Amino acids radical electron transfer

Electron transfer from the amine to flavin would result in the aminium radical which is expected to rearrange rapidly to radical 61. Inactivation of the enzyme would then occur via coupling of the radical with the flavin radical anion resulting in the formation of 66. Coupling of the aminium radical with an amino acid radical would result in the formation of 65. By use of radioactive labeling techniques Silverman et al. have confirmed the formation of 65 and 66 this confirms the role of electron transfer in the oxidation process. Similar studies have been performed using 1-phenylcyclobutylamine (Scheme 18) [198]. 

Type A PCET reactions describe amino acid radical generation steps in many enzymes, since the electron and proton transfer from the same site as a hydrogen atom [188]. Similarly, substrate activation at C-H bonds typically occurs via a Type A configuration at oxidized cofactors such as those in lipoxygenase [47, 48] galactose oxidase [189-191] and ribonucleotide reductase (Y oxidation at the di-iron cofactor, vide infra) [192]. Here, the HATs are more akin to the transition metal mediated reactions of Section 17.3.1 since the final site of the electron and proton are on site differentiated at Ae (redox cofactor) and Ap (a ligand). 

Many enzymes use redox centers to store and transfer electrons during catalysis. These redox centers can be composed of metals such as iron or cobalt, or organic cofactors such as quinones, amino acid radicals, or flavins. In order to fully appreciate the catalytic mechanisms of these enzymes, it is often necessary to determine the free energy required to reduce or oxidize their protein redox centers. This is called the redox potential. The measurement of enzyme redox potentials can be performed by either direct or indirect electrochemical methods. The type of electrochemistry suitable for a particular protein system is simply dictated by the accessibility of its redox center to the electrode surface. Because most reactions catalyzed by enzymes occur within hydrophobic pockets of the protein, the redox sites are often far from the surface of the protein. Unless an electron transfer path exists from the protein surface to the redox center, it is not feasible to use direct electrochemistry to measure the redox potential. Since only a few enzymes (most notably certain heme-containing enzymes) have such electron transferring paths and... 

Another interesting mechanism of the repair of amino acid residues of LDL apolipoprotein B100 by flavonoids has been recently described [75]. The authors of this work suggested that LDL-bound quercetin (but not rutin) repaired the tyrosine free radical by intramolecular electron transfer. 

The oxidation of peroxidases by hydroperoxide leads to a ferryl iron-oxo species as well as a radical cation on the porphyrin ring, which is sometimes transferred to an adjacent amino acid. This species is referred to as compound I. Compound I can oxidize substrates directly by a two-electron process to regenerate the native peroxidase, but, more commonly, it oxidizes substrates by an one-electron process to form compound II where the porphyrin radical cation has been reduced. Compound II, in turn, can perform a second one-electron... 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

Alanine dosimeters are based on the ability of 1-a alanine (a crystalline amino acid) to form a very stable free radical when subjected to ionizing radiation. The alanine free radical yields an electron paramagnetic resonance (EPR) signal that is dose dependent, yet independent of the dose rate, energy type, and relatively insensitive to temperature and humidity. Alanine dosimeters are available in the form of pellets or films and can be used for doses ranging from 10 Gy to 200 kGy. A reference calibration service using the alanine EPR system was developed and the scans were sent to the service center by mail. Currently the available system allows transferring the EPR scan to a NIST server for a calibration certificate. This way the procedure has been shortened from days to hours. ... 

The COs radical anion is a strong one-electron oxidant ( 7-1.7 V vs NHE [15]) that oxidizes appropriate electron donors via electron transfer mechanisms [103]. Detailed pulse radiolysis studies have shown that carbonate radicals can rapidly abstract electrons from aromatic amino acids (tyrosine and tryptophan). However, reactions of CO3 with S-containing methionine and cysteine are less efficient [104-106]. Hydrogen atom abstraction by carbonate radicals is generally very slow [103] and their reactivities with other amino acids are negligible [104-106]. 

Diagram 1. Proposed mechanism ferric Hbl after laser exposure, reduction process could occur by an electron transfer from the orbital a2ub2u to the eg(n ) [11]. A n radical cation is formed in the porphyrin after an electron pass from eg(rt ) to the metal d orbitals. It cannot be excluded the possibility of that some amino acid might take place in the process, Sample photoinduced reduction is a step involved in ferric Hbl processes after photoexcitation, which could or could not led to heme-ligand bond breakage. 

A long-known characteristic of D-amino acid oxidase is its tendency to form charge-transfer complexes with amines, complexes in which a nonbonding electron has been transferred partially to the flavin. Complete electron transfer would yield a flavin radical and a substrate radical which could be intermediates in a free radical mechanism, as discussed in the next section.256... 

An alternative route for the synthesis of TV-methyl amino acids without racemization is shown in Scheme 8.[98 This method includes the use of TBPB in the presence of copper(I) octanoate. The proposed mechanism of this free radical reaction is given in Scheme 8. Electron transfer from copper(I) to TBPB affords the copper(II), benzoate, and tBuO radical 4, which undergoes (3-scission to acetone and methyl radical 5. In turn, electron transfer from the urethane to the copper(II) ion, followed by proton transfer, affords the corresponding urethane radical 6, which reacts with the methyl radical 5 to give the desired product in overall yields of 54% (Z derivative) or 57% (Boc derivative), respectively. 

Amino acid side-chains may have a role in electron transfer in proteins through the well-known hopping pathway . In this process electrons could move between certain residues such as tyrosine and tryptophan, with the generation of free radical intermediates. Such free radical residues are known in certain high oxidation state species of hemoproteins. 

Fujita S, Steenken S (1981) Pattern ofOFI radical addition to uracil and methyl-and carboxyl-substituted uracils. Electron transfer ofOFI adducts with N,N, Ar, Ar -tetramethyl-p-phenylenediamine and tetranitromethane. J Am Chem Soc 103 2540-2545 Fujita S, Nagata Y, Dohmaru T (1988) Radicals produced by the reactions of SO4 with uridine and its derivatives. Studies by pulse radiolysis and y-radiolysis. Int J Radiat Biol 54 417-427 Fujita S, Horii FI,Taniguchi R, Lakshmi S, Renganathan R (1996) Pulse radiolytic investigations on the reaction of the 6-yl radicals of the uracils with Cu(ll)-amino acid complexes. Radiat Phys Chem 48 643-649... 

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