Amino Acids (for BSCI 2520)

From Table 4-1 in the Voet textbook, 5th ed. All structures here are predominant at pH = 7 (except for His).

Glycine, Gly, G

Nonpolar

Alanine, Ala, A

Nonpolar

Valine, Val, V

Nonpolar

Leucine, Leu, L

Nonpolar

Isoleucine, Ile, I

Nonpolar

Methionine, Met, M

Nonpolar

Proline, Pro, P

Nonpolar

Phenylalanine, Phe, F

Nonpolar

Tryptophan, Trp, W

Nonpolar

Serine, Ser, S

Polar, uncharged

Threonine, Thr, T

Polar, uncharged

Asparagine, Asn, N

Polar, uncharged

Glutamine, Gln, Q

Polar, uncharged

Tyrosine, Tyr, Y

Polar, uncharged

Cysteine, Cys, C

Polar, uncharged

Lysine, Lys, K

Positively charged (basic residue)

Arginine, Arg, R

Positively charged (basic residue)

Histidine, His, H

Positively charged (basic residue)

The structures for all the other amino acids in this set depict what the amino acids look at pH = 7. However, at pH = 7, histidine’s neutral and protonated (positively charged) forms are BOTH present. This is because the pK of histidine’s R group is 6.04 (close to pH = 7)

Aspartate / Aspartic acid, Asp, D

Negatively charged (acidic residue)

Glutamate / Glutamic acid, Glu, E

Negatively charged (acidic residue)

Asparagine OR aspartic acid (aspartate)

Asx, B

Glutamine OR glutamic acid (glutamate)

Glx, Z

Undetermined or nonstandard amino acid

X

Significance of pH = 7 on charge of amino acids

The pK of the carboxylic acid group is usually around pH = 2. The pK of the amino group is usually around pH = 9.

Thus, at pH = 7, the carboxylic acid group is predominately deprotonated. The amino group is predominately protonated.

If pH < pK of R group: protonated form of R group predominates (considered positively charged AA residues)

If pH > pK of R group: deprotonated form of R group predominates (considered negatively charged AA residues)