Q.19 Consider the two 𝑝𝐾𝑎values of valine as 2.32 and 9.62. The isoelectric point (pI)
of this amino acid is ______. (rounded off to two decimal places)
The isoelectric point (pI) of valine, given its pKa values of 2.32 and 9.62, is 5.97. This value represents the pH at which the amino acid carries no net charge, calculated as the average of the two pKa values for neutral amino acids like valine.
Calculation Method
Valine is a neutral amino acid with two ionizable groups: the α-carboxyl group (pKa1 = 2.32) and the α-amino group (pKa2 = 9.62). The pI formula for such amino acids is pI = (pKa1 + pKa2)/2.
Step-by-step:
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Add the pKa values: 2.32 + 9.62 = 11.94
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Divide by 2: 11.94 / 2 = 5.97 (rounded to two decimal places)
This arithmetic mean identifies the pH where the zwitterion form predominates, with equal concentrations of positively and negatively charged species.
Charge States Explained
At pH < pI (e.g., pH 2), valine is protonated (+1 net charge: ⁻OOC-CH(NH₃⁺)-(CH(CH₃)₂)-COOH form).
At pH = pI (5.97), net charge is zero (zwitterion: ⁻OOC-CH(NH₃⁺)-(CH(CH₃)₂)-COO⁻).
At pH > pI (e.g., pH 12), deprotonated (-1 net charge: ⁻OOC-CH(NH₂)-(CH(CH₃)₂)-COO⁻).
No options are provided in the query, but common MCQ distractors include unaveraged pKa values (2.32 or 9.62), incorrect rounding (6.00), or side-chain misuse (valine lacks ionizable R-group).
Introduction to Valine Isoelectric Point (pI)
The valine isoelectric point (pI) is crucial for CSIR NET Life Sciences aspirants tackling amino acid properties. With pKa values of valine at 2.32 (carboxyl) and 9.62 (amino), the pI of valine calculates to 5.97—rounded to two decimal places. This isoelectric point calculation helps predict protein behavior in electrophoresis, a frequent exam topic.
Why pI Matters in Amino Acid Chemistry
The isoelectric point of valine marks the pH where net charge is zero, maximizing solubility and aiding separation techniques. For neutral amino acids, pI averages the two pKa values, unlike acidic/basic ones requiring side-chain inclusion.
Valine’s nonpolar isopropyl side chain confirms the simple formula—no third pKa needed.
Detailed pI Calculation for Valine
Isoelectric point calculation formula:
pI=2pKa1+pKa2/2
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pKa₁ (α-COOH) = 2.32
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pKa₂ (α-NH₃⁺) = 9.62
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Sum: 11.94
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pI = 11.94 / 2 = 5.97
This matches standard references for valine pI.
Charge Forms Across pH Range
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Low pH (<2.32): Cationic (+1)
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pH 2.32–9.62: Zwitterion (0) peaks at pI 5.97
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High pH (>9.62): Anionic (-1)
CSIR NET Exam Tips
Round to two decimals as specified—5.97, not 6.0. Practice with glycine (pI 5.97) for pattern recognition. Memorize: neutral amino acids average α-pKa pair.
