Biological chromophores

Amino acids and proteins: Among the 20 amino acids that constitute the proteins, tryptophan, tyrosine, and phenylalanine absorb in the near UV region. All the three amino acids show structured absorption spectra. The absorption by phenylalanine is weak with an ε_max of ~200 M^-1cm^-1 at ~250 nm. Molar absorption coefficients of ~1400 M^-1cm^-1 at 274 nm and ~5700 M^-1cm^-1 at 280 nm are observed for tyrosine and tryptophan, respectively. Disulfide linkages, formed through oxidation of cysteine resides, also contribute to the absorption of proteins in near UV region with a weak ε_max of ~300 M^-1cm^-1 around 250-270 nm. The absorption spectra of proteins are therefore largely dominated by Tyr and Trp in the near UV region. In the far UV region, peptide bond emerges as the most important chromophore in the proteins. The peptide bond displays a weak n → π* transition (ε_max ≈ 100 M^-1cm^-1) between 210-230 nm, the exact band position determined by the H-bonding interactions the peptide backbone is involved in. A strong π → π* transition (ε_max ≈ 7000 M^-1cm^-1) is observed around 190 nm. Side chains of Asp, Glu, Asn, Gln, Arg, His also contribute to the absorbance in the far UV region. Figure 5.5 shows an absorption spectrum of a peptide.

 

Figure 5.5 Absorption spectrum of a peptide. The absorption band ~280 nm is due to aromatic residues. Absorption band in the far UV region arises due to peptide bond electronic transitions.