Tyrosine spectrophotometric estimation

For the reason discussed below, the spectrophotometric estimation of tryptophan and tyrosine in proteins is carried out in strongly alkaline solution, usually 0.1 V KOH. The effects of this medium on the absorption spectra of the aromatic amino acids and of cystine are therefore of interest. 

The Spectrophotometric Estimation of pK Values of Tyrosine Derivatives and Peptides... 

Apart from the intrinsic interest that the phenomenon of bound tyrosine residues has in relation to the stabilization of the configuration of native proteins, further work will be required to ascertain if the time effects in strongly alkaline solution are of common occurrence and of sufficient magnitude to invalidate the spectrophotometric estimation of tyrosine and tryptophan in native proteins. The results listed in Table VII suggest that under the conditions recommended for this estimation, the possible complications that might arise from the presence of bound tyrosine residues do not in fact result in serious error. 

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. 

There are several recorded determinations of the absorption curves of the aromatic amino-acids. Most of these were obtained with photographic methods of spectrophotometry which have been superceded by more accurate photoelectric methods. It will be shown that in the spectrophotometric analysis of tyrosine and tryptophan in proteins, the photometric error is magnified in the final estimate of tyrosine and tryptophan contents. This fact is inevitably bound up with the form of the equations of mixture analysis. It is therefore important that the absorption constants be measured as accurately as possible. 

Price, M.L. and Butler, L.G., Rapid visual estimation and spectrophotometric determination of tannin content of sorghum grain, J. Agric. Food Chem., 25, 1268, 1277, 1977. Folin, O. and Ciocalteu, V., On tyrosine and tryptophan determinations in proteins, J. Biol. Chem., 27, 627, 1927. 

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