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Chapter 18 Homework Key
1. Which of the following forms of glutamic acid would you expect to predominate at a low pH, at a neutral pH, and at a high pH? Explain your reasoning.
a. It is a neutral pH, since the positive and negative charges cancel out, and the molecule would be electrically neutral. / b. It is a basic (high) pH, since the molecule has two negative charges, which means a base has pulled off the acidic hydrogens. / c. It is an acidic (low) pH, since there is a net positive charge, meaning the basic groups all reacted with H+ from the acid.2. Draw structures of the following amino acids at the indicated pH’s.
a. Valine at a neutral pH / b. Serine at a very basic pH / c. Lysine at a very acidic pHAt a neutral pH, the amine has pulled the acidic H off of the carboxylic acid group to form the zwitterion form. / At a very basic pH, the acidic groups have lost their acidic H’s, and the molecule is negatively charged. / At an acidic pH, all of the basic groups are protonated, and the molecule is positively charged.
3. Draw the structures of the following peptides. Don’t worry about pH.
a. Ser-Ser-Val
b.
Draw in the disulfide bridge connecting the two cysteines.
4. How do the following interactions help stabilize tertiary and quaternary structures of proteins. Give an example of a pair of amino-acids that could give rise to each such interaction. DO NOT USE THE EXAMPLES I USED IN MY POWERPOINT: I WILL COUNT THEM WRONG!
a. Hydrophobic interactions: London Dispersion Forces attracting two non-polar groups - two of any amino acid with a non-polar side chain, such as leucine or isoleucine.
b. Salt bridges: An amino-acid with a positively charged side-chain, such as arginine or histidine, and an amino acid with a negatively-charged side chain, such as aspartic acid. These attract each other.
c. Side chain hydrogen-bonding: two amino acids with alcohol groups, such as threonine and tyrosine. The hydrogen-bonding attracts the amino acids.
5. What is denaturing of a protein? Overcoming the hydrogen-bonding, salt bridges and hydrophobic attractive forces, so that the protein unfolds from its native shape.
Explain how the following could denature a protein:
a. Heat: the more energy you apply to a protein, the more energy you have to overcome the above attractive forces.
b. A detergent: Detergents have negatively charged ends that can be attracted to the positively-charged side-chains of a basic amino-acid, and disrupt salt bridges. Detergents also have long non-polar parts that can interact with the non-polar side-chains of many amino-acids, and disrupt hydrophobic attractions.
c. A concentrated solution of sodium chloride: The sodium and chloride ions can disrupt salt bridges between charged side chains. Na+ associates with negative charges, and Cl- associates with positive charges.
6. Which atoms in the following molecules are chiral? Star (*) them.
The following is a Fischer projection of 2-deoxy ribose, the sugar found in DNA.