(and the BDFE) of tBu3PhOH.40 The EPR equilibration method provides a high degree of precision and the values are, in general, internally consistent.122 The values obtained agree very well with those from other methods, such as from E?and pKa measurements. For example, the adjusted Pedulli values for BDFE(PhOH) and BDFE(2,6-tBu2PhOH) in C6H6, = 83.8 and 78.3 kcal mol-1 (Table 4), agree very closely with our conversion of Bordwell’s BDFEs in DMSO (from E?and pKa values)116 to C6H6 using the Abraham method, 83.7 and 78.1 kcal mol-1, respectively. 5.2.3 Tyrosine–Redox reactions of the amino acid tyrosine are involved in biological energy transduction, charge transport, oxidative stress, and enzymatic catalysis.123 The 1H+/1e- oxidized form, the tyrosyl radical, has been implicated in a variety of enzymatic systems, including ribonucleotide reductases,109 photosystem II,106 galactose get ARA290 oxidase,124 prostaglandin-H-synthase125 and perhaps cytochrome c oxidase.126 Furthermore, tyrosineNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Pageoxidation products are thought to play deleterious roles in various disease states, including atherosclerosis and aging.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptThe HS-173MedChemExpress HS-173 proton-coupled redox chemistry of tyrosine (TyrOH) and related compounds has been widely reported.128?29130131 In aqueous solutions, the Pourbaix diagram shows a clear 59 mV per pH dependence for the oxidation of tyrosine below pH 10, indicative of a 1e-/ 1H+ redox couple. As for phenol, above pKa(tyrosine) the redox potential does not depend on pH because this is the proton-independent TyrO?TyrO- redox couple. Other, more detailed, discussions of aspects of proton-coupled redox chemistry of tyrosine can be found in other contributions to this issue. As an aside, we encourage biochemical studies of PCET to use a nomenclature that explicitly shows the proton, such as `TyrOH’ for tyrosine, to avoid ambiguity. For instance, the commonly used “Y? for tyrosyl radicals could refer either to neutral radical TyrO?or to the typically high-energy radical cation TyrOH?. 5.2.4 -Tocopherol and Related Phenols—Tocopherol (a main component of Vitamin E) is thought to be a key chain breaking antioxidant in biological systems. Since its discovery in 1922,132 vitamin E has received considerable attention from chemists, biologists, and clinicians, among others.110 Due to its insolubility in water, several small water soluble analogs such as Trolox C ((?-6-hydroxy-2,5,7,8-tetramethylchromane-2carboxylic acid) and HPMC (6-hydroxy-2,2-5,7,8-pentamethylchroman) have been developed (Scheme 8; see references 133 and 134). As shown in Table 4, these three phenols show similar thermochemistry in the same solvent. This is in good agreement with their solution kinetic behavior and indicates that the analogs lacking the greasy phytyl tails are good models for the redox chemistry of tocopherol. The BDFEs of these phenols are much lower than those of other phenols, by more than 10 kcal mol-1 vs. unsubstituted phenol and by 2 kcal mol-1 vs. tBu3PhOH in the same solvent. This relatively weak bond is the origin of the good biological reducing power of vitamin E. The weak bond is a result of the electron-donating substituents, which also reduces the acidity of these phenols. The combination of a weak O bond, low acidity, and a high outer-sphere redox.(and the BDFE) of tBu3PhOH.40 The EPR equilibration method provides a high degree of precision and the values are, in general, internally consistent.122 The values obtained agree very well with those from other methods, such as from E?and pKa measurements. For example, the adjusted Pedulli values for BDFE(PhOH) and BDFE(2,6-tBu2PhOH) in C6H6, = 83.8 and 78.3 kcal mol-1 (Table 4), agree very closely with our conversion of Bordwell’s BDFEs in DMSO (from E?and pKa values)116 to C6H6 using the Abraham method, 83.7 and 78.1 kcal mol-1, respectively. 5.2.3 Tyrosine–Redox reactions of the amino acid tyrosine are involved in biological energy transduction, charge transport, oxidative stress, and enzymatic catalysis.123 The 1H+/1e- oxidized form, the tyrosyl radical, has been implicated in a variety of enzymatic systems, including ribonucleotide reductases,109 photosystem II,106 galactose oxidase,124 prostaglandin-H-synthase125 and perhaps cytochrome c oxidase.126 Furthermore, tyrosineNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Pageoxidation products are thought to play deleterious roles in various disease states, including atherosclerosis and aging.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptThe proton-coupled redox chemistry of tyrosine (TyrOH) and related compounds has been widely reported.128?29130131 In aqueous solutions, the Pourbaix diagram shows a clear 59 mV per pH dependence for the oxidation of tyrosine below pH 10, indicative of a 1e-/ 1H+ redox couple. As for phenol, above pKa(tyrosine) the redox potential does not depend on pH because this is the proton-independent TyrO?TyrO- redox couple. Other, more detailed, discussions of aspects of proton-coupled redox chemistry of tyrosine can be found in other contributions to this issue. As an aside, we encourage biochemical studies of PCET to use a nomenclature that explicitly shows the proton, such as `TyrOH’ for tyrosine, to avoid ambiguity. For instance, the commonly used “Y? for tyrosyl radicals could refer either to neutral radical TyrO?or to the typically high-energy radical cation TyrOH?. 5.2.4 -Tocopherol and Related Phenols—Tocopherol (a main component of Vitamin E) is thought to be a key chain breaking antioxidant in biological systems. Since its discovery in 1922,132 vitamin E has received considerable attention from chemists, biologists, and clinicians, among others.110 Due to its insolubility in water, several small water soluble analogs such as Trolox C ((?-6-hydroxy-2,5,7,8-tetramethylchromane-2carboxylic acid) and HPMC (6-hydroxy-2,2-5,7,8-pentamethylchroman) have been developed (Scheme 8; see references 133 and 134). As shown in Table 4, these three phenols show similar thermochemistry in the same solvent. This is in good agreement with their solution kinetic behavior and indicates that the analogs lacking the greasy phytyl tails are good models for the redox chemistry of tocopherol. The BDFEs of these phenols are much lower than those of other phenols, by more than 10 kcal mol-1 vs. unsubstituted phenol and by 2 kcal mol-1 vs. tBu3PhOH in the same solvent. This relatively weak bond is the origin of the good biological reducing power of vitamin E. The weak bond is a result of the electron-donating substituents, which also reduces the acidity of these phenols. The combination of a weak O bond, low acidity, and a high outer-sphere redox.
