Explain why precipitation takes place at the isoelectric point of an amino acid

Question

Explain why precipitation takes place at the isoelectric point of an amino acid or protein. Why are lead compounds poisonous? Explain why dilute silver nitrate is applied to the eyes of newborn infants. Explain why dilute, rather than concentrated, silver nitrate must be used when applied to the eyes. Why is the isopropyl alcohol that is used in rubbing alcohol a disinfectant? Why are tannic acid and picric acid used in commercial products for the treatment of burns? Based on your laboratory observations, how might ninhydrin be used in forensic chemistry to solve crimes?

Explanation / Answer

QUESTION 1

For precipitation to occur, the protein/amino acid molecules have to attract each other.

At the isoelectric point, the net primary charge on the protein/amino acid molecule becomes zero.

At a pH above the isoelectric point, the protein/amino acid molecules are negatively charged and at a pH below the isoelectric point, they are positively charged. Like charges repel each other, and the protein/amino acid molecules remain in solution.

At the isoelectric point the charge on the protein/amino acid molecule cancel each other and reduce the repulsive forces, in turn increasing the attractive forces and resulting in precipitation.

QUESTION 2

Once lead enters the human body, it is distributed throughout the body other non-poisonous minerals (such as iron, calcium, etc). Lead mainly affects the nervous system by causing the degeneration of mylein coats of axons in nerve cells. Lead poisoning can adversely affect kidneys, cardiovascular system and the reproductive system causing nephropathy, high blood pressure, coronary heart disease, low sperm count in males and miscarriages in women.

QUESTION 3

A) Dilute solutions of silver nitrate is used to prevent bacterial eye infections in new borns. The mother's birth canal might contain gonorrhea or chlamydia and many other bacteria, which cause eye infections in new borns. Thus, dilute silver nitrate drops is used to prevent these infections.

B) Concentrated silver nitrate, if applied, damages the eyes of the new born. This has resulted in replacing silver nitrate with antibiotics.

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