At pH 6.0, the charge state of HSN CH_COI H;N_CH_CO2 HNN_CH_CO5 (CH3)4 CH; CH2 +NH2 CO2 net +1 charge net 0 charge net = [charge lysine alanine aspartic acid (a basic amino acid) neutral amino acid) (an acidic amino acid) Homework: Indicate if the following peptides are acidic, basic, or neutral at pH = 6.0: Gly-Leu-Val Leu-Trp-Lys-Gly-Lys N-Acetyl-Asp-Val-Ser-Arg-Arg
The Correct Answer and Explanation is:
Here are the classifications for each peptide at pH 6.0:
- Gly-Leu-Val: Neutral
- Leu-Trp-Lys-Gly-Lys: Basic
- N-Acetyl-Asp-Val-Ser-Arg-Arg: Neutral
Explanation
The overall charge of a peptide at a specific pH is determined by the sum of the charges on its N-terminal amino group, its C-terminal carboxyl group, and any ionizable amino acid side chains. At a pH of 6.0, we can establish the charge of these key groups. The N-terminal alpha-amino group (pKa ~9.5) will be protonated, carrying a +1 charge. The C-terminal alpha-carboxyl group (pKa ~2.5) will be deprotonated, carrying a -1 charge.
For the side chains, we must consider acidic and basic amino acids. Acidic side chains like aspartic acid (pKa ~3.9) will be deprotonated and have a -1 charge. Basic side chains like lysine (pKa ~10.5) and arginine (pKa ~12.5) will be protonated, each carrying a +1 charge. Amino acids with neutral, non-ionizable side chains do not contribute to the charge.
- Gly-Leu-Val: This peptide has a +1 charge at its N-terminus and a -1 charge at its C-terminus. The amino acids glycine, leucine, and valine all have non-ionizable side chains. Therefore, the net charge is (+1) + (-1) = 0, making the peptide neutral.
- Leu-Trp-Lys-Gly-Lys: This peptide has a +1 charge at the N-terminus and a -1 charge at the C-terminus. It contains two lysine residues. The side chain of each lysine is basic and will be protonated, contributing a +1 charge each. The other amino acids, leucine, tryptophan, and glycine, have neutral side chains. The total charge is (+1) + (-1) + (+1 from first Lys) + (+1 from second Lys) = +2. This positive net charge makes the peptide basic.
- N-Acetyl-Asp-Val-Ser-Arg-Arg: The N-terminus of this peptide is modified with an acetyl group. This acetylation forms an amide bond, which neutralizes the N-terminal amino group, so its charge is 0. The C-terminus has a -1 charge. The peptide contains one acidic residue, aspartic acid (-1 charge), and two basic residues, arginine (+1 charge each). Summing these charges gives (0) + (-1) + (-1 from Asp) + (+1 from first Arg) + (+1 from second Arg) = 0. The peptide has a net charge of zero and is therefore neutral. the C-terminal carboxyl group, and the side chains of any ionizable amino acids. At a pH of 6.0, we can establish the charge of these groups based on their pKa values. The N-terminus (pKa ~9-10) will be protonated and have a +1 charge. The C-terminus (pKa ~2-3) will be deprotonated and have a -1 charge.
For Gly-Leu-Val, the peptide has a +1 charge at its N-terminus (Glycine) and a -1 charge at its C-terminus (Valine). The side chains of Glycine, Leucine, and Valine are all nonpolar and do not ionize, so they have a charge of 0. The total net charge is (+1) + (-1) = 0, making the peptide neutral.
For Leu-Trp-Lys-Gly-Lys, the peptide has a +1 charge from the N-terminus and a -1 charge from the C-terminus. However, this peptide contains two basic Lysine (Lys) residues. The side chain of Lysine has a pKa of about 10.5. Since the pH of 6.0 is well below this pKa, both Lysine side chains will be protonated, each contributing a +1 charge. The net charge is (+1 for N-terminus) + (+1 for first Lys) + (+1 for second Lys) + (-1 for C-terminus) = +2. A net positive charge makes the peptide basic.
For N-Acetyl-Asp-Val-Ser-Arg-Arg, the analysis is more complex. The N-terminus is modified with an acetyl group, which forms an amide bond. This N-acetylated group is uncharged at pH 6.0. The peptide contains one acidic residue, Aspartic acid (Asp), whose side chain (pKa ~4) is deprotonated at pH 6.0, contributing a -1 charge. It also has two basic Arginine (Arg) residues. The Arginine side chain (pKa ~12.5) is protonated at pH 6.0, with each contributing a +1 charge. Finally, the C-terminus has a -1 charge. The net charge is (0 for N-terminus) + (-1 for Asp) + (+1 for first Arg) + (+1 for second Arg) + (-1 for C-terminus) = 0. The peptide is therefore neutral.izable groups. These groups include the N-terminal amino group, the C-terminal carboxyl group, and the side chains of acidic or basic amino acids. The charge of each group depends on its pKa value relative to the solution’s pH, which in this case is 6.0.
For a group to be protonated, the solution’s pH must be below its pKa. For it to be deprotonated, the pH must be above its pKa. At pH 6.0, the N-terminal amino group (pKa ~9.5) is protonated, carrying a +1 charge. The C-terminal carboxyl group (pKa ~2.5) is deprotonated, carrying a -1 charge.
- Gly-Leu-Val: This peptide only has two ionizable groups: its N-terminus (+1) and its C-terminus (-1). The side chains for Glycine, Leucine, and Valine are nonpolar and do not have a charge. The total charge is (+1) + (-1) = 0. Therefore, this peptide is neutral.
- Leu-Trp-Lys-Gly-Lys: This peptide has a +1 charge from its N-terminus and a -1 charge from its C-terminus. The key additions are the two Lysine (Lys) residues. The side chain of Lysine is basic, with a pKa of about 10.5. Since the pH of 6.0 is well below this pKa, both Lysine side chains will be protonated, each contributing a +1 charge. The total net charge is calculated as (+1 from N-terminus) + (-1 from C-terminus) + (+1 from first Lys) + (+1 from second Lys) = +2. A net positive charge makes this peptide basic.
- N-Acetyl-Asp-Val-Ser-Arg-Arg: This peptide has unique modifications. The “N-Acetyl” group at the beginning means the N-terminal amino group has been modified and is no longer ionizable; its charge is 0. The C-terminus is deprotonated, contributing a -1 charge. The peptide contains one acidic residue, Aspartic acid (Asp), and two basic residues, Arginine (Arg). At pH 6.0, the Asp side chain (pKa ~3.9) is deprotonated, giving it a -1 charge. The two Arg side chains (pKa ~12.5) are both protonated, each contributing a +1 charge. The total net charge is (0 from N-acetyl) + (-1 from Asp) + (+1 from first Arg) + (+1 from second Arg) + (-1 from C-terminus) = 0. Consequently, this peptide is neutral.
