Activation and transfer of oxygen—IX : Nonenzymic hydroxylation of phenylalanine by model systems of dihydroalloxazine/O2 dihydroalloxazine/H2O2 and alloxazinium cation/H2O

Efficient hydroxylations were effected without addition of metal compounds. In the dihydroalloxazine system HO· radicals were the hydroxylating species according to the stoichiometry and the distribution of the hydroxyphenylalanine isomers. The OH radicals were generated in one-electron reductions of A^R-OOH or H₂O₂, in which a dihydroalloxazine or a semiquinone acted as the reducing agent. The yield of hydroxylation varied in dependence on the further oxidation of the hydroxycyclohexadienyl radicals. Quantitative disproportionation occurred in 6N H₂SO₄, while an attack by O₂, H₂O₂ or A^R-OOH predominated in the pH region 0–7. The influence of a HO- consuming aliphatic compound e.g. EDTA was studied.Hydroxylating species are also formed in the attack of an alloxazinium cation by H₂O₂, depending on the acidity of the medium.     

Computational Analysis of the Oxygen Addition at the C4a Site of Reduced Flavin in the Bacterial Luciferase Bioluminescence Reaction

The energetic characteristics of selected reaction steps in the bacterial luciferase-catalyzed luminescence reaction were examined by computation using the MNDO-PM3 method. Specifically, a three-step model was proposed to account for the reaction between oxygen and reduced riboflavin 5'-phosphate (1,5H2-FMN) to generate first the 5-hydroFMN-4a-peroxide (5H-FMN-4aOO-) and then the 5-hydro-4a-hydroperoxyFMN (5H-FMN-4aOOH) intermediates. Lysine (Lys-H+) and aspartate (Asp-) were chosen as representative catalytic residues involved in the protonation and deprotonation processes. Results show that deprotonation at the N1 site of 1,5H2-FMN by a basic amino acid residue at the luciferase active site would efficiently accelerate the reaction rate of O2 addition to form 5H-FMN-4aOO-. The most favored site of oxygen attack is at the flavin C4a. With the aid of a catalytic acid group, the 5H-FMN-4aOO- so formed tends to undergo a spontaneous protonation reaction to yield the 5H-FMN-4aOOH.     

Synthesis of functionally substituted cyanoacetates

Some new cyanoacetates were synthesized and characterized. They are precursors for -cyanoacrylates used as rapidly polymerized, cold-hardening adhesives.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 523–525, March, 1993.     

Activation and transfer of oxygen—XI : Autoxidative behaviour of some N1,3,5-methylated tetrahydrolumazines and dihydroalloxazines

The autoxidation of N_1,3,5-methylated tetrahydrolumazines and dihydroalloxazines and the covalent hydration of the oxidized (6,7-dihydrolumazinium and alloxazinium) species are described. Autoxidation led to ring contractions in which either the original C₄^a- or C₈^a_(10^a)-atom became the spiro center. In covalent hydration of the N_1,3,5-methylated cationic species a bridge position was not appreciably attacked, in contrast with the N_1,3,8(10)-methylated series. Instead, the N₅-substituent was preferably attacked by HO^? to give N₅-hydroxymethyl transients decomposing into formaldehyde and N₅-dealkylated tetrahydrolumazines or dihydroalloxazines, respectively. 4^a-Hydroxy derivatives were neither found as final products in the autoxidation nor in the covalent hydration.The two types of ringcontraction suggest that in some peroxy intermediates both C_4^a and C_8^a(10^a) may be linked to oxygen. The ringcontractions and N₅-demethylation are conversions on the level of the pseudobase. They must be distinguished from two other new intramolecular nucleophilic rearrangements on peroxide level leading respectively to: (1) the conversion of the N₅-methyl into a N₅-formyl group, indicating that the peroxy group may migrate to the N₅-carbon; (2) a cleavage of the C₄C_4^a and N₃C₄ bonds with the formation of CO₂ and a 2-oxo-3-ureido-tetrahydropyrazine derivative showing that the peroxy group may also be linked to C₄ to give other types of endoperoxides or cyclic peresters (peroxylactones).Nonenzymic hydroxylation of phenylalanine by the use of 1,3,5,7,8-pentamethyl-5,10-dihydroalloxazine was studied and the results compared with those obtained from a N_1,3,10-methylated derivative.