Відмінності між версіями «Drogen bonds are far better H?donors than analogous species devoid of intramolecular»

Поточна версія на 07:02, 15 березня 2018

five.2.6 Ascorbate--Ascorbic acid (Vitamin C) is often a ubiquitous biological cofactor that is definitely required for human well being.175 purchase (R)-K-13675 Ascorbate has traditionally been believed of as a (R)-K-13675 biological activity oneelectron reductant, but redox 1.64028E+14 reactions of ascorbate almost generally involve the loss of an electron as well as a proton (or perhaps a hydrogen atom), so it's truly a PCET reagent. Hence, the gas phase O BDE in methanol (96.4 kcal mol-1)188 is ca. eight kcal mol-1 stronger that the analogous BDE in phenol (88 kcal mol-1, see above). The alcohol O bond is normally stronger than the C bonds in the exact same molecule. Again working with methanol as an instance, the O BDE is greater than eight kcal mol-1 stronger than the C BDFEg for H-CH2OH, 87.9 kcal mol-1.37 For thisNIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptChem Rev. Author manuscript; readily available in PMC 2011 December 8.Warren et al.Pagereason, hydrogen atom abstractors react with alcohols to give a hydroxyalkyl radical including 1568539X-00003152 H2OH, rather than the alkoxyl radical (CH3O?.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript5.three.1 tert-Butanol and tert-Butoxyl Radical--The tert-butoxyl radical (tBuO? has received think about.Drogen bonds are far better H?donors than analogous species without having intramolecular hydrogen bonding. This can be opposite for the thermochemistry in water exactly where BDFE(catechol) > BDFE(hydroquinone). 5.two.six Ascorbate--Ascorbic acid (Vitamin C) is actually a ubiquitous biological cofactor that is definitely needed for human well being.175 Ascorbate has traditionally been thought of as a oneelectron reductant, but redox 1.64028E+14 reactions of ascorbate nearly usually involve the loss of an electron and a proton (or perhaps a hydrogen atom), so it is actually seriously a PCET reagent. Njus176 and Tsubaki177 have shown that ascorbate donates hydrogen atoms in its reactions with cytochrome b561. Njus has also demonstrated this for other ascorbate using enzyme systems.178 Ascorbate can also be probably oxidized by loss of H+ + e- inside the catalytic cycle of ascorbate peroxidase (APX).179 HAT from ascorbate may play a role in regeneration of vitamin E (tocopherol) radicals.135,180 Investigations from our group have shown that 5,6isoproylidene ascorbate, a convenient, commercially offered organic-soluble analog of ascorbate, reacts with TEMPO, tBu3PhO?and iron-porphyrin models by means of concerted transfer of H?181,182 The aqueous thermochemistry of ascorbate is effectively understood (Figure 4).135,183,184 In principle, a nine-membered square could be constructed for ascorbic acid because two electrons and two protons could be removed to make dehydroascorbate. However, comparable to hydroquinones, the oxidized types which have not lost a proton are high-energy species (extremely acidic) and are usually not relevant to ascorbate chemistry. Ascorbic acid becomes a stronger lowering agent at higher pH since it is converted to ascorbate (AscH-) after which the doubly deprotonated form (Asc2-).184,185 At physiological pH, AscH- is definitely the predominant species and the ascorbyl radical (Asc?) is deprotonated (the pKa of AscH? is -0.45). Hence, by far the most crucial reaction is AscH- Asc? + H+ + e-.