Biomedical subjects

Ching Yeh Lin

Publications and source records attributed to Ching Yeh Lin.

3 recordsLinked to original sources

PubMed2006

Significant differences in the electrochemical behavior of the alpha-, beta-, gamma-, and delta-tocopherols (vitamin E).

Alpha-, beta-, gamma-, and delta-tocopherols can be oxidized in dry CH2Cl2 or CH3CN by one electron to form cation radicals that deprotonate to form the neutral phenoxyl radicals, which are then immediately further oxidized by one electron to the phenoxonium cations (an ECE electrochemical mechanism, where E signifies an electron transfer and C represents a chemical step, with the electrochemical mechanism having been determined by in situ spectroscopic analysis). The principal difference in the electrochemical behavior of the tocopherols relates to the stability of their associated phenoxonium cations. The phenoxonium cation of alpha-tocopherol is stable in solution for at least several hours, the phenoxonium cation of beta-tocopherol is stable for several minutes, and the phenoxonium cations of gamma- and delta-tocopherol are stable for <1 s. In dry CH2Cl2 containing >0.75 M acid (CF3COOH), the deprotonation reaction of the cation radicals can be completely inhibited resulting in the cyclic voltammetric behavior of the tocopherols appearing as chemically reversible one-electron oxidation processes (an E mechanism). In dry acid conditions, the cation radicals can be further oxidized by one electron to form the dications, which are unstable and immediately deprotonate. The high stability of the phenoxonium cation of alpha-tocopherol compared to the other tocopherols (and most other phenols) is a chemically important feature that may shed new light on understanding alpha-tocopherol's unique biological properties.

Electrochemistry↗

PubMed2006

Advances in methods and algorithms in a modern quantum chemistry program package.

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

Algorithms↗

PubMed2005

Transformation of alpha-tocopherol (vitamin E) and related chromanol model compounds into their phenoxonium ions by chemical oxidation with the nitrosonium cation.

[reaction: see text] Alpha-tocopherol (alpha-TOH), the main oil component making up vitamin E, and its nonnatural solid 6-hydroxy-2,2,5,7,8-pentamethylchroman and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid structurally related analogues were oxidized quantitatively with 2 mol equiv of NO+ SbF6(-) in CH3CN at 233 K to form phenoxonium cations (alpha-TO+ SbF6(-)) in a chemically reversible two-electron/one-proton process. Solution-phase infrared spectroscopy, 1H and 13C NMR spectroscopy, and corresponding theoretical calculations of the spectroscopic data using density-based and wave-function-based models support the identity of the remarkably stable phenoxonium cations. The presence of an oxygen atom in the para position to the hydroxyl group and the chromanol ring structure appear to be important factors in stabilization of the phenoxonium ions, which raises the interesting possibility that the cations play a crucial role in the mode of action of vitamin E in biological systems. Although the phenoxonium cations are reactive toward nucleophiles such as water, they may be moderately stable in the hydrophobic (lipophilic) environment where vitamin E is known to occur naturally.

Antioxidants↗