Absorption, tyrosine

Procedure. The method can be tested using the matrix of concentrations, in micromoles per liter (pmol L ), of tryptophan and tyrosine at 280 nrrr suitably rrrodified to take into account constant absorption at 280 nrrr of some absorber that is neither tryptophan nor tyrosine... 

Absorption of proteins in the 230-300 nm range is determined by the aromatic side chains of tyrosine (Xmax = 274 am), tryptophan (Xmax = 280 nm), and phenylalanine (Xmax = 257 nm). Because the difference in the absorption spectra of native and unfolded protein molecules is generally small, difference spectra can... 

Tyrosine contains a phenolic side chain with a pKa of about 9.7-10.1. Due to its aromatic character, tyrosine is second only to tryptophan in contributing to a protein s overall absorptivity at 275-280nm. Although the amino acid is only sparingly soluble in water, the ionizable nature of the phenolic group makes it often appear in hydrophilic regions of a protein—usually... 

Protein peroxidation Modified tyrosines GC/MS, HPLC, immunoassays Protein carbonyls Atomic absorption spectroscopy, fluorescence spectroscopy, HPLC... 

L-Amino acid oxidase has been used to measure L-phenylalanine and involves the addition of a sodium arsenate-borate buffer, which promotes the conversion of the oxidation product, phenylpyruvic acid, to its enol form, which then forms a borate complex having an absorption maximum at 308 nm. Tyrosine and tryptophan react similarly but their enol-borate complexes have different absorption maxima at 330 and 350 nm respectively. Thus by taking absorbance readings at these wavelengths the specificity of the assay is improved. The assay for L-alanine may also be made almost completely specific by converting the L-pyruvate formed in the oxidation reaction to L-lactate by the addition of lactate dehydrogenase (EC 1.1.1.27) and monitoring the oxidation of NADH at 340 nm. 

The aromatic amino acids each have two major absorption bands in the wavelength region between 200 and 300 nm (see reviews by Beaven and Holiday(13) and Wetlaufer(14). The lower energy band occurs near 280 nm for tryptophan, 277 nm for tyrosine, and 258 nm for phenylalanine, and the extinction coefficients at these wavelengths are in the ratio 27 7 l.(14) As a result of the spectral distributions and relative extinction coefficients of the aromatic amino acids, tryptophan generally dominates the absorption, fluorescence, and phosphorescence spectra of proteins that also contain either of the other two aromatic amino acids. 

Ionization of the phenol hydroxyl group in tyrosine shifts the 277-nm absorption peak to 294 nm and the 223-nm peak to 240 nm. The molar extinction coefficient for the peak of the lower energy band increases from about 1350 M l cm-1 to about 2350 M em-1 and for the higher energy band from about 8200 M cm-1 to about. 1,000 M l cm-1.113 141 In addition, the lower energy absorption band of tyrosine shows vibrational structure that is lost upon ionization of the phenol side chain. 

Eisinger(55) also noted that it is difficult to obtain accurate data with phenylalanine as the donor and either tryptophan or tyrosine as the acceptor. The source of this problem is the weak S, - S0 absorption of phenylalanine compared to that of tyrosine or tryptophan, which leads to considerable experimental uncertainty in measuring the sensitized acceptor emission. This error may account for the finding of Kupryszewska et al.<56> that the sensitization of the acceptor fluorescence was less than the quenching of the donor fluorescence in their study of phenylalanine-to-tyrosine energy transfer... 

In an investigation of the physical basis of the interaction of histones with DNA, De Petrocellis et al.< 95 > have examined the effect of phosphate ions on histone HI. Binding results have shown that there are high-affinity sites for phosphate ions. In addition, phosphate ions were found to perturb the absorption spectra of HI and quench tyrosine fluorescence. Binding of the phosphate group resulted in positive difference absorption bands near 275 and 293 nm, which are similar to those produced at acid and alkaline pH, respectively. 

Muller et al.understand better the role of tyrosine in the structure and biological function of MDH. Resolution of the protein absorption spectrum, using iV-acetylphenylalanine ethyl ester in dioxane and A-acetyltyrosine ethyl ester in dioxane or 0.1 M phosphate buffer to model the effect of the local environments of the chromophoric groups, indicated that both the pig and the... 

---