Question

5. Which of the 20 standard amino acids are a. Cyclic b. Aromatic C. Sometimes charged at physiological pH d. Technically not hydrophobic, polar, or charged e. Basic f. Acidic . Sulfur containing 6. Estimate the net charge of a His-His-His-His tetrapeptide at pH 6.0 7. The artificial non-nutritive sweetener aspartame is a dipeptide with the sequence Asp-Phe. The carboxyl terminus is methylated. Draw the structure of aspartame at pH 7.0. Sketch a titration curve for the amino acid cysteine, and indicate the pKa values for all titratable groups. Also indicate the pH at which the amino acid has no net charge. 8.

Answer 1

Ans. #5. a. Cyclic: Proline

b. Aromatic: Tyrosine, phenylalanine

c. Sometimes charged at physiological pH: Tyrosine, Cysteine. The side chain of these two amino acids are uncharged in their pure solutions. However, when incorporated in proteins, sometimes these residues may become charged and participate catalytic activities.

d. Technically not hydrophobic, polar, or charged- Glycine. Because of having a single –H atom as its side chain, glycine is technically not hydrophobic or hydrophilic or charged.

e. Basic- Arginine, Lysine, Histidine

f. Acidic- Glutamic acid, Aspartic acid

g. Sulfur containing- Methionine, Sulfur

#6. Net charge on the peptide = +2.091

#7.
NH2 он Aspartame

#8. Cysteine has three ionizable groups- pKa1 = 1.91, pKa2 = 10.28, pKa3 = 8.14

At pH 5.0, cycteine has a net charge of 0.0

10.28---- pKa2. N-ter NH2 group 8.14 t pKa3. -SH group of side chain pH 5.0 pl 191キ pKa1. C-terminal -COOH group Volume of Base (mL)