Question

Isoelectric point of Aspartic acid is (A) 2 (B) 5 (C) 3 (D) \(6.5\)

Short answer

The isoelectric point (pI) of Aspartic acid is approximately 2.8, which is closest to the option (A) 2.

Step-by-step solution

Identify pKa values for Aspartic Acid

We begin by identifying the pKa values for Aspartic acid's ionizable groups: α-carboxyl group (\(pKa1\)), α-amino group (\(pKa2\)), and side chain carboxyl group (\(pKa3\)). These values are: - \(pKa1 = 1.88\) for the α-carboxyl group - \(pKa2 = 9.60\) for the α-amino group - \(pKa3 = 3.65\) for the side chain carboxyl group

Determine ionization order

Next, we need to determine the order of ionization to figure out which pKa values will be used to calculate the isoelectric point. Given that we are dealing with an acidic amino acid (Aspartic acid), the ionization order is typically α-carboxyl group, side chain carboxyl group, and then the α-amino group. From the values identified in Step 1, we can see this to be true: - \(pKa1 < pKa3\): α-carboxyl group ionizes before the side chain carboxyl group - \(pKa3 < pKa2\): side chain carboxyl group ionizes before the α-amino group

Calculate the isoelectric point (pI)

Now that we have the correct ionization order, we need to average the pKa values either side of the zwitterion form (where the net charge is zero). For Aspartic acid, we'll average the pKa values of the ionized carboxyl groups (\(pKa1\) and \(pKa3\)): \(pI = \frac{pKa1 + pKa3}{2}\) Plug in the pKa values: \(pI = \frac{1.88 + 3.65}{2}\)

Solve for pI

We'll solve the equation for pI: \(pI = \frac{5.53}{2}\) \(pI = 2.765\) Our final answer is approximately 2.8, which is closest to the option (A) 2. So, the correct answer is: (A) 2

Key concepts

Understanding pKa Values

The concept of pKa values is essential when discussing acid and base chemistry, particularly in the context of amino acids. The pKa value is a measure of the strength of an acid in solution. More specifically, it indicates the pH at which a given proton (from a hydrogen atom in a compound) is 50% disassociated from the compound. Therefore, each pKa value corresponds to a specific ionizable group in an amino acid.

For a molecule like Aspartic acid, there are multiple pKa values to consider because it contains more than one ionizable group. Generally:

• The lower the pKa, the stronger the acid, meaning it more readily donates protons.
• In the case of Aspartic acid, it has three relevant pKa values: one for the α-carboxyl group, one for the α-amino group, and one for the β-carboxyl (side chain) group.

Understanding these values is imperative for calculating the isoelectric point (pI), where the zwitterionic form of the amino acid predominates. This exercise involves identifying and using the relevant pKa values, depending on the ionization state that is being considered at neutrality.

Characteristics of Amino Acids

Amino acids are organic compounds that combine to form proteins, and they play many critical roles in the body. They are composed of two fundamental functional groups: an amine group (-NH2) and a carboxyl group (-COOH), bonded to a central carbon (the alpha carbon), along with a distinctive side chain.

The side chain, often referred to as the "R group," varies between different amino acids and determines their properties, such as their polarization, acidosis, or granular characteristics. In the case of Aspartic acid:

• The side chain contains an additional carboxyl group, which gives Aspartic acid its properties as an acidic amino acid.
• This additional carboxyl group leads to a unique set of ionization patterns and pKa values, which must be considered for calculations like determining the isoelectric point.

Zwitterion: The Unique Ionization State

The term "zwitterion" refers to the unique ionization state of a molecule that contains both a positive and a negative charge, resulting in an overall neutral charge. Most commonly, amino acids in solutions will naturally adopt this form at a certain pH range called the isoelectric point (pI).

For Aspartic acid:

• The zwitterionic form involves the carboxyl groups being largely deprotonated (negatively charged) and the amino group being protonated (positively charged).
• At the isoelectric point, the net charge of the molecule is zero because the positive and negative charges balance each other out.

Determining the pI is crucial for understanding how an amino acid behaves in different pH environments, as this affects its solubility and interaction with other molecules. The isoelectric point of amino acids is calculated specifically, often using the relevant pKa values that flank the zwitterionic state.