Über Peptidsynthesen, 31. Chromatographische Trennung einiger diastereomerer Dipeptide und Betrachtungen zur Konformation

Effects of amino acid-derived chiral ionic liquids on cyclodextrin-mediated capillary electrophoresis enantioseparations of dipeptides

The synergistic effect of chiral ionic liquids composed of tetraalkylammonium ions and the amino acids Asn, Asp or Pro on the enantioseparations of dipeptides mediated by β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis was studied. Addition of a chiral ionic liquid resulted in a concentration-dependent increase in the enantioresolutions compared to the sole presence of a cyclodextrin in the background electrolyte. The extent varied with the tetraalkylammonium cation (tetramethylammonium versus tetrabutylammonium) as well as the amino acid component of the ionic liquid. The presence of a chiral ionic liquid did not counteract the pH-dependent reversal of the enantiomer migration order of the dipeptides Ala-Phe, Ala-Tyr and Phe-Phe when increasing the pH of the background electrolyte from 2.5 to 3.5. Comparing the effect of a chiral ionic liquid based on Asp with the addition of equimolar concentrations of the individual components of the ionic liquid, a diverse picture was observed. In some cases, higher resolution values were obtained with the chiral ionic liquid, while for other cases superior enantioseparations were obtained upon separate addition of the amino acid component and a tetraalkylammonium chloride. With regard to the stereochemistry of the amino acid, a superior effect was typically observed using the l-configured amino acid, but in some cases higher resolution values were found in the presence of d-Asp. The rationale for the diverse observations is not obvious and may be due to the zwitterionic nature of analytes as well as the amino acid component of the chiral ionic liquid.

Chlorofluoroacetic Acid as a Highly Versatile Derivatizing Agent: Assignment of Stereochemistry to Esters of Chiral Alcohols

Stereochemistry of a series of diastereomeric esters obtained from chiral alcohols 1a-21a by derivatization with (S)- or (R)-chlorofluoroacetic acid was correlated with their LC and GC separation (∆tr) and NMR resolution (∆δ). Both the chromatographic and NMR spectral behavior of respective diastereomers was found to follow systematic rules reflecting their steric arrangement. Moreover, identical conformations of the esters seem to be preferred in solution as well as in the chromatographic processes. Reasons underlying this behavior are discussed.

Amino acid composition and sequence of dermorphin, a novel opiate-like peptide from the skin of Phyllomedusa sauvagei

Dermorphin, a heptapeptide with very potent opiate-like activity, has been isolated from methanol extracts of the skin of the South American frog Phyllomedusa sauvagei. The amino acid sequence of the peptide is: H-Try-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2. Dermorphin presents striking differences from the known enkephalins; it offers a surprising example of a peptide from Vertebrata containing a D-amino acid residue in its sequence.

A technique for the determination of enantiomeric amino acids in biological samples

Racemic amino acids can be separated into their enantiomers by means of gas-liquid chromatography. The most applied technique, today, is the conversion of chiral compunds into diastereoisomers with optically active reagents and subsequent chromatography on conventional optically inactive stationary phases. In previous studies it has been realized that this technique is associated with various problems. We studied the use of optically active stationary phases for separating enantiomers directly via a diastereoisomeric association complex. The optically acitve stationary phases employed are N- and C-terminal substituted dipeptides of the type N-trifluoroacetyl-dipeptide-cyclohexyl esters and have been synthesised by the I-hydroxibenztriazole dicyclohexylcarbodiimide method. The quality of these phases with respect to separation factors, resolution factors, and thermodynamical properties have been evaluated. All synthetic phases show excellent properties; however, when attempting separation of mixtures of naturally occurring amino acids extensive overlap in the elution diagram was detected. Only one phase - N-TFA-L-chi-amino-n-butyryl-L-chi-amino butyric acid cyclohexyl ester - gave complete resolution of the naturally occurring amino acids alanine, valine, glycine, threonine, leucine, isoleucine, serine and proline on a 400 ft x 0.02 in capillary column. Less volatile amino acids such as aspartic acid, phenylalanine, methionine, glutamic acid, tyrosine, arginine, and tryptophan can be resolved at a 100 ft x 0.02 in column.