Ultraviolet spectra Aromatic amino acids

Side chains of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan absorb ultraviolet light in the 240- to 300-nm region, while histidine and cystine absorb to a lesser extent. Figure 3-13 shows the absorption spectrum of a "reference compound" for tyrosine. There are three major absorption bands, the first one at 275 nm being a contributor to the well-... 

Phenylalanine s phenyl side chain classifies it as an aromatic amino acid. The aromatic amino acids, like most compounds carrying conjugated rings, exhibit strong absorption of light in the near-ultraviolet region of the spectrum (Figure 5.6). This absorption is frequently used for the analytical detection of proteins. 

The application of low-temperature techniques to the investigation of protein spectra in the ultraviolet region was initiated by Lavin and Northrop (1935) who investigated the ultraviolet absorption spectra of pepsin, serum albumin, and ovalbumin in glycerol, and showed that the fine structure of the protein spectrum was enhanced at — 100°C. Preliminary reports of similar work have been published by Randall and Brown (1949) on thin films of sublimed tryptophan and phenylalanine at 90°C., and by Sinsheimer et al. (1949) for tryptophan at 77.6°K. Loof-bourow and his coworkers (Sinsheimer et al., 1950) have begun publication of a series of papers reporting much more comprehensive work on the influence of low temperature on the spectra of amino acids and proteins in thin films and in solid solution. Beaven et al. (1950) have reported a few results on thin Aims of the aromatic amino acids. 

Absorbance spectroscopy (difference spectroscopy) monitors conformational transitions in macromolecules by measuring absorbance changes, usually in the aromatic region of the ultraviolet (UV) spectrum. The amino acids tryptophan and tyrosine are the most important chromophores in the UV region for proteins. As mentioned earlier, tryptophan residues are often engineered into proteins as reporters of local and/or global environment. 

In the preceding Section (5.3) the CD bands in the near ultraviolet region are assigned to those of tyrosyl and phenylalanyl residues, whose thermal motions are restricted sterically keeping their geometry constant around the main chain in the proteins. The CD spectrum of aromatic moieties provides a good measure for the analysis of motional freedom around a given amino acid residue. Better examples have been reported on several kinds of amino acids and their polymers. 

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