Azines are less than the BDFEs of arylamines, presumably because of stabilization of the radical by the delocalized system. 5.6 Tryptophan, Flavins and Nucleosides The nitrogen containing heterocycles tryptophan, flavin and the nucleotide guanine are important in biological redox chemistry. Tryptophan is thought to be important in longrange electron transfer in proteins123,285 and its oxidation products are often observed in oxidatively stressed proteins.286 Guanine is the most easily oxidized nucleoside and is therefore implicated in the much-studied long-range hole transfer through DNA. Guanine oxidation is also thought to be important in DNA damage/repair.287 Flavins are critical biological cofactors that mediate charge transfer in a variety of proteins.288,289 Although these cofactors are widely discussed in terms of electron transfer, their pH dependent redox potentials indicate that they should be viewed as PCET reagents, at least in certain circumstances. 5.6.1 Indole and Tryptophan–The biological importance of electron transfer reactions of tryptophan has prompted thorough studies of its solution thermochemistry (Table 14). Mer yi and co-workers have reported aqueous redox potentials and pKa values for a series of indoles,290 although their measurement of E?TrpH?/0) is different from the value reported by both Harriman128 and from DeFilippis.131 (Table 14 does not give the pKas for the amine or the carboxylate moieties of tryptophan.) Indoles and tryptophan are more acidic than alkylamines and anilines, but are still less acidic than phenols [in DMSO, pKa(indole) = 20.9291 while pKa(phenol) = 18.0116 (see Tables 4 and 14 for more extensive data)]. The more striking difference between indole and phenol is the acidity of the radical cation: PhOH? is a very strong acid (aqueous pKa = -2115) while indole? is a weak acid (aqueous pKa = 4.9290). Thus oxidations of indoles and tryptophan often form the radical order Tariquidar cation (like the amines discussed above), while oxidations of phenols typically form the neutral Z-DEVD-FMK structure phenoxyl radical. This comparison of indole and phenol is particularly interesting because tryptophan and tyrosine are the most important redox-active amino acids, and their thermochemistry provesChem Rev. Author manuscript; available in PMC 2011 December 8.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptWarren et al.Pagethe framework for understanding their roles in biological catalysis and charge transfer. Tyrosine radical cations (TyrOH?) are too high in energy to be involved in a biological system, even in photosystem II that is said to contain the strongest oxidant in biology.292 Thus, in biological systems (and in the large majority of chemical systems as well) tyrosine and other phenols are oxidized to the neutral phenoxyl radical. However, TrpH? is a much more accessible species, being much less acidic than TyrOH? and having a reduction potential 0.25 V lower than that of TyrOH?. Therefore oxidations of tryptophan (and indoles) often involve the radical cation. In this way, indoles resemble the alkylamines and anilines discussed in Section 5.5.3. While tryptophan is easier to oxidize by outer-sphere electron transfer, tyrosine is easier to oxidize by PCET because its BDFE is about 3 kcal mol-1 weaker than the N BDFE in tryptophan. Again, given the critical importance of the proton in these chemical transformations, we strongly encourage those working on redoxactive amino acids to not just refer to.Azines are less than the BDFEs of arylamines, presumably because of stabilization of the radical by the delocalized system. 5.6 Tryptophan, Flavins and Nucleosides The nitrogen containing heterocycles tryptophan, flavin and the nucleotide guanine are important in biological redox chemistry. Tryptophan is thought to be important in longrange electron transfer in proteins123,285 and its oxidation products are often observed in oxidatively stressed proteins.286 Guanine is the most easily oxidized nucleoside and is therefore implicated in the much-studied long-range hole transfer through DNA. Guanine oxidation is also thought to be important in DNA damage/repair.287 Flavins are critical biological cofactors that mediate charge transfer in a variety of proteins.288,289 Although these cofactors are widely discussed in terms of electron transfer, their pH dependent redox potentials indicate that they should be viewed as PCET reagents, at least in certain circumstances. 5.6.1 Indole and Tryptophan–The biological importance of electron transfer reactions of tryptophan has prompted thorough studies of its solution thermochemistry (Table 14). Mer yi and co-workers have reported aqueous redox potentials and pKa values for a series of indoles,290 although their measurement of E?TrpH?/0) is different from the value reported by both Harriman128 and from DeFilippis.131 (Table 14 does not give the pKas for the amine or the carboxylate moieties of tryptophan.) Indoles and tryptophan are more acidic than alkylamines and anilines, but are still less acidic than phenols [in DMSO, pKa(indole) = 20.9291 while pKa(phenol) = 18.0116 (see Tables 4 and 14 for more extensive data)]. The more striking difference between indole and phenol is the acidity of the radical cation: PhOH? is a very strong acid (aqueous pKa = -2115) while indole? is a weak acid (aqueous pKa = 4.9290). Thus oxidations of indoles and tryptophan often form the radical cation (like the amines discussed above), while oxidations of phenols typically form the neutral phenoxyl radical. This comparison of indole and phenol is particularly interesting because tryptophan and tyrosine are the most important redox-active amino acids, and their thermochemistry provesChem Rev. Author manuscript; available in PMC 2011 December 8.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptWarren et al.Pagethe framework for understanding their roles in biological catalysis and charge transfer. Tyrosine radical cations (TyrOH?) are too high in energy to be involved in a biological system, even in photosystem II that is said to contain the strongest oxidant in biology.292 Thus, in biological systems (and in the large majority of chemical systems as well) tyrosine and other phenols are oxidized to the neutral phenoxyl radical. However, TrpH? is a much more accessible species, being much less acidic than TyrOH? and having a reduction potential 0.25 V lower than that of TyrOH?. Therefore oxidations of tryptophan (and indoles) often involve the radical cation. In this way, indoles resemble the alkylamines and anilines discussed in Section 5.5.3. While tryptophan is easier to oxidize by outer-sphere electron transfer, tyrosine is easier to oxidize by PCET because its BDFE is about 3 kcal mol-1 weaker than the N BDFE in tryptophan. Again, given the critical importance of the proton in these chemical transformations, we strongly encourage those working on redoxactive amino acids to not just refer to.
