Test for Peptides and Proteins
22nd Nov 2024
Experiment 178
Objective: Test for Peptides and Proteins
Introduction:
Peptide bonds are formed between the carboxyl and amino groups of adjacent amino acid residues. These bonds are found in peptides, polypeptides, and proteins, which all yield a positive result in the biuret test. Collectively, these compounds are referred to as proteins. A peptide is a short chain of amino acids, while a polypeptide is a longer chain. Proteins consist of one or more polypeptides folded into specific structures that enable biological function.
Foods tested in this experiment may contain a mixture of peptides, polypeptides, and proteins. As it is difficult to differentiate between these, quantitative tests measure total protein content.
Chemicals and Materials Required:
· 1 M Sodium Hydroxide (NaOH)
· 1 M Aqueous Copper (II) Sulfate
· Glycine
Glassware Required:
3 Graduated conical measures, 500 mL1 Beaker, 500 mL3 Beakers, 50 mLMeasuring cylinder, 100 mL2 Volumetric pipettes, 5 mL1 Volumetric pipette, 10 mL4 Volumetric pipettes, 20 mL7 Pipette bulbs5 Snap-cap vials, 20 mL3 Glass stirring rods
Experimental Procedure:
1. Preparation of the Egg Albumen Solution: Mix the albumen (egg white) with 300 mL of aqueous NaCl to prepare the egg albumen solution.
2. Conical Measure Setup: Prepare three conical measures as described in the following table. Each mixture should be stirred with a glass stirring rod.
Conical Measure | Contents | Copper Sulfate | Sodium Hydroxide (NaOH) |
1 | 400 mL of water | 5 mL of 1 M | 5 mL of 1 M |
2 | 400 mL of water + one spatula of glycine | 5 mL of 1 M | 5 mL of 1 M |
3 | 400 mL of water + 30 mL egg albumen solution | 20 mL of 1 M | 40 mL of 1 M |
Observations:
Conical Measure
Conical Measure | Contents | Result |
1 | Water | Light blue precipitate |
2 | Glycine | Blue solution |
3 | Egg Albumen | Blue-violet solution |
Discussion Questions:
1. What Color is Formed from Protein? The presence of protein results in a blue-violet color when the biuret test is applied, indicating peptide bonds in the solution.
2. Why is NaOH Used in the Test? Sodium hydroxide (NaOH) provides an alkaline environment, which is necessary for the biuret test to work, as it facilitates the interaction between copper ions and peptide bonds.
3. What Does 1M NaOH Mean? A 1M (1 molar) NaOH solution contains 1 mole of sodium hydroxide dissolved in 1 liter of solution. This concentration ensures the required reactivity for the biuret test.