UV absorption of amino acids at 280 nm is primarily governed by aromatic rings and disulfide bonds:

• Tryptophan (D) has the highest molar extinction coefficient at 280 nm ($\varepsilon_{280} \approx 5,500 \text{ M}^{-1}\text{cm}^{-1}$) due to its indole ring.
• Tyrosine (C) has an intermediate absorption at 280 nm ($\varepsilon_{280} \approx 1,490 \text{ M}^{-1}\text{cm}^{-1}$) due to its phenolic hydroxyl ring.
• Cystine/Cysteine (A) exhibits weak absorption around 280 nm ($\varepsilon_{280} \approx 125 \text{ M}^{-1}\text{cm}^{-1}$ for disulfide bonds).
• Phenylalanine (B) absorbs primarily at 257 nm with virtually negligible absorption at 280 nm ($\varepsilon_{280} \lt 10 \text{ M}^{-1}\text{cm}^{-1}$).