Introduction• The food we consume is divided into three

main classes: carbohydrates, the body’s most readily available energy source; lipids, the body’s principal energy reserve; and proteins, the body’s source of energy for growth and cellular maintenance.

• Proteins also make up the second largest portion of cells, after water.

• They are large polymeric compounds that cells synthesize from various building blocks called amino acids.

• Note that all amino acids contain carboxylic acid groups (-COOH), amino groups (-NH2), and substituent, or replaceable, side chains (-R).

• The general structural formula for an amino acid is shown in this figure.

• Twenty different amino acids, which differ only in the structure of their side chains, are used by human cells to build proteins.

• The side chain structure determines the class of the amino acid: non-polar, neutral, basic, or acidic.

• Human cells can synthesize most the amino acids needed to build proteins. However, about 9 amino acids, called essential amino acids cannot be synthesized by human cells and must be obtained from food.

• Amino acids incorporated into proteins are covalently linked by peptide bonds. Peptide bonds are amide bonds formed between the carboxylic acid group of one amino acid and the amino group of a second amino acid.

Chemical Reactions of Amino Acids and Protein Functional Groups

• Certain functional groups in amino acids and proteins can react to produce characteristically colored products.

• The color intensity of the product formed by a particular group varies among proteins in proportion to the number of reacting functional, or free, groups present .

• In this part of experiment, various color-producing reagents (dyes) will be used to qualitatively detect the presence of certain functional groups in amino acids and proteins.

Ex (1). Millon Reaction

• Principle: Millon's reagent (Hg/HNO3) gives positive results ( pink to dark-red color) with proteins containing the phenolic amino acid “tyrosine”

• Purpose: To detecte the amino acid that have phenol group of tyrosin.

Note: some proteins containing tyrosine will initially form a white precipitate that turns red when heated, while others form a red solution immediately. Note that any compound with a phenol group will yield a positive test, so one should be certain that the sample being tested does not contain any phenols other than those present in tyrosine.

Procedure:1. In test tube add 2 ml of protein into separate labeled test

tubes.

2. Add 3-4 drops of Millon’s reagent, and immerse the tubes in a boiling water bath for 5 minutes.

3. Cool the tubes and record the colors formed.

• Result:• A white ppt is formed with albumin and casein (but not gelatin) Why???

• The ppt gradually turns into red.

Ex (2). Xanthoprotic Reaction

• Some amino acids contain aromatic groups that are derivatives of benzene.

• These aromatic groups can undergo reactions that are characteristic of benzene and its derivatives.

• One such reaction is the nitration of a benzene ring with nitric acid.

• The amino acids tyrosine and tryptophan contain activated benzene rings and readily undergo nitration.

• The amino acid phenylalanine also contains a benzene ring, but the ring is not activated and therefore does not undergo readily nitration.

• Principle: Nitric acid gives color when heated with proteins containing tyrosine (yellow color) or tryptophan (orange color); the color is due to nitration.

• • Purpose: used to identify the presence of an

activated benzene ring.

Note: If one spills a concentrated solution of nitric acid onto someone’s skin. The proteins in skin contain tyrosine and tryptophan, which become nitrated and turn yellow.

Procedure

1. Add 2 ml of protein solution in a test tube and add 2 drops of concentrated nitric acid.

2. The formed white precipitate, will turn yellow upon heating, and finally will dissolve giving a yellow color to the solution.

3. Cool the solution down. Carefully add 3 ml of 6 N NaOH. Note that the yellow color turns orange.

Ex (3). Glyoxylic Acid Reaction(Hopkins-Colé test) :

• Principle: The indole ring reacts with glyoxylic acid in the presence of a strong acid to form a violet cyclic product.

• Purpose: The Hopkins-Cole test is specific for tryptophan, the only amino acid containing indole group. .

Procedure:

• Add 1 ml of protein solution in a test tube, add 1 ml of Hopkins-Colé reagent and mix well.

• Incline the test tube and slowly add 1 ml of concentrated H2SO4 on the inner wall of the test tube thus forming a reddish - violet ring at the interface of the two layers.

Result:

• A reddish violet ring is formed at the junction between the 2 layers with albumin and casein.

• Gelatin gives negative results.

Ex (4). Reduced sulfur test (Test for (-SH) group)

• Principle: Proteins containing sulfur (like: cysteine and cystine) give a black deposit of lead sulfide (PbS) when heated with lead acetate in alkaline medium.

• Purpose: To detect amino acid which containe sulfer group.

• Sulfur-containing protein ----> NaOH----> S2- ----Pb2+----> PbS

Procedure:

1. Add 1 ml of protein solution in a test tube, add 2 drops of 10% sodium hydroxide solution and 2 drops (or few) of lead acetate.

2. Stopper the tubes and shake them. Remove the stoppers and heat in a boiling water bath for 5 minutes.

3. Cool and record the results.

Result:

• A black deposit is formed with albumin• while a slight black turbidity is obtained with

casein due to its lower content of sulfur. • Gelatin gives negative result.

Ex (5). Sakaguchi Test for Arginine:

• Principle: α- naphthol and sodium hypobromite/chlorite react with the above mentioned compound to form red orange complexes.

• Purpose: used for the detection of a specific type of protein with the amino acid containing the guanidinium group (e.g. arginine).

Procedure

1. To 1 ml of the protein solution add 1 ml of 3 N NaOH solution and 0.5 ml of 0.1 % α- naphthol solution, and a few drops of 2 % hypobromite solution.

2. The presence of a guanidinium group in the compound under examination will be confirmed by the formation of a red color.