The double bond character of the carbon-nitrogen bond results in a shorter bond compared to the normal length of a pure bond ( vs. ). The double bond resonance of peptide bonds aids to increase stability, while subsequently decreasing the rotation about that bond. The partial double bond results in the amide group being planar, thus allowing for either a cis or trans conformation of a peptide bond. Due to the preference for the trans orientation over the cis orientation, there is less steric hindrance between groups attached to the alpha-carbon atoms. Thus, the correct answer is that the carbon-nitrogen bond has partial double bond character.

On the MCAT you must be able to recognize the following as basic amino acids: lysine, arginine, and histidine. Important acidic amino acids include aspartic acid (aspartate) and glutamic acid (glutamate). Important nonpolar amino acids include: methionine, alanine, isoleucine, proline, phenylalanine, tryptophan, valine, and leucine.

Every amino acid contains a carboxyl group and an amino group. These two functional groups are essential for amino acid binding and breaking.

While sulfide groups contribute to higher protein structure by forming disulfide bonds, they do not exist in every amino acid.

Amino acids are categorized as nonpolar, polar, acidic, or basic. The category that an amino acid is placed into gives you an idea of where you might find the amino acid within a protein. For example, polar amino acids are commonly found on the outside of proteins, where other polar molecules (water) are likely to be found.

Each amino acid has an N and a C terminus. The N terminus contains an amino group and the C terminus contains a carboxylic acid group. In order to make a peptide linkage (and eventually create a polypeptide), a bond must form between the amino and carboxylic groups, with water as a byproduct.

Since alanine is nonpolar, we know that the only parts of the amino acid that can be charged are the N-terminus and the C-terminus.

In an acidic solution, there is an excessive amount of protons available to protonate the amino acid. As a result, the carboxylic acid end and the amine end will both be fully protonated. This will result in an overall charge of +1, due to the nitrogen having three hydrogens attached.

Nucleotides are composed of a pentose sugar, phosphate group, and nitrogenous base. Nucleotides are the building blocks of nucleic acids, such as DNA and RNA. Phosphodiester bonds form between the phosphate group of one nucleotide and the 3' carbon of the pentose sugar on a second nucleotide to form a linkage. A nucleoside describes the molecule that is formed by a pentose sugar and nitrogenous base.

Sulfate groups are not found in nucleotides or nucleosides. 

Basic amino acids are those containing an amine group, while acidic contain a carboxylic acid group.

The basic amino acids are lysine (the correct answer), arginine, and histidine.

The acidic amino acids are glutamic acid (glutamate) and aspartic acid (aspartate).

Disulfide bonds are disrupted by the introduction of mercaptoethanol. Disulfide bonds are created by the interaction of two cysteine amino acids on different parts of the amino acid chain during the development of tertiary protein folding. As a result, a protein with few to no cysteine amino acids would be least affected by mercaptoethanol.

Alpha-helices are linked to secondary structure, and do not involve disulfide bonds. Similarly, quaternary structure is not determined by disulfide bonds; a protein without quaternary structure could still have disulfide bonds, which would be disrupted by mercaptoethanol. Proline is not involved in disulfide bonds, and its frequency would not affect the potency of mercaptoethanol to the protein.

At low pH levels, we expect amino acids to exist in their cationic form. At a pH level equal to the isoelectric point (pI) we expect amino acids to exist as zwitterions, and at high pH levels we expect them to exist in their anionic forms.

Low pH causes protonation of the amino groups; high pH causes deprotonation of the carboxyl groups.