Coupling constants thiols

A comparison of 13C and JH spin-coupling constants for pyrazine-2( l//)-thione establishes the thione tautomer as the dominating structure. A small amount (ca. 5%) of the thiol tautomer is also detected in DMSO-J5. In CD3N02 solution, the tautomeric mixture contains about 3% of the thiol form (82TL4785). The relative energies of tautomers of 2-mercaptopyrazine were calculated by the AMI method. The predominance of the thiol tautomer is predicted for the gas phase however, the significant contribution of the thione tautomer in polar solvents is indicated (92MI7). 

Tautomerism in hydroxy-, thiol- and amino-substituted pyrazines has been verified by C, H spin-spin coupling constants, and the results are supported by N NMR data <82TL4785>. 

Chemical Shifts and Coupling Constants of Thiols (8 in ppm relative to TMS, J in Hz)... 

FIGURE 1.24. Potential-dependent forward and backward rate constants of the ferrocene-ferrocenium couple attached to a gold electrode hy a long-chain alkane thiol assembled together with unsubstituted alkane thiols of similar length. Solid line use of Equations (1.37) to (1.39) with X, = 0.85 eV, ks — 1.25 s 1. Adapted from Figure 4A in reference 65, with permission from the American Association for the Advancement of Science. 

The lability of the thiol bonds, and the energy dissipation processes resulting from their motion, will have very short time constants, and would likely be stochastic in nature. Even at steady-state, bond fluctuations may have an effect. For this reason, we now turn our attention to stochastic coupling. 

Orthophthalaldehyde (OPA) in combination with a thiol is the reagent of choice for derivatization, despite its inability to react with proline, hydroxyproline, and the sulfur-containing amino acids. Another drawback of the reagent is the instability of the reaction products, making an automated derivatization system coupled to an automated injector, and constant retention times an absolute necessity. Taking into account these considerations, the HPLC analysis will be of use to every biochemical genetics laboratory for biological fluids other than urine. The system has also a... 

Similarly, David et al carried out the thiol-ene radical coupling of vinylphosphonic acid in the presence of sulfanylacetic acid in water at 70 °C (Scheme 3.4). They showed that a low amount of the thiol compound was required, according to its very high chain transfer constant, and this reaction returned good yields. 

Ring synthesis by radical-radical coupling, as well as intramolecular radical substitution, is an alternative method for producing rings, which does not rely on the addition of a radical to an unsaturated bond. Cyclopropanes have been prepared from diiodides by the reaction of a catalytic amount of a fluorinated tin hydride with NaCNBHs in a fluorinated-organie solvent mixture. The rate constant for the reaction was determined using PhsSiH as a chain carrier and found to be 5 x 10 at 80 °C (Scheme 18). The cyclization of 2,2-diethylpropane-1,3-thiol and 4,4-diethyl-1,2-dithiolane in the presence of 2,5-dimethylhexa-2,4-diene and AIBN has been shown to proceed via a radical-chain mechanism (Scheme 19). ... 

Misleading conclusions can be drawn if only the reduction potentials of, for example, the GS, HVGSH and PhO, H /PhOH couples are considered and the law of mass action is ignored. Rate constants for repair of phenoxyl radicals by GSH, producing thiyl radicals, of < 10 dm moP s have been estimated in the cases of 1-naphthol [67] or acetaminophen [68]. The latter phenol was oxidized by thiyl radicals (cysteine) with 5 ca. 7x10 dm moP s at pH 7, an estimate of the reduction potential of the acetaminophen phenoxyl radical, (PhO, H /PhOH) = 0.71 V confirming that reaction (5), as an equilibrium, is well over to the left [69]. In spite of this, thiyl radicals are undoubtedly formed during peroxidase catalysed oxidation of acetaminophen in the presence of thiols [64,70]. Similar results were obtained with 4-ethoxyaniline (p-pheneti-dine) [71]. 

---