The peptide sequence ATHMARAMEN (Alanine-Threonine-Histidine-Methionine-Alanine-Arginine-Methionine-Glutamic acid-Asparagine) has ionizable groups including the N-terminal amino (pKa ≈9.0), C-terminal carboxyl (pKa ≈2.2), His side chain (pKa ≈6.0), Arg side chain (pKa ≈12.5), and Glu side chain (pKa ≈4.3)[web:7][web:1]. At pH 3.7, groups with pKa > pH are mostly protonated (+1 for bases: N-term, His, Arg), while acidic groups with pKa < pH are mostly deprotonated (-1), but precise states depend on pH-pKa differences[web:2]. The net charge approximates +2, as N-term (+1), His (+1), Arg (+1) contribute positively, C-term is mostly neutral (≈0), and Glu is partially negative (≈-0.4 to -0.8), yielding +3.8 to +2 overall[web:1].

Ionizable Groups Analysis

• N-terminal (Ala-NH₃⁺): pKa 9.0 >> 3.7 → fully protonated, +1 charge[web:7].
• C-terminal (Asn-COOH): pKa 2.2 < 3.7 → mostly deprotonated, but ΔpH=1.5 means ~97% deprotonated (-0.97 to -1)[web:21].
• His (position 3): pKa 6.0 > 3.7 → protonated, +1[web:7].
• Arg (position 6): pKa 12.5 >> 3.7 → protonated, +1[web:7].
• Glu (position 8): pKa 4.3 > 3.7 → mostly protonated/neutral (≈+0.2 to 0 charge), or partial using Henderson-Hasselbalch[web:1].

Other residues (A, T, M, N) lack ionizable side chains at this pH[web:7].

Option Explanations

Using approximation (fully charged if |pH-pKa| >2 units):

• a. +1: Too low; ignores multiple positives (N-term, His, Arg: at least +3 minus partial negatives)[web:2].
• b. +2: Correct; balances +3 positives with partial deprotonation of C-term/Glu (~+3 -1)[web:1].
• c. -1: Impossible at acidic pH 3.7; positives dominate acidic groups[web:3].
• d. -2: Wrong; requires more negatives, but only 2 acidic groups partially ionized[web:2].

pKa Table for Key Groups[web:7]

CSIR NET Preparation Tips

• At low pH (<4), count basic groups (+1 each) minus acidic (-1 if pKa<<pH)
• Practice with peptide calculators for exact values[web:13]
• Master Henderson-Hasselbalch equation for partial charges[web:12]