Precipitation of proteins with salts of heavy metals.
Proteins interacting with salt of heavy metals (copper, mercury, lead, etc.), denature and form insoluble complex compounds due to heavy metal adsorption on the surface of protein molecule. This allows to use proteins as an antidote for poisoning by heavy metals.
Pour 0.5 ml of protein solution in two test tubes. Add 2 drops of 5% copper sulfate solution in the first one, and 2 drops of 5% solution of lead acetate solution in the second.
Observations:
Conclusion:
Precipitation of proteins with concentrated mineral acids.
Precipitation of protein with concentrated mineral acids is caused by dehydration of protein molecules, the formation of insoluble protein complex salts. The protein precipitate is dissolved in an excess of sulfuric and hydrochloric acids. Excess of nitric acid does not dissolve the precipitated protein. Reaction with nitric acid is used in clinical studies of urine to reveal presence of protein in it (Geller reaction).
Pour 1 ml of concentrated nitric acid in the test tube. Tilt the tube and add 1 ml of protein solution by the wall carefully. Check the result. Than shake the tube and add an excess of nitric acid.
Observations:
Conclusion:
Precipitation of proteins with organic acids.
Reactions with trichloroacetic and sulfosalicylic (2-hydroxy-5-sulfobenzoic) acids are specific and sensitive. They are used in clinical laboratories for detecting protein in the urine and other biological liquids. Sulfosalicylic acid can precipitate peptides, which are breakdown products of proteins. Trichloroacetic acid precipitates only proteins. It is used in determining non-protein (residual) nitrogen of blood, which consists of protein metabolism products.
Pour 1 ml of protein solution in two test tubes. Add 2 drops of sulfosalicylic acid in one tube, and trichloroacetic acid in another.
Observations:
Conclusion:
Test Questions
1. What are amino acids properties?
2. What color reactions for proteins are known to you from the course of bioorganic chemistry?
3. Give the definition of for levels of protein structure. What types of bonds stabilize primary, secondary, tertiary and quaternary structures of proteins?
4. Give definitions of isoelectric and isoionic points of proteins.
5. What is the practical application of the chromatography method in medicine?
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6. What is the practical application of the precipitation and salting out of proteins in medicine?
7. Why do phosphoproteins play a special role for a fetus, embryo, and newborn?
8. What is the difference between glycoproteins and proteoglycans? What types of bonds are between the carbohydrate and polypeptide chain?
9. Why glycoproteins are less sensitive to the denaturation factors than simple proteins?
10. Describe the structure and functions of hemoglobin A.
11. Why hemoglobin of an embryo and adult person has different affinity to oxygen?
12. What is more soluble in water: nucleic acids or nucleoproteins? Why?
13. What types of chemical bonds hold 2 polynucleotide strands in DNA molecule?
14. What is meant by the denaturation and renaturation of protein?
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