**Question:** All of the following Aminoacids absorb light at 280 nm, except:

A. Tryptophan
B. Tyrosine
C. Proline
D. Glycine

**Core Concept:** UV-Visible spectroscopy is a technique used to study the interaction of molecules with ultraviolet and visible light. Amino acids contain aromatic rings (indole in Trp, benzene ring in Tyr, and imidazole ring in His) which can absorb light at specific wavelengths. The absorption spectrum of these rings typically occurs between 200-400 nm, with peaks at around 280 nm.

**Why the Correct Answer is Right:** Glycine does not contain any aromatic rings or significant chromophores that would allow it to absorb light at 280 nm. Its structure consists of a simple alanine-like backbone with a methyl group attached to the alpha-carbon. No aromatic rings or chromophores are present in glycine, hence it does not absorb light at 280 nm.

**Why Each Wrong Option is Incorrect:**

A. Tryptophan (Trp) is an essential amino acid containing an indole ring, which is an aromatic ring. The indole ring of Trp can absorb light at 280 nm, making Trp the correct answer to exclude.

B. Tyrosine (Tyr) contains a benzene ring, which is another aromatic ring. The benzene ring of Tyr can also absorb light at 280 nm, making Tyr the correct answer to exclude.

C. Proline (Pro) is an essential amino acid with a unique alpha-carbon structure. Pro does not contain any aromatic rings or significant chromophores that would allow it to absorb light at 280 nm, making Pro the correct answer to exclude.

D. Glycine (Gly) does not contain any aromatic rings or chromophores that would allow it to absorb light at 280 nm, making Gly the correct answer to exclude.

**Clinical Pearls:**

1. UV-Visible spectroscopy is a valuable tool in biochemistry and molecular biology to study the interaction of amino acids with ultraviolet and visible light.
2. The absorption spectrum of amino acids is used to determine their concentration in biological samples, as well as to study their structural and functional properties.
3. The exclusion of Gly, Trp, and Tyr from the correct answer provides an understanding of the structural features of these amino acids that allow them to absorb light at 280 nm:
- Gly contains a simple structure with no aromatic rings or chromophores.
- Trp has an indole ring, which is an aromatic ring capable of absorbing light at 280 nm.
- Tyr has a benzene ring, which is another aromatic ring capable of absorbing light at 280 nm.

**Why You Should Care:**

Understanding the structural features of amino acids that allow them to absorb light at 280 nm is essential for:

1. Biochemistry students studying UV-Visible