Tyrosine, optical absorption

Primary photochemical events in two site-directed mutants YF(M208) and YL(M208) of RC from Blastochloris viridis, in which tyrosine at position M208 is replaced by phenylalanine and leucine, respectively, were investigated with the use of 1H-ENDOR as well as optical absorption spectroscopy (Mue et al., 2000). The residue at M208 is in close proximity to the primary electron donor, P, the (BChl), and the BPh. Analysis of the experimental data revealed two torsional isomers of the 3-acetyl group of... 

Fig. 16. Spectroscopic characterization of the oxidized apogalactose oxidase free radical, (a) Optical absorption spectrum for the radical-containing apoprotein, (b) X-band EPR spectrum of the metal-free protein following Ir(IV) oxidation, (c) Expansion of the region near g = 2 comparing experimental data (Exp) with a theoretical simulation (Sim) based on coupling of the unpaired electron spin with one and one Hp proton of a tyrosine phenoxyl. Simulation parameters g = 2.0017, g2 = 2.0073 Ai Ha) = 8.4 G, A2(Hc,) = 8.8 G di(Hp) = 12.7 G, A2(Hp) = 13.8 G.

The following amino acids have been definitely excluded as part of a common structure of the Type 1 center Tryptophan has been eliminated as a ligand for the reasons given in Section IIAl. Arginine is absent in the plastocyanins. Tyrosine has been eliminated by optical absorption studies of azurin and by a recent analysis of the resonance enhanced Raman spectrum of stellacyanin 206). 

Intermediates occurring in these mechanisms have been identified by ESR measurements and by flash photolysis studies using optical absorption detection. For example, ESR measurements on wool keratins revealed the formation of sulfur-centered radicals of the structure RCH2S, which, in this case, are assumed to result from a reaction of electronically excited tyrosine moieties with cystine residues [11]. In many proteins, cross-links are formed. In the case of keratin and collagen, the cross-links are of the tryptophan-histidine and dityrosine types [11]. Cross-links formed by the combination of R-S or R-S-S radicals, both intermolecularly and intramolecularly, with incorrect sites are considered to be an important source of photoaggregation effects [8]. ESR measurements have also yielded evidence of C-H and C-N bond ruptures [8]. 

However, it is possible to detect a tyrosine radical optically in ribonucleotide reductase, as there is only a relatively weak competing absorption from the binuclear non-haem iron centre [164]. A distinct sharp peak is seen that is not present in proteins that have been treated with the radical scavenger hydroxyurea [165,166] nor is it present in proteins such as haemerythrin or methane monooxygenase, which have similar active-site structures, but lack... 

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. 

The potential advantages of selective nitration of tyrosyl residues in native proteins are numerous. The reaction is performed under mild conditions, giving rise to a 3-nitrotyrosyl derivative (pK 7), which in the acid form absorbs intensely at 350 nm. Hence, the nitrotyrosine content may be readily determined spectrophotometrically, as well as by amino acid analysis ( 2.2.3). The absorption spectrum of 3-nitro-tyrosine is highly sensitive to solvent polarity and exhibits significant optical activity in the long wavelength absorption band. Consequently, nitrotyrosyl residues can be utilized as indicators of conformational change, or of interactions of proteins with other macromolecules or small molecules (e.g. Kirschner and Schachman 1973). Any perturbation in the pK of nitrotyrosyl residues is readily determined spectrophotometrically. 

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