Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?
Alanine is a non-polar, aliphatic amino acid without an aromatic ring. If we replace it with an amino acid that has an aromatic ring, that should increase the absorbance. Let me think about the options. The correct answer must be one of the aromatic amino acids. Tryptophan has the highest molar extinction coefficient at 280 nm, so replacing alanine with tryptophan would definitely increase UV absorbance. Tyrosine also contributes, but maybe less than tryptophan. Phenylalanine does too, but again, maybe not as much as tryptophan.
Wait, the options aren't provided, but the correct answer is supposed to be one of the amino acids. So the question is testing the knowledge that aromatic residues are responsible for UV absorption in proteins. The wrong options would be amino acids without aromatic rings. For example, if the options included glutamic acid (polar, no ring), lysine (polar, no ring), or valine (non-polar, aliphatic, no ring), those would be incorrect. So the key is that the correct answer must be an aromatic amino acid. The answer is likely tryptophan, as it's the strongest contributor.
Now, the explanation needs to cover the core concept of UV absorption by aromatic amino acids. The correct answer is one of these, and the incorrect ones are non-aromatic. The clinical pearl would be to remember that Trp, Tyr, and Phe are the main contributors to 280 nm absorbance. So replacing alanine (non-aromatic) with any of them would increase absorption. But since the question asks which one would do so, the answer depends on which of these is an option. Assuming the correct answer is tryptophan, as it's the most significant.
I need to make sure that in the explanation, the wrong options are addressed. For example, if an option is another aliphatic amino acid like leucine, that's non-aromatic and won't help. So each incorrect option is wrong because it lacks an aromatic ring. The clinical pearl is to memorize the three aromatic amino acids and their role in UV absorption.
**Core Concept**
UV absorbance of proteins at 280 nm is primarily due to aromatic amino acids: **tryptophan**, **tyrosine**, and **phenylalanine**. These amino acids contain conjugated aromatic rings that absorb UV light. Replacing a non-aromatic amino acid like alanine (which lacks an aromatic ring) with one of these increases absorbance.
**Why the Correct Answer is Right**
Tryptophan has the highest molar extinction coefficient at 280 nm among amino acids. Its indole ring system allows strong absorption of UV light. Replacing alanine (a non-aromatic, aliphatic amino acid) with tryptophan introduces this aromatic structure, directly increasing