7. Aspartic acid has a side chain bearing a carboxylic acid group; its pK, is 4. The a-amino and a-carboxylic groups have pK, values similar to those of alanine. Determine the net charge on aspartic acid at the following pH values: pH 1 pH 2 pH 4 pH 9 pH 13

5. Refer to Model 2 for information relevant to this question. A sample of the peptide Lys-Glu-Ser has a net charge of zero between what two pH values? What is the pl of Lys-Glu-Ser? Model 2: The pl of a peptide is determined by examining the ionizable groups. The protonated and unprotonated forms of each ionizable group are in equilibrium. Consider the peptide Lys-Glu-Ser shown below at pH 7.2. The complete structure is on on the left and a stylized structure with just the ionizable groups is on the right. While the N-terminal is depicted as protonated, a sample of Lys-Glu-Ser is composed of a population of molecules and within that population some molecules contain a non-protonated N-terminal group at pH 7.2. may H3N* COO- H3N-CH-CIN-CH C N-CH C- H. pK 3.5 OH H. pK= 8.5 COO- CH2 CH2 CH2 NH, pK 10.0 pK 4.2 CH2 CH2 OH CH2 c=0

Choose A if the statement is CORRECT B if the statement is WRONG This amino acid is/has: polar charged basic side chain 6. 7. 8. 9. aromatic side chain 10. K H H H N-C-C CH₂ CH ₂ CH₂ CH ₂ -Z N I NH₂ =O OH

Which of the amino acid side-chains would you expect to be protonated (i.e. has not lost its proton) under neutral pH conditions? Include reasons supporting your prediction.