What percent of imidazole is protonated at physiological pH (7.4)?

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Determine the pKa of imidazole. The pKa of the protonated form of imidazole is approximately 6.0. This value is essential for calculating the ratio of protonated to unprotonated species at a given pH.

Use the Henderson-Hasselbalch equation to relate pH, pKa, and the ratio of protonated to unprotonated species. The equation is:

pH = pKa + \log (\frac{[A⁻]}{[HA]})

, where [A⁻] is the concentration of the unprotonated form and [HA] is the concentration of the protonated form.

Rearrange the Henderson-Hasselbalch equation to solve for the ratio of [HA] (protonated form) to [A⁻] (unprotonated form):

\frac{[HA]}{[A⁻]} = 10

^{(pKa - pH)}.

Substitute the known values into the equation. Use pKa = 6.0 and pH = 7.4 to calculate the ratio of [HA] to [A⁻].

Convert the ratio into a percentage. The percentage of imidazole that is protonated can be calculated using the formula:

Percent = \frac{[HA]}{[HA] + [A⁻]} × 100

. Substitute the ratio from the previous step to find the final percentage.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Protonation and Deprotonation

Protonation refers to the addition of a proton (H+) to a molecule, while deprotonation is the removal of a proton. In organic chemistry, the protonation state of a compound can significantly affect its reactivity and properties. Understanding these processes is crucial for predicting how molecules behave in different pH environments, such as physiological pH.

pKa and pH Relationship

The pKa of a compound is the pH at which half of the species are protonated and half are deprotonated. This relationship is essential for determining the protonation state of molecules like imidazole at a given pH. By comparing the pKa of imidazole to the physiological pH of 7.4, one can assess the extent of protonation in biological systems.

Imidazole Structure and Function

Imidazole is a five-membered aromatic ring containing two nitrogen atoms, which can act as both a base and an acid. Its ability to be protonated or deprotonated is vital for its role in biological systems, particularly in enzyme active sites and as a buffer. Understanding its structure helps in predicting how it interacts with protons at physiological pH.