Tyrosine ultraviolet spectrum

The aromatic rings in the protein absorb ultraviolet light at an absorbance maximum of 280 nm, whereas the peptide bonds absorb at around 205 nm. The unique absorbance property of proteins could be used to estimate the level of proteins. These methods are fairly accurate with the ranges from 20 p,g to 3 mg for absorbance at 280 nm, as compared with 1 to 100 p,g for 205 nm. The assay is non-destructive as the protein in most cases is not consumed and can be recovered. Secondary, tertiary and quaternary structures all affect absorbance therefore, factors such as pH, ionic strength, etc can alter the absorbance spectrum. This assay depends on the presence of a mino acids which absorb UV light (mainly tryptophan, but to a lesser extent also tyrosine). Small peptides that do not contain such a mino acids cannot be measured easily by UV. 

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-... 

A solvent which has been foimd to be of great interest in connection with protein conformation studies is ethylene glycol. Sage and Singer (1958, 1962) have investigated in some detail the properties of RNase in pure ethylene glycol, containing added neutral electrolyte. They examined the ultraviolet absorption spectrum, the ionization behavior of the tyrosine residues by spectrophotometric titration experiments, and the optical rotatory dispersion of the system. 

Electronic (ultraviolet) absorption spectrum of L-tyrosine, at pH 12.55 the logarithm of the absorbance, i , is plotted against wavelength. 

Apart from this the interest and application of ultraviolet spectra of proteins are analytical. On a microscale the absorption spectrum may be the simplest and best evidence for the recognition of a protein. It is possible that, with care, it will be the best means of obtaining an estimate of tyrosine and tryptophan in a protein. The instability of tryptophan under the conditions required for protein hydrolysis gives weight in favor of a method such as the spectrophotometric which allows a direct determination of tryptophan to be made (on a protein) without hydrolysis. 

Fig. 1.5. Ultraviolet absorption spectrum of tyrosine as affected by pH. (according to Luebke, Schroeder and Kloss, 1975) — 0.1 mol/1 HCl, — 0.1 mol/1 NaOH...

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. 

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