The Question is from neet-chemistry nitrogen-compounds amino-acids, and it was created by the author Mr. S M HUSSAIN

Question

Which of the following statements regarding the isoelectric point (pI) of amino acids is INCORRECT?

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
	**Primary Concept**: Isoelectric Point (pI)
	**Related Topics**: Amino acids, Zwitterions, Acid-base chemistry, pKa
	**Prerequisites**: Understanding of amino acid structure, acid-base equilibria, and the concept of pKa.
## SOLUTION PATHWAY
**Given Information**:
	We are analyzing the properties of the isoelectric point (pI) of amino acids.
**Method & Steps**:
	1. **Understanding Isoelectric Point**: The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For amino acids, this is the pH where the amino acid exists predominantly as a zwitterion.
		Reasoning: At the pI, the positive and negative charges on the amino acid balance each other out.
	2. **pI Calculation**: The pI is calculated based on the pKa values of the ionizable groups in the amino acid. For neutral amino acids with only two ionizable groups (amino and carboxyl), the pI is approximately the average of the pKa values of these two groups. However, for amino acids with ionizable side chains, the pI is the average of the two pKa values that correspond to the protonation and deprotonation of the zwitterionic form.
		Key Point: The statement 'The pI is *always* the average of *two* pKa values' is incorrect. It can be the average of two pKa values in *some* cases (simple amino acids), but not all.
	3. **Factors Affecting pI**: The pI is affected by temperature and, to a lesser extent, ionic strength.
		Result: Option B is the incorrect statement.
## OPTION ANALYSIS
[A] ✓: The amino acid is least soluble at its pI because the zwitterionic form has minimal interaction with polar water molecules, leading to aggregation and precipitation.
[B] :x: While the pI is the average of two pKa values for *some* amino acids (those with only two ionizable groups), this is not a universal rule.  Amino acids with ionizable side chains (e.g., lysine, arginine, histidine, aspartic acid, glutamic acid) have more complex pI calculations involving three pKa values. The pI is determined by the average of the pKa values corresponding to the equilibrium between the neutral and the positive or negative charged species.
[C] :x: Ionic strength can affect the pI, although the effect is usually small compared to the influence of pKa values. High ionic strength can shield charges and slightly alter the pKa values, thus affecting the pI.
[D] :x: Temperature affects the ionization constants (pKa values) and, consequently, the pI.  Increased temperature generally leads to slightly higher pKa values and thus a shift in pI.
## LEARNING AIDS
	**Common Mistakes**:
		1. Assuming pI is always the average of *any* two pKa values: Remember it's the average of the two pKa values that bracket the zwitterionic form.
		2. Ignoring the effect of ionizable side chains:  These significantly impact the pI calculation and make the simple average incorrect for amino acids with charged side chains.
	**Quick Tips**:
		1. For simple amino acids, the pI is approximately the average of the $pK_a$ of the carboxyl group and the $pK_a$ of the amino group.
		2. For amino acids with acidic side chains, the pI is lower, and for those with basic side chains, the pI is higher.

Mr. S M HUSSAIN · Answer

The pI is the pH where the amino acid is least soluble.

Mr. S M HUSSAIN · Answer

The correct Option is "The pI is always the average of two pKa values."

Mr. S M HUSSAIN · Answer

The pI is affected by the solution's ionic strength.

Mr. S M HUSSAIN · Answer

The pI is temperature-dependent.

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