**Core Concept:** Amino acids are organic compounds containing an amino group (-NH2) and a carboxyl group (-COOH). Proteins are composed of amino acids, which are linked by peptide bonds to form polypeptide chains. The absorbance of proteins at 280 nm wavelength is due to the presence of aromatic amino acids, particularly tryptophan, tyrosine, and phenylalanine. These amino acids have an indole, phenol, and benzene ring, respectively, which absorb UV light at around 280 nm.

**Why the Correct Answer is Right:** The correct answer is **tryptophan** because it is an essential amino acid that contains an indole ring. This indole ring allows tryptophan to absorb UV light at 280 nm, contributing to the increased absorbance of the protein at this wavelength when compared to other amino acids.

**Why Each Wrong Option is Incorrect:**

A. Alanine is a non-polar amino acid without any aromatic rings, so replacing it with alanine will not increase the UV absorbance at 280 nm.

B. Valine is a non-polar amino acid without any aromatic rings, so replacing alanine with valine will not increase the UV absorbance at 280 nm.

C. Cysteine has a sulfhydryl group (-SH), but it lacks an aromatic ring, so replacing alanine with cysteine will not increase the UV absorbance at 280 nm.

D. Leucine is a non-polar amino acid without any aromatic rings, so replacing alanine with leucine will not increase the UV absorbance at 280 nm.

**Clinical Pearl:** The UV absorbance of proteins at 280 nm is a common method to determine protein concentration in biochemistry and molecular biology experiments. By knowing the molar absorptivity (e) of a protein at 280 nm, the concentration of a protein solution can be calculated based on its absorbance. This method is particularly useful when working with samples containing hemoglobin or other colored molecules that can interfere with the absorbance at 280 nm.

**Correct Answer:** Tryptophan