A peptide bond is a covalent bond that connects adjacent amino acid molecules. The bond occurs between a carboxylic acid group on one amino acid and an amino group on the other. Specifically, the carbon atom from the carboxylic acid and the nitrogen atom from the amino group form a peptide bond; therefore, only carbon and nitrogen participate in a peptide bond.

Nitrogen and hydrogen make up the amino group (), whereas carbon and oxygen make up the carboxylic acid group (). Remember that an amino acid has a central carbon. The central carbon always has an amino group, a carboxylic acid, and a hydrogen atom. The fourth group is different for each amino acid, which gives them unique properties. 

A protein molecule can have four different structural levels. Primary structure consists of the sequence of amino acids making up the polypeptide. Secondary structure consists of the sequence of amino acids and the intermolecular forces and covalent bonds that form between amino acid backbone components. Examples of intermolecular forces in secondary structures include hydrogen bonding, van der Waals forces, and dipole-dipole interactions.

Tertiary structure involves covalent bonds, dipole-dipole interactions, and hydrophobic interactions between amino acid R-groups. Tertiary structure is responsible for the 3-dimensional shape of the polypeptide. An example of a covalent bond found in tertiary structure is the disulfide bond. This bond occurs between sulfur molecules in cysteine amino acids; therefore, disrupting the bonds stated in the question will alter the tertiary structure.

Quaternary structure occurs when two or more polypeptide chains interact with each other and form bonds. 

Denaturing a polypeptide is the process of disrupting the secondary, tertiary, and quaternary structures. This means that denaturing a protein will lead to disruption in intermolecular forces such as hydrogen bonds. Recall that hydrogen bonds occur between a hydrogen atom and either a nitrogen, oxygen, or fluorine atom; therefore, denaturing a polypeptide will cause a disruption in the intermolecular forces between nitrogen and hydrogen (hydrogen bonds).

Secondary structures can form unique structures called beta-pleated sheets or alpha helices. The beta-pleated sheets are formed when a polypeptide chain folds in such a way that it loops back to lie adjacent to an earlier segment. Alpha helices are formed when a polypeptide chain twists and forms a helical structure. Note that both of these structures involve intermolecular forces (hydrogen bonds, van der Waals, etc.) between amino acids. Denaturing a polypeptide will disrupt both of these structures.

Recall that a denatured polypeptide will not lose its peptide bonds; therefore, the polypeptide will have its original number and sequence of amino acids (primary structure). The side chains of amino acids will not change during denaturation. The intermolecular forces and disulfide bonds between adjacent amino acids will change, but the composition of each amino acid won’t change.

To answer this question you need to know the single letter codes for the given amino acids.

D-K-A-R-H-F is aspartic acid - lysine - alanine - arginine - histidine - phenylalanine.

R-A-A-D-P-P is arginine - alanine - alanine - aspartic acid - phenylalanine - phenylalanine.

E-P-H-D-V-W is glutamic acid - phenylalanine - histidine - aspartic acid - valine - tryptophan.

To solve this question we need to figure out the overall charges of the three polypeptide chains. Recall that five amino acids are charged under physiological conditions (pH of 7.4). These five amino acids are glutamic acid (E), aspartic acid (D), arginine (R), lysine (K), and histidine (H). Glutamic acid and aspartic acid have an overall  charge under physiologic pH. This occurs because their side chains are deprotonated. On the other hand, arginine, lysine, and histidine have a protonated side chain and have a  charge. Knowing this information we can solve for the overall net charge on the three polypeptide chains.

Polypeptide I: overall net charge of 

Polypeptide II: overall net charge of 

Polypeptide III: overall net charge of 

The amino acids histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine are considered essential amino acids as they cannot be made by the body. They need to obtained from outside sources/nutrients.

During ion-exchange chromatography, as you increase the pH of the mobile phase (the solution flowing through the column), you elute out proteins based on their isoelectric point (pI). So in this case, you first pass the pI of glutamic acid (Glu), and then that of leucine (Leu). You would never elute out the lysine, because the pI increase stops at 7 and never gets as high as lysine’s pI, 9.74. You have, however, successfully separated the three amino acids from each other, as the lysine remains in the column.

Nucleic acids absorbs light at . The aromatic amino acids phenylalanine, tyrosine, and tryptophan absorb light at .

The opposite of the false statement is true: 18 of the 19 amino acids are in the S configuration, while cysteine is the only amino acid in the R configuration.

Diethylaminoethyl (DEAE) cellulose is a positive weak ion exchanger. Any molecules with a positive charge will elute through the column first, whereas molecules with a negative charge will adhere to the DEAE column and elute after the salt wash. Since protein III has a pI greater than the pH of the column, it has a positive charge and elutes first. Protein II elutes second because its pI is less than the pH but close in value. Therefore protein II contains less of a negative charge than protein I.

Hydrogen bonding stabilizes secondary structure to form alpha helices and beta sheets. Hydrophobic interactions between nonpolar side groups dictate a protein's tertiary structure. Disulfide bonds are involved in the determination of a protein's tertiary and quaternary structures, but they are not the primary contributors.