The net charge on the peptide Ala-Glu-Val-Asn-Ile-Asp-Pro-Asp-Gln-Gly-Asp at pH 1.0 is +1.

This calculation relies on the protonation states of all ionizable groups, as pH 1.0 is below the pKa of every group in the peptide. All amino and carboxyl groups are fully protonated, yielding one positive charge from the N-terminus and neutral carboxyl groups.

Ionizable Groups

The peptide has 12 amino acids, so 11 peptide bonds form internally. Ionizable groups include:

• N-terminal α-NH₃⁺ (Ala): +1 charge
• C-terminal α-COOH (Asp): neutral at pH 1 (protonated)
• Side chain COOH in Glu (position 2): neutral, pKa ~4.3
• Side chain COOH in Asp (positions 6,8,12): neutral each, pKa ~3.9

No basic side chains or other ionizable groups (Asn/Gln amides neutral, Pro no charge). Acidic residues: 1 Glu + 3 Asp = 4 side chain COOH + 1 C-terminal COOH = 5 COOH total, all protonated (neutral) at pH 1. Only 1 positive from N-term.

Step-by-Step Calculation

1. Identify all ionizable sites: N-term NH₃⁺ (pKa~9.5, protonated +1), C-term COOH (pKa~2.2, protonated 0), 1 Glu side COOH (pKa 4.3, 0), 3 Asp sides COOH (pKa 3.9, 0 each).
2. At pH 1.0 << lowest pKa (~2), acids protonated (neutral), bases protonated (+1).
3. No Lys/Arg/His, so only 1 positive group. Net charge = +1.

Why pH 1.0 Matters

All pKa >1: α-COOH ~2.0-2.3, Asp side ~3.9, Glu side ~4.3, α-NH₃⁺ ~9-10. Fully protonated form: H₃N⁺-peptide-(COOH)₅, charge +1. For CSIR NET, approximate without Henderson-Hasselbalch as 10^(pKa-pH) >>1 for acids.

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Detailed Solution for CSIR NET

• List ionizable groups: 1 N-terminal NH₃⁺ (+1 at pH1), 5 COOH (4 acidic sides +1 C-term, 0 at pH1).
• Protonation rule: pH < pKa → acids protonated (COOH, 0), bases protonated (NH₃⁺, +1).
• Net charge: +1 – 0 = +1. No options needed as fill-in-blank; common trap is assuming deprotonated COOH (- charges).

This matches standard biochemistry for low pH where peptide resembles cationic form with single + from N-terminus. Practice similar for exams.