Which of the following aminoacids have maximum buffering capacity at PH 7?
**Core Concept**
The buffering capacity of an amino acid at a given pH is determined by its pKa value, which is the pH at which the amino acid is 50% protonated. Amino acids with a pKa value close to the desired pH have maximum buffering capacity.
**Why the Correct Answer is Right**
The buffering capacity of an amino acid is influenced by its side chain and the presence of an alpha-amino group and an alpha-carboxyl group. At pH 7, the alpha-amino group is mostly protonated, while the alpha-carboxyl group is mostly deprotonated. The side chain of histidine has a pKa value of approximately 6.0, which is close to pH 7. This makes histidine an effective buffer at pH 7, as it can accept or donate protons easily.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine has a pKa value of approximately 2.3 for its alpha-carboxyl group, making it a poor buffer at pH 7.
* **Option B:** Alanine has a pKa value of approximately 2.3 for its alpha-carboxyl group, similar to glycine, making it a poor buffer at pH 7.
* **Option D:** Valine does not have a pKa value close to pH 7, making it a poor buffer at this pH.
**Clinical Pearl / High-Yield Fact**
The buffering capacity of amino acids is an important concept in biochemistry, as it helps maintain the pH of the body's internal environment. Histidine, with its unique pKa value, plays a crucial role in buffering the pH of the blood and tissues.
**Correct Answer: C. Histidine. Histidine.**