Lab Activity 8Proteins part II IUG, Spring 2014 Dr. TarekZaida
Experiments • A.A can be characterized qualitatively by using several dyes that will react with certain groups of the A.A. • Seven Tests: • Ninhydrin 4. Xanthoproteic 7. Sakaguchi • Biuret 5. Hopkin’s- Cole • Millon’s 6. Sulfur
1. Ninhydrin Test • For amino acids containing a free NH2 & free COOH. • Reaction with ninhydrin to produce a colored product. • When NH2 is attached to α-C on the amino acid’s carbon chain, the amino group’s N is part of a blue-purple product. • Amino acids that have N-H (a secondary amino group (e.g. proline) also react with ninhydrin, but they yield a yellow product.
Procedure.. 1. Label 6 cleaned, drained test tubes with the names of the following solutions: 2 % glycine, 1 % tyrosine, 2 % proline, 2 % casein, 2 % gelatin, 2 % albumin. 2. Add 15 drops of each solution in the corresponding test tube. • To each of the test tubes add 5 drops of 0.5 % ninhydrin reagent solution. • Place the test tubes into the boiling-water bath for 5 minutes. Remove the test tubes from the water bath and place then in a test tube rack. Record your observations!
2. Biuret • For detecting peptide bonds (hence peptides or proteins).. • How it works? • The copper atoms of Biuret solution (CuSO4 ) in a basic environment will react with peptide bonds (-CO ---NH) to form a chelate of a deep violet color, indicating the presence of proteins. • A light pink color indicates the presence of peptides..
Procedure.. 1. To 1 ml of a solution containing protein add 4 ml of a biuret reagent. 2. Mix well, then let to stand at RT for about 30 min. 3. Record your observations!
3. Sulfur Test • For the detection of sulfur-containing amino acids such as cysteine. • Is done by converting S to an inorganic sulfide ( S2-) through cleavage by a base. • When the resulting solution is combined with lead acetate (CH3COOPb), a black precipitate of lead sulfide is formed. Sulfur-containing protein ----> NaOH----> S2- ----Pb2+----> PbS Cysteine
Procedure.. 1. Place 1 ml of 2% casein, 2% egg albumin, 2% peptone, 2% gelatine and 0.1 M cysteine into separate, labeled test tubes. 2. Add 2 ml of 10 % aqueous sodium hydroxide. Add 5 drops of 10 % lead acetate solution. 3. Stopper the tubes and shake them. Remove the stoppers and heat in a boiling water bath for 5 minutes. Cool and record the results.
7. Sakaguchi • For detection of the amino acid containing the guanidinium group (e.g. arginine). • In basic conditions, α- naphthol and sodium hypobromite/chlorite react with the guanidinium group to form red orange complexes. Guanidinium group Arginine
Procedure • Add 1 ml of 3 N NaOH solution to 1 ml of the protein solution, followed by addition of 0.5 ml of 0.1 % α- naphthol solution, and a few drops of 2 % hypobromite solution (NaOBr). 2. The formation of a red color indicates the presence of a guanidinium group in the compound under examination.