Dušan Koval

Determination of dissociation constant of phosphinate group in phosphinic pseudopeptides by capillary zone electrophoresis

Capillary zone electrophoresis (CZE) was used for determination of dissociation constant of phosphinate group in phosphinic pseudopeptides, i.e. peptides where one peptide bond is substituted by phosphinic acid moiety -PO2--CH2-. The dissociation constants were determined for a set of newly synthesized pseudopeptides derived from a structure N-Ac-Val-Ala(psi)(PO2--CH2)Leu-His-NH2 by nonlinear regression of experimentally measured pH dependence of their effective electrophoretic mobilities. CZE experiments were carried out in Tris-phosphate background electrolytes in the pH range 1.4-3.2. The pseudopeptides were synthesized as a mixture of four diastereomers, the separation of which was achieved in most cases. Moreover, differences of the effective mobilities of the pseudopeptide diastereomers enabled simultaneous determination of the dissociation constant of their phosphinate group without necessity of previous isolation of individual isomers.

Chiral analysis of helquats by capillary electrophoresis: resolution of helical N-heteroaromatic dications using randomly sulfated cyclodextrins.

Enantiomers of helical N‐heteroaromatic dications, helquats, were separated by CE. An acidic 22/35 mM sodium/phosphate background electrolyte, pH 2.4, with addition of randomly sulfated α‐, β‐ and γ‐ cyclodextrins allowed enantioresolution of a series of helquats, which comprised 5, 6 and 7 fused rings participating in the helical backbone. In general, at least one of the chiral selectors was found to provide baseline separation for 22 out of 24 helquats and partial separation for the remaining two. Individually, the sulfated γ‐cyclodextrin turned out to separate 79% of the helquats, followed by the β‐ and α‐congeners with 54 and 42% of the resolved compounds, respectively. Migration order of enantiomers was inspected for selected helquats and a relation of molecular size of the analytes to a cavity of the cyclodextrin selectors is discussed.

Resolution of a configurationally stable [5]helquat: enantiocomposition analysis of a helicene congener by capillary electrophoresis

Racemic [5]helquat as a triflate salt has been synthesized using a robust, three-step procedure. Subsequent exchange of triflate anions for inexpensive (R,R)-dibenzoyltartrate anions via an ion exchange resin afforded two diastereoisomeric salts. Crystallization led to the resolution of the helquat (ee > 98%). This is the first time that a non-racemic helquat has been obtained; its helicity having been assigned and its racemization barrier determined. Capillary electrophoresis with a sulfated β-cyclodextrin chiral selector is introduced for the first time as a straightforward method to analyze the enantiocomposition of charged, helicene-like species.

[6]Saddlequat: a [6]helquat captured on its racemization pathway

A dicationic [6]helicene congener captured on the racemization pathway in its saddle-shaped geometry is introduced. Synthesis, structure, resolution, and dynamic properties of this chiral [6]saddlequat in-between and its highly stereocontrolled transformation into enantiopure [6]helquat are discussed and demonstrated. The dynamic aspects established by experiment and supported by detailed DFT-D calculations are presented visually in the form of a movie (electronic table-of-contents and electronic supplementary information). The title [6]saddlequat was found to be an isolable chiral species on the entirely chiral enantiomerization pathway of a [6]helquat that is discussed as an example of Mislows “rubber glove” molecule.

Heart‐cutting 2‐D CE using multiple detection points for chiral analysis of native amino acids

A new methodology based on heart‐cutting 2‐D CE in a single capillary was developed for the chiral separation of a mixture of 22 underivatized amino acids. The first dimension is performed in an achiral BGE (2.3 M acetic acid, pH 2.1) allowing the separation of the analytes as a function of their charge‐to‐radius ratio. A selected fraction from the first dimension is then separated in the second dimension in the presence of a chiral selector (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid. Since the introduction of the second electrolyte is performed by pressure mobilization, capillaries with small id of 10 μm are used to limit the peak broadening due to Taylor dispersion. Double capacitively coupled contactless conductivity detector is used for monitoring the selection and the isolation of the fraction at the different steps of the analysis (voltage and pressure mobilization steps). This double detection allows calculating the voltage and pressure stop times in real‐time analysis. This new methodology is applied with success for the chiral separation of different amino acids (d,l‐Tyr, d,l‐Trp and d,l‐Thr) contained in a mixture of 22 native amino acids.

Heart-Cutting Two-Dimensional Capillary Electrophoresis for the On-Line Purification and Separation of Derivatized Amino Acids

Heart-cutting two-dimensional (2D) capillary electrophoresis (CE) in a single capillary was used for analysis of derivatized amino acids. A mixture of 12 amino acids derivatized with UV-active benzyl 4-(3-(2-chloroethyl)-3-nitrosoureido)butylcarbamate label served as a model of a moderately complex sample due to the presence of numerous derivatization byproducts. The first step of the heart-cutting 2D approach was sample cleanup by capillary zone electrophoresis (CZE) in borate electrolyte. Then, only a selected portion of the first-dimension separation was transferred into the second dimension of the separation by a specific voltage and pressure program. Finally, the zone of derivatized amino acids was separated by micellar electrokinetic chromatography in a borate-sodium dodecyl sulfate system. The whole 2D process can be performed in a conventional CE analyzer without any interface for connection of the two separation modes. Intraday repeatability of the total migration time was 2%. In general, the heart-cutting 2D-CE methodology in a single capillary can be adapted for any CE mode regardless of the direction and velocity of electroosmotic flow and position of the fraction of interest in the first dimension (i.e., first, last, or intermediate fraction).

Analysis and characterization of phosphinic pseudopeptides by capillary zone electrophoresis

Capillary zone electrophoresis (CZE) was applied to analysis and characterization of phosphinic pseudopeptides with the general structure N‐Ac‐Val‐Alaψ(

Search for conglomerate in set of [7]helquat salts: multigram resolution of helicene-viologen hybrid by preferential crystallization.

PO_2^ -

Study of deoxyribonucleic acid-ligand interactions by partial filling affinity capillary electrophoresis.

‐CH2) Leu‐Xaa‐NH2, where Xaa represents one of 20 proteinogenic amino acid residues. Pseudopeptides containing neutral or acidic amino acid residues in position Xaa were analyzed as anions in weakly alkaline (pH 8.1) Tris‐Tricine background electrolyte (BGE), pseudopeptides with basic amino acid residues in position Xaa were analyzed as cations in acid BGEs (Tris‐phosphate buffers). Acidity of phosphinic acid moiety in peptides with basic amino acid residues was determined from the dependence of effective mobility of these peptides on pH in the acid pH region (pH 1.4–2.8). Additionally, separation of diastereomers of some peptides was achieved.

Analysis of water extracts from airborne dust samples by capillary isotachophoresis

Investigation of a set of 12 [7]helquat salts by X-ray crystal diffraction led to identification of conglomerate behavior in bis(trifluoroacetate) salt [2][CF(3)CO(2)](2). This is to demonstrate that a systematic search for conglomerates can be performed for a given helicenoid enabling straightforward multigram resolution via preferential crystallization. Subsequently, preferential crystallization of this chiral helicene-viologen hybrid has been established to obtain pure P and M enantiomers on a multigram scale, 5 g each. Furthermore, preparation of nonracemic samples of [7]helquat 2 via diastereomeric (R,R)-dibenzoyltartrate salts is described, and determination of absolute configuration and racemization barrier is also reported.