Gas chromatography of alpha-keto acids as their O-trimethyl silylquinoxalinol derivatives.

The O-trimethylsilyl (TMS) quinoxalinols are very useful derivatives for the gas chromatography of alpha-keto acids because of their high stability and the absence of stereoisomerism and because of the presence of specific, common and abundant fragments in electron impact mass spectra, which allows the low-level detection of whole groups of keto acids by single-ion detection. In this paper, the chromatographic properties of eleven O-TMS-quinoxalinols on OV-1, OV-17 and Dexsil 300 are reported in terms of methylene units. Also by use of methylene units, the chromatographic isotope effect is analyzed in detail for nine perdeutero-TMS derivatives. The effect is explained by the diminished interaction of the deuterated compounds with the unlabelled liquid phase.     

Analysis of amino acids as their tert.-butyldimethylsilyl derivatives by gas-liquid chromatography and mass spectrometry

A gas-liquid chromatographic procedure is described which will separate and quantitate the seventeen amino acids typically found in protein acid hydrolyzates. If present, tryptophan, cysteine and carboxymethylcysteine can also be simultaneously assayed via this method. The amino acids, as their tert.-butyldimethylsilylated derivatives, are readily separable on SE-30 (wall-coated open-tubular) and OV-17 (bonded) capillary columns, as well as on packed gas-liquid chromatographic columns coated with the same liquid phases. Retention times and responses for all amino acids and internal standards are given. Mass spectral analysis of all tert.-butyldimethylsilylated amino acids is presented and displays a characteristic and unique [M- 57] fragment ion for each amino acid which often dominates the mass spectrum. Application of this method is demonstrated using four well characterized proteins.     

Gas chromatography of dimethylalkylsilyl ether derivatives of bile acid methyl esters

The gas chromatographic separation of the DMAS ether derivatives of ten bile acid methyl esters, namely DMES, DMnPS and DMiPS ethers, have been studied by use of an open tubular glass capillary column, coated with SE-30. The DMAS ether derivatives were eluted in sequence according to the number of hydroxyl groups except for 12-KCDCA, and separated. This contrasts with the poorly resolved gas chromatographic peaks produced by the TMS ether derivatives. The DMES ether derivatives were resolved with baseline separation in 30 min, but those of HCA and 12-KCDCA had similar methylene unit values.     

Evaluation of the lipophilicity of bile acids and their derivatives by thin-layer chromatography and principal component analysis

The lipophilic character of bile acids and their glyco- and tauro-conjugates was studied. The classical R(Mo) values were measured by means of reversed thin-layer chromatography using a mixture of methanol-water as the solvent system and compared with the factors scores obtained by principal component analysis based also onto the TLC-retention data. The reliability of the factor scores values as lipophilic indices are shown by their high correlation with the classical R(Mo) values. In addition, a better correlation was observed between scores corresponding to the first principal components and the partition coefficients (log P) of bile acids. Finally, the "lipophilicity chart" described by the first two components has the effect of separating compounds from each other most effectively from the congeneric aspect point of view.     

Gas chromatography of fatty acids.

Lipids in foods contain a wide variety of fatty acids differing in chain length, degree of unsaturation, position and configuration of double bonds and the presence of special functional groups. Modern capillary gas chromatography offers excellent separation of fatty acids. Fused-silica capillary columns with stationary phases of medium polarity and non-polar methylsilicone stationary phases successfully separate most of the natural fatty acids. Special applications, such as the separation of complex cis-trans fatty acid mixtures and cyclic fatty acids, required particular chromatographic conditions, including the use of very long capillary columns or more polar stationary phases. The derivatization methods for the preparation of fatty acid esters also need to be optimized to obtain accurate quantitative results. This paper reviews the derivatization techniques, capillary columns and stationary phases commonly used in the gas chromatography of fatty acids in food.     

Gas chromatography of amino acids.

This review summarizes all papers that have appeared on the gas chromatography of amino acids (including the iodoamino acids) and their enantiomers in the period 1956-mid-1974. It has been found that the methods used for analysis of amino acids can be divided into three classes: (1) degradative procedures and techniques for converting the amino acid into another chemical compound; (2) procedures based on esterification of the carboxyl group and derivatization of the a-amino and other reactive groups in at least two steps; and (3) procedures based on a simultaneous derivatization of the carboxyl and a-amino groups in one reaction medium. For the treatment of the amino acid or its alkyl ester, three approaches can be distinguished for the two latter cases, i.e., acylation, alkylation (including silylation) and condensation. Of the procedures used for the resolution of optical antipodes, two methods are discussed, namely analysis of diastereoisomers on optically inactive stationary phases and separation of enantiomers on optically active stationary phases.     

Separation and quantitation of fatty acids, sterols and bile acids in feces by gas chromatography as the butyl ester-acetate derivatives

A system allowing the separation and quantitation of individual species of fecal fatty acids, sterols and bile acids in a single chromatographic step is described. The system is based on the butylation of carboxyl groups and acetylation of free hydroxyls of the compounds in fecal lipid extracts, followed by their resolution by temperature-programmed gas chromatography. As the butyl ester-acetate derivatives, fatty acids, sterols and bile acids elute separately and with no overlap on a variety of chromatographic columns, obviating the need for prior separation of each class by thin-layer or column chromatography. All common bile acids, a wide variety of sterols and keto-steroids, as well as saturated and unsaturated fatty acids may be routinely resolved. Quantitation is facilitated by the addition of the internal standards, heptadecanoic acid and nor-deoxycholic acid to each sample. With an automatic sample injector, the rapid assessment of a wide range of potential risk factors for colorectal cancer may be carried out in large numbers of samples.     

Capillary gas chromatography of amino acids as their tert-butyldimethylsilyl derivatives in the analysis of serum amino acids in metabolic diseases

Reaction of amino acids with N-methyl-N-(tert-butyldimethylsilyl)trifluoroaceamide (MTbSTFA) in acetonitrile affords good yields of amino acid derivatives with excellent gas chromatographic and mass spectrometric properties. The single-step derivatization procedure is highly reproducible. The TBDMS amino acids are stable at room temperature for at least three days. Only a single peak is observed for each amino acid. The procedure allows simultaneous analysis of asparagine and glutamine together with other serum amino acids. Separation is achieved on a borosilicate glass capillary coated with OV-1. The mass spectra of the TBDMS amino acids possess characteristic diagnostic ions. These properties were used in the sensitive detection by GC-MS and SIM-GC-MS of GABA and pipecolic acid in the serum of a newborn suspected of a Zellweger-type syndrome, which could not be detected by other methods.     

Synthesis of proline derivatives of bile acids and their evaluation as organocatalysts in the asymmetric direct aldol reaction

A new family of bile acid derived organocatalysts was obtained by linking l- or d-proline to amino derivatives of cholic and deoxycholic acids, which were used to promote the asymmetric direct aldol reaction between acetone and 4-nitrobenzaldehyde. Both the activity and enantioselectivity of the organocatalytic systems were dependent not only on the position of the proline moiety on the cholestanic backbone and its absolute configuration, but also on the presence of free hydroxyl group on the steroidal skeleton. The cholic acid derivative bearing a d-prolinamide moiety at the 12-position and free hydroxyl groups at the 3- and 7-positions emerged as the best organocatalytic system giving complete conversion of the substrate, even when using only 2% of catalyst loading and ee up to 80%.