Publications (37) View all
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Article: Isomerization and epimerization of the aspartyl tetrapeptide Ala-Phe-Asp-GlyOH at pH 10 - a capillary electrophoresis study.
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ABSTRACT: Isomerization and enantiomerization of Asp in the tetrapeptide Ala-Phe-Asp-GlyOH was studied at pH 10 and 80°C as well as 25°C. CE-MS allowed the distinction between α-Asp and β-Asp linkages in degradation products based on the ratio of the b and y fragment ions. Besides isomerization and enantiomerization of Asp, enantiomerization of Ala and Phe was also observed at both temperatures by chiral amino acid HPLC analysis using Marfey's reagent for derivatization. The rate of enantiomerization of the amino acids proceeded in the order Asp > Ala > Phe. The CE assay was validated with respect to linearity, LOQ, LOD and precision and employed to characterize the time course of the degradation of the tetrapeptide upon incubation in borate buffer, pH 10. Isomerization to β-Asp peptides was identified as the major degradation reaction. The configuration of Asp or Ala affected the half life of the starting peptide to a minor extent but did not influence the distribution of the individual products under equilibrium conditions at 80°C. Degradation at 25°C proceeded very slowly so that the equilibrium was not reached after 245 days.Electrophoresis 03/2013; · 3.30 Impact Factor -
Dataset: anie 201107011 sm miscellaneous information
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Article: Capillary electrophoretic study of the degradation pathways and kinetics of the aspartyl model tetrapeptide Gly-Phe-Asp-GlyOH in alkaline solution.
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ABSTRACT: The aim of the present study was the investigation of the isomerization and epimerization kinetics of the aspartyl tetrapeptide Gly-Phe-Asp-GlyOH at alkaline conditions. Incubations of the model tetrapeptide in sodium borate buffer, pH 10 and ionic strength 0.2M, at 25°C and 80°C were analyzed by a validated CE-UV assay and fitted according to a pharmacokinetic model. CE-ESI-MS was used for peptide identification. Enantiomerization and isomerization of the aspartyl residue of the model tetrapeptide was observed under all experimental conditions applied. Differences in the velocity and the ratios of the rates of the degradation reactions indicated different effects of temperature on the individual reactions. At 80°C, a rapid formation of β-Asp and d-Asp containing isomers from Gly-l-Phe-α-l-Asp-GlyOH was monitored. Rate constants of the hydrolysis of the succinimide (Asu) intermediate generally exceeded the formation of the intermediate from α/β-Asp peptides. A higher rate constant was observed for the enantiomerization from l-configured Asu compared to d-Asu. At 25°C, epimerization and isomerization equilibrium was not reached within 5208h. Compared to 80°C different ratios of the individual reaction rates were noted. Moreover, inversion of the sequence of the first 2 amino acids was noted as a minor side reaction at 80°C.Journal of pharmaceutical and biomedical analysis 12/2012; 76C:96-103. · 2.45 Impact Factor -
Article: Degradation kinetics of an aspartyl-tripeptide-derived diketopiperazine under forced conditions.
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ABSTRACT: The aim of the present study was the determination of the stability of a diketopiperazine (DKP) derived from the aspartyl tripeptide Phe-Asp-GlyOH at pH 10 and 80°C and 25°C as well as at pH 7.4 and 80°C. The analysis was performed using a validated capillary electrophoresis assay. Rapid epimerization of the incubated cis-DKP to the trans-DKP was observed under all conditions. Linear diastereomeric α-l/d-Asp and β-l/d-Asp peptides were the primary reaction products at pH 10 and 80°C, indicating that a cyclic succinimide is formed in the process. In contrast, the DKPs were by far the major compounds in the incubation solutions at pH 10 and 25°C and at pH 7.4 and 80°C, whereas the linear Asp peptides were found only at low concentrations. A kinetic model was derived to fit the concentration versus time data, which consider the succinimide as central intermediate for DKP formation and for isomerization and enantiomerization of the linear Asp peptides. Besides the back reaction of the DKPs to the succinimides, an additional hydrolysis reaction of the DKP ring was considered to obtain the fit of the experimental data, indicating that additional degradation reactions have to be considered for Asp-derived DKPs. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:4178-4190, 2012.Journal of Pharmaceutical Sciences 08/2012; 101(11):4178-90. · 3.06 Impact Factor -
Article: Structurally diverse μ-conotoxin PIIIA isomers block sodium channel NaV 1.4.
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ABSTRACT: The one and only fold? Three chemically synthesized μ-conotoxin PIIIA isomers, which contain different disulfide connectivity, block the skeletal muscle voltage-gated sodium channel Na(V) 1.4 with similar, yet distinguishable potency. Hence, bioactivity of this μ-conotoxin is not strictly coupled to its native fold. Future development of conotoxin-derived analgesics may benefit from such a widened structural repertoire.Angewandte Chemie International Edition 03/2012; 51(17):4058-61. · 13.45 Impact Factor
