Hydroorganic mixtures

As previously observed, for most free amino acids and small peptides unbuffered hydroorganic mixtures are enough to yield good enantioseparations however, for some bifunctional amino acids and most other compounds, an aqueous buffer is usually necessary to enhance resolution. TEAA and ammonium nitrate are the most effective buffer systems, while sodium citrate was also effective for the separation of 2-arylpropionic acids (profens) on vancomycin CSPs [78], and ammonium acetate is the most widely used and appropriate in view of LC-MS applications (see Section 2.3.1.4). Small changes in ammonium acetate concentration of MeOH-water (90 10) mobile phases scarcely affected retention and— to a lesser extent—enantioselectivity of carnitine derivatives [61]. 

Fig. 1. Density (p) of some hydroorganic mixtures as a function of temperature. OlOII, glycerol DMSO, dimethyl sulfoxide EGOH, ethylene glycol PrOH, propylene glycol MPD, 2-methyl-2,4-pentanediol MeOH, methanol. From Travers et al. (1975). Reprinted with permission of Biochimie. Copyright by the Societe de Chimie Bio-logique.

The values of paH as a function of temperature for various buffers in different hydroorganic mixtures have been measured by the indicator method (Hui Bon Hoa and Douzou, 1973a Maurel et al., 1975 Travers et al., 1975). Results are given in Tables XIV-XXII and Figs. 9-15. 

When prepared from the substrate-free ferric cytochrome, the oxy-ferrous compound Fe2+—02 decays much faster than the camphor-bound compound. The activation energy of the autoxidation process is rather small 14.5 kcal mob1 for Fe2+—02 in the hydroorganic mixture, and 19 kcal mol-1 for Fes2+—02 in the same solvent) see Fig. 37. 

Compact conformation Non-electrostatic interactions Hydrophobic interactions Conformational transition Extended form Free energy change Tetraalkylammonium counter-ions Hydroorganic mixture Urea solution... 

The electrochemical reduction of pyraz- Similar behaviors were observed for ines in an alkaline hydroorganic medium quinoxaline [246, 247] and pyridopyrazine gives the corresponding 1,4-dihydropyraz- derivatives [248, 249], ines that are easily reoxidizable. Generally, Triazines are reducible in two 2-e steps l,4-dihydropyrazinesisomerizeintol,2-or in slightly acidic media [250], The first 1,6-dihydropyrazines (Scheme 135) [245]. reduction leads to a mixture of two... 

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