. Color reactions of amino acids

Box1. Selenocysteine as the 21st amino acid

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تنظّم محاضرة توعوية لطالبات كلية الأسوة للبنات في باكستان

الهيأة العليا لإحياء التراث تقيم دورة متخصّصة عن الفقه الكلامي

قسم التطوير يشرع بمقابلة المتقدمين للانضمام إلى أكاديمية التطوير الإداري

قسم الشؤون الفكرية يعلن انضمام جامعة كربلاء إلى المستودع الرقمي العراقي للأطاريح

المزيد

الآخبار الصحية

اختراع صيني.. لاصق يصلح كسور العظام خلال دقائق

اكتشاف حلقات عملاقة من الحمض النووي.. تقلل خطر السرطان

آباء أميركيون يطالبون بتنظيم روبوتات الدردشة لحماية الأطفال ؟

4 إشارات من القلب لا يجب تجاهلها أبدا

المزيد

الآخبار التكنلوجية

"تحليل المحادثات" يكشف كيف يستخدم الناس تشات جي بي تي

الدخان يكشف المستور.. التحنيط بدأ خارج مصر

مشهد مدهش للأرض من الفضاء.. جمجمة بيولوجية بعينين محدّقتين!

أول قطار يعمل بالهيدروجين في أميركا يدخل الخدمة رسميًا

المزيد

مواضيع ذات صلة

The Chemical Nature of RNA Differs From That of DNA

DNA Provides A Template For Replication & Transcription

DNA Can be Reversibly Denatured & Specifically Renatured, Both in the Test Tube & in Living Cells

DNA Contains Four Distinct Deoxynucleotides

Overproduction of Pyrimidine Catabolites

Disorders of Purine Metabolism

Biochemical Characteristics and Biological Function of Cardiac Natriuretic Peptides

The Deoxyribonucleosides of Uracil & Cytosine are Salvaged

Biosynthesis of Pyrimidine Nucleotides

Hepatic Purine Biosynthesis is Stringently Regulated

Inosine Monophosphate (IMP) is Synthesized From Amphibolic Intermediates

DNA & RNA are Polynucleotides