Enantiomeric resolution of underivatized small peptides by HPLC with a chiral crown ether stationary phase

Factors influencing the stereoisomeric resolution of underivatized dipeptides and a representative tripeptide on Crownpak (CR) columns have been investigated. The elution order and relative retention suggest that a combination of chiral, steric, and hydrophobic interactions effects the extent of chiral recognition and the retention achieved during separations. Some dipeptides whose amine terminus is located three atoms from the asymmetric center (such as dipeptides of D ,L -glycine) were resolved, but the elution order was the opposite of that expected for the type of Crownpak column used (CR(+)). Peptides containing hydrophobic substituents were strongly retained, but their retention times could be significantly reduced, and detectability improved, by use of gradient elution. Analysis of a commercial sample of D ,L -leucine-D ,L -alanine revealed the stereoisomers to be present in an unexpected quantitative ratio and demonstrated the utility of these separations for quality assurance and quantitative analyses.     

Characterization of low-molecular weight peptides in champagne wine by liquid chromatography/tandem mass spectrometry

This paper reports the use of an on-line LC–ESI–MS/MS method for the identification and quantification of di- and tripeptides in champagne wine without laborious sample pretreatment. The identification of these compounds, in their underivatised form, is based on identical retention times and ESI–MS spectra to those of reference standards. The presence of nine dipeptides (Arg–Ile, Ile–Arg, Ile–Val, Lys–Phe, Lys–Tyr, Phe–Lys, Tyr–Gln, Tyr–Lys, Val–Ile) and the absence of two tripeptides (Phe–Arg–Arg and Lys–Met–Asn) have been evidenced in the matrix. Calibration curves for each analyte were established using Phe–Arg as internal standard. The calibration curves were linear in the concentration range 0.1–10 mg L⁻¹ with a determination coefficient, r², better than 0.992. The accuracy for the calibration standard was estimated at between 92 and 102%. This method allows high recovery and satisfies the necessary requirements with respect to accuracy, repeatability and sensitivity. The first application of this analytical method to the measurement of di- and tri-peptides in different vintages of champagne wine is reported. Compositional changes in the peptides occurred depending on the vintage.     

Toward stereocontrolled, chemoenzymatic synthesis of unnatural peptides

An efficient, chemoenzymatic method for the multicomponent synthesis of unnatural tripeptides is presented. Development of a previously described procedure combines the diversity offered by multicomponent reactions with the selectivity of biocatalysts and allows the convenient introduction of varied amino acid moieties into the tripeptide scaffold, with control of the stereochemistry. Additionally, it allows the introduction of a methyl group to the amide nitrogen, leading to derivatives of N-methylated amino acids.     

Investigation of the adsorption mechanism of a peptide in reversed phase liquid chromatography,from pH controlled and uncontrolled solutions

The single-component equilibrium adsorption of the tripeptide Leucyl-Leucyl-Leucine (LLL) on a high-efficiency Jupiter Proteo column (C₁₂$C_{12}$) was investigated experimentally and modeled theoretically. The experimental equilibrium isotherms of LLL for adsorption on a C₁₂$C_{12}$ packing material from an aqueous solution of methanol (48%) and trifluoroacetic acid (0.1%) were measured by frontal analysis (FA). The FA measurements were done with two solutions, one in which the pH was controlled, the other in which it was not. Two solutions of LLL in the mobile phase were prepared (4.3 and 5.4 g/L) and their pH measured (2.94 and 2.88), respectively. The first solution was titrated with TFA to match the pH of the mobile phase (2.03), so its pH was controlled. The pH of the other solution was left uncontrolled. In both cases the isotherms could be modeled by a bi-Langmuir equation, a choice consistent with the bimodal affinity energy distribution (AED) obtained for LLL. The isotherm parameters derived from the inverse method (IM) of isotherm determination under controlled pH conditions (by fitting calculated profiles to experimental breakthrough profiles) are in a good agreement with those derived from the FA data. Under uncontrolled pH conditions, the application of IM suggests the coexistence of two different adsorption mechanisms. According to the isotherm parameters found by these three methods (FA, AED and IM), the C₁₂$C_{12}$-bonded silica can adsorb around 500 and 70 g/L of LLL under controlled and uncontrolled pH conditions, respectively. The adsorption of LLL on the C₁₂$C_{12}$ material strongly depends on the pH of the mobile phase and on the quantity of TFA added, which plays the role of an ion-pairing agent.     

Studies on the application of the Passerini reaction and enzymatic procedures to the synthesis of tripeptide mimetics

A new, efficient method for the multicomponent synthesis of tripeptide mimetics is presented. Simple, chemoenzymatic transformations of Passerini reaction products enable the introduction of varied amino acid moieties into the tripeptide scaffold, with control of the stereochemistry. Additionally, this method allows the convenient introduction of a methyl group to the amide nitrogen, leading to derivatives of N-methylated amino acids—compounds of interest for medicinal chemistry.     

Conformational heterogeneity of tripeptides containing Boc-Leu-Aib as corner residues in the solid state

A critical analysis of single crystal X-ray diffraction studies on a series of terminally protected tripeptides containing a centrally positioned Aib (α-aminoisobutyric acid) residue has been reported. For the tripeptide series containing Boc-Ala-Aib as corner residues, all the reported peptides formed distorted type II β-turn structures. Moreover, a series of Phe substituted analogues (tripeptides with Boc-Phe-Aib) have also shown different β-turn conformations. However, the Leu-modified analogues (tripeptides with Boc-Leu-Aib) disrupt the concept of β-turn formation and adopt various conformations in the solid state. X-ray crystallography sheds some light on the conformational heterogeneity at atomic resolution.