7 research outputs found
Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids
The
kinetics of the photooxidation of aromatic amino acids histidine (His),
tyrosine (Tyr), and tryptophan (Trp) by 3,3′,4,4′-benzophenonetetracarboxylic
acid (TCBP) has been investigated in aqueous solutions using time-resolved
laser flash photolysis and time-resolved chemically induced dynamic
nuclear polarization. The pH dependence of quenching rate constants
is measured within a large pH range. The chemical reactivities of
free His, Trp, and Tyr and of their acetylated derivatives, <i>N</i>-AcHis, <i>N</i>-AcTyr, and <i>N</i>-AcTrp, toward TCBP triplets are compared to reveal the influence
of amino group charge on the oxidation of aromatic amino acids. The
bimolecular rate constants of quenching reactions between the triplet-excited
TCBP in the fully deprotonated state and tryptophan, histidine, and
tyrosine with a positively charged amino group are <i>k</i><sub>q</sub> = 2.2 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (4.9 < pH < 9.4), <i>k</i><sub>q</sub> = 1.6 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (6.0 < pH < 9.2), and <i>k</i><sub>q</sub> = 1.5
× 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> (4.9
< pH < 9.0), respectively. Tryptophan, histidine, and tyrosine
with a neutral amino group quench the TCBP triplets with the corresponding
rate constants <i>k</i><sub>q</sub> = 8.0 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (pH > 9.4), <i>k</i><sub>q</sub> = 3.0 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (pH > 9.2), and <i>k</i><sub>q</sub> = (4.0–10.0) × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> (9.0 < pH < 10.1) that are close to
those for the N-acetylated derivatives. Thus, it has been established
that the presence of charged amino group changes oxidation rates by
a significant factor; i.e., His with a positively charged amino group
quenches the TCBP triplets 5 times more effectively than <i>N</i>-AcHis and His with a neutral amino group. The efficiency of quenching
reaction between the TCBP triplets and Tyr and Trp with a positively
charged amino group is about 3 times as high as that of both Tyr and
Trp with a neutral amino group, <i>N</i>-AcTyr and <i>N</i>-AcTrp
Oxidation of Purine Nucleotides by Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: pH-Dependence
Deprotonation of Transient Guanosyl Cation Radical Catalyzed by Buffer in Aqueous Solution: TR-CIDNP Study
Dihydropyrans by Cycloadditions of Oxadienes
International audience3,4‐Dihydro‐2H‐pyrans are present in the skeletons of several natural products, and these versatile synthetic intermediates are readily transformed into tetrahydropyrans, pyridines, or 1,5‐dicarbonyl units. Among the strategies developed to access 3,4‐dihydro‐2H‐pyrans, the hetero‐Diels‐Alder reaction between an oxadiene and a dienophile is particularly valuable because up to three contiguous stereogenic centers can be created diastereo‐ and/or enantioselectively in a single step. This review addresses the mechanism of the reaction and presents methods for preparing the product dihydropyrans enantio‐ and diastereoselectively. Thermal and Lewis acid promoted cycloadditions are discussed, as is the role of activating groups on the oxadiene