The predominant form of histamine present in human blood is (`rho K_(a)` Histidine - 6.0)

Options

A.
B.
C.
D.

Explanation

To determine the predominant form of histamine present in human blood, we need to analyze the pKa values and the pH of blood. Here’s a step-by-step solution:

### Step 1: Understand the Structure of Histidine
Histidine has an imidazole side chain, which contains a nitrogen atom that can accept a proton (H+). The pKa of the imidazole nitrogen in histidine is given as 6.0.

**Hint:** Remember that the pKa value indicates the pH at which half of the species are protonated and half are deprotonated.

### Step 2: Know the pH of Human Blood
The normal pH of human blood is approximately 7.34. This pH is slightly above the pKa of the imidazole group in histidine.

**Hint:** Compare the pH of blood with the pKa values to determine the protonation state of the functional groups.

### Step 3: Analyze the Protonation State
Since the pH (7.34) is higher than the pKa of the imidazole nitrogen (6.0), the imidazole group will predominantly be in its deprotonated form. However, we also have an amino group (NH2) with a pKa of about 9.7.

**Hint:** A functional group is protonated when the pH is lower than its pKa.

### Step 4: Determine the Form of Histamine
At a pH of 7.34, the amino group (NH2) will be protonated to form NH3+ because the pH is lower than its pKa (9.7). The imidazole group will remain neutral (deprotonated) because the pH is above its pKa.

**Hint:** Consider which groups are protonated based on their pKa values relative to the pH.

### Step 5: Draw the Predominant Structure
The predominant form of histamine in blood will have the imidazole ring in its neutral form and the amino group in its protonated form (NH3+). This can be represented as follows:

- Imidazole ring: Neutral
- Amino group: Protonated (NH3+)

**Hint:** Visualize the structure to confirm the protonation states of the groups.

### Conclusion
The predominant form of histamine present in human blood is the one where the imidazole is neutral and the amino group is protonated (NH3+).

**Final Answer:** The correct option is D, which represents the structure of histamine with the amino group as NH3+.