Questions: curve? Try this for one amino acid of your preference. If you were given the abbreviations for 4-6 amino acids, could you draw the polypeptide chain AND determine its isoelectric point? Do this for the amino ac Asp-Arg-Val-Tyr-Ile

To address the question, let's break it down into two parts: drawing the polypeptide chain and determining its isoelectric point (pI).

The given sequence of amino acids is Asp-Arg-Val-Tyr-Ile. Here are the structures of these amino acids:

1. Asp (Aspartic Acid): D
2. Arg (Arginine): R
3. Val (Valine): V
4. Tyr (Tyrosine): Y
5. Ile (Isoleucine): I

To draw the polypeptide chain, we need to connect these amino acids via peptide bonds. A peptide bond is formed between the carboxyl group of one amino acid and the amino group of the next amino acid.

H2N-CH(COOH)-CH2-CO-NH-CH(COOH)-(CH2)3-NH-C(NH2)2-CO-NH-CH(COOH)-CH(CH3)2-CO-NH-CH(COOH)-CH2-Ph-OH-CO-NH-CH(COOH)-CH(CH3)-CH2-CH3-COOH

This is a simplified linear representation of the polypeptide chain Asp-Arg-Val-Tyr-Ile.

The isoelectric point (pI) is the pH at which the polypeptide has no net charge. To determine the pI, we need to consider the pKa values of the ionizable groups in the amino acids:

• Aspartic Acid (Asp): pKa1 (α-COOH) ≈ 2.1, pKa2 (α-NH3+) ≈ 9.8, pKa3 (side chain COOH) ≈ 3.9
• Arginine (Arg): pKa1 (α-COOH) ≈ 2.2, pKa2 (α-NH3+) ≈ 9.0, pKa3 (side chain NH2) ≈ 12.5
• Valine (Val): pKa1 (α-COOH) ≈ 2.3, pKa2 (α-NH3+) ≈ 9.6
• Tyrosine (Tyr): pKa1 (α-COOH) ≈ 2.2, pKa2 (α-NH3+) ≈ 9.1, pKa3 (side chain OH) ≈ 10.1
• Isoleucine (Ile): pKa1 (α-COOH) ≈ 2.3, pKa2 (α-NH3+) ≈ 9.7

To find the pI, we need to average the pKa values of the groups that will be ionized around the pI. For this polypeptide, the relevant pKa values are:

• The α-COOH of Asp (2.1)
• The α-COOH of Arg (2.2)
• The α-COOH of Val (2.3)
• The α-COOH of Tyr (2.2)
• The α-COOH of Ile (2.3)
• The α-NH3+ of Asp (9.8)
• The α-NH3+ of Arg (9.0)
• The α-NH3+ of Val (9.6)
• The α-NH3+ of Tyr (9.1)
• The α-NH3+ of Ile (9.7)
• The side chain COOH of Asp (3.9)
• The side chain NH2 of Arg (12.5)
• The side chain OH of Tyr (10.1)

The pI is typically calculated by averaging the pKa values of the groups that are ionizable near the pI. For simplicity, we will consider the most relevant pKa values:

• The side chain COOH of Asp (3.9)
• The side chain NH2 of Arg (12.5)

The pI is approximately the average of these two values:

\[ \text{pI} = \frac{3.9 + 12.5}{2} = 8.2 \]

• Polypeptide Chain: The sequence Asp-Arg-Val-Tyr-Ile is connected via peptide bonds.
• Isoelectric Point (pI): The pI of the polypeptide chain Asp-Arg-Val-Tyr-Ile is approximately 8.2.