Acid scavengers, analysis

Not every excess acidity mechanistic analysis has been an outstanding success. For instance, several enolization studies have used this technique. The enolization of acetophenone was one of the reactions originally studied by Zucker and Hammett 146 their sulfuric acid rate constant data, obtained by iodine scavenging (the reaction is zero-order in halogen), was used in an excess acidity analysis,242 together with additional results obtained for some substituted acetophenones (using bromine scavenging).243... 

BASIL CIS CV CVD DSSC ECALE EC-STM EDX, EDS, EDAX EIS EMF EQCM FAB MS FFG-NMR Biphasic Acid Scavenging Utilizing Ionic Liquids Copper-indium-selenide Cyclic Voltammetry Chemical Vapor Deposition Dye Sensitized Solar Cell Electrochemical Atomic Layer Epitaxy Electrochemical in situ scanning tunnelling microscopy Energy Dispersive X-ray analysis Electrochemical Impedance Spectroscopy Electromotive Force Electrochemical Quarz Crystal Microbalance Fast atom bombardment mass spectroscopy Fixed Field Gradient Nuclear Magnetic Resonance... 

Yields in the above reactions can often be improved by the addition of 1 mole of triphenylphosphine directly to the trifluoroacetic acid solution of the reactants immediately before final work-up. It would appear that the triphenylphosphine functions as a scavenger for TTFA released in the metal-metal exchange reaction, thus protecting the final phenol from further electrophilic thallation and/or oxidation. Validation of the metal-metal exchange mechanism was obtained indirectly by isolation and characterization of an ArTlX2/LTTFA complex directly from the reaction mixture. NMR analysis revealed that this complex still possessed an intact aryl-thallium bond, indicating that it was probably the precursor to the transmetallation products, an aryllead tristrifluoroacetate and TTFA. 

The monomeric peptides [Cys-I], [Cys,Cys(Acm-II], [Cys(Acm),Cys-III], and [Cys-IV] were synthesized by Fmoc chemistry on Rink-amide resin as 5-Mob derivatives and cleaved/deprotected with 1M TMSBr/thioanisole in TFA in the presence of m-cresol and 1,2-ethanedithiol as scavengers. Following gel filtration on Sephadex G-10 with 1M AcOH as solvent and HPLC purification the peptides were obtained in 30—40% yield. Each product was characterized by LSIMS, HPLC, and amino acid analysis. 

The electronic property of the amino acid on the C-terminus also has an effect on antioxidant activity (Li et al., 2011), that is, the larger the electronic property, the higher is the activity. The C-terminus is a polar position that is thus affected by its electrostatic potential, to some extent therefore, the amino acids Trp, Glu, Leu, lie, Met, Val, Tyr, etc. are suitable at the C-terminus. Some researchers have speculated that the identity of the amino acid on the C-terminus would play an important role in its activity. Suetsuna (2000) separated and identified a radical scavenging peptide, Tyr-Phe-Tyr-Pro-Glu-Leu, from casein hydrolysate, and it was confirmed that the Glu-Leu on the C-terminus mainly contributed to its antioxidant activity. Kim et al. (2009) speculated that the hydrophobic property of the amino acid on the C-terminus, for example, Val and Leu, had a distinct effect on the activity, as determined from the analysis of antioxidative peptides derived from venison hydrolysate. 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

Salts usually have a dual effect on cationic polymerization (cf., Section VII.E.3). Tetrabutylammonium chloride coordinates strongly with Lewis acids, e.g., with BCI3 or SnCl4 to form tetrabutylammonium tetrachlo-roborate and pentachlorostannate, respectively. In the latter case, an excess of salt may lead to the formation of hexachlorostannate dianions. Thus, salts scavenge Lewis acid and reduce the polymerization rate in a way similar to nucleophiles. Recent analysis of model reactions by 119Sn NMR spectroscopy demonstrate a transformation of SnCl4 in the presence of the added salt ... 

However, 0.5 g p-thiocresol and 0.5 mL m-cresol were used as scavengers as recommended in the literature for chloroacetylated peptide compounds (33). The crude peptide was purified by RP-HPLC and, after lyophilization, was characterized by analytical HPLC, mass spectrometry, and amino acid analysis. The pure product was obtained at 65% yield. 

A multisyringe spectroscopic flow injection analysis (FIA) system was proposed for evaluating the antioxidant capacity of solutions, as a function of the solvent and the pH. The method is based on UVD at 517 nm of the scavenging capacity of the test solution in the presence of the 2,2-diphenyl-1-picrylhydrazyl free radical (14) during 3 min after stopping the flow. The method was validated for ascorbic acid and a series of phenolic antioxidants. ... 

AAA showed 66 of 82 crude products to be compositionally correct (Trp was not quantitated). Only one product had a complete deletion, corresponding to a des(Val,Lys,Arg) peptide synthesized by Boc chemistry. Of the other 15 samples that were not compositionally correct, 13 showed at least a low Lys value, often accompanied by a low Ser, Thr, and/or Arg value. Based on sequence analysis and mass spectrometric results (see later discussion), neither the low Lys nor other low amino acid values could be demonstrated, indicating that AAA of crude peptides may sometimes suffer interference from scavengers used during peptide-resin cleavage. A similar effect was seen in the 1993 ABRF PS study (3). 

---