Photochemically Induced Solid State Dimerisation of Resveratrol Analogues: A Greener Synthetic Process

The photochemical dimerisation of resveratrol analogues in the solid state to generate chiral phenyl substituted cyclobutanes is described. NMR spectroscopic and X-ray crystallographic methods have confirmed that the dimerisation leads to the head to tail orientation of the phenyl group substituents in the cyclobutane derivative. Interestingly, the parent compound, resveratrol, in the solid state, did not form a cyclobutane dimer, but the O-acetyl analogues gave the corresponding cyclobutane dimers in high yield, suggesting that the close packing of molecules together with the electron density through the conjugated double bond of the resveratrol structure are important determinants for photodimerisation to occur in the solid state.

Origin of the selectivity differences of aromatic alcohols and amines of different n-alkyl chain length separated with perfluorinated C8 and bidentated C8 modified silica hydride stationary phases

Perfluorinated C8-(PerfluoroC8) and bidentate anchored C8-(BDC8)-modified silica hydride stationary phases have been employed for the isocratic separation of homologous phenylalkanols and phenylalkylamines differing in their n-alkyl chain length, using aqueous-acetonitrile (ACN) mobile phases of different ACN contents from 10 to 90% (v/v) in 10% increments. These analytes showed reversed-phase (RP) retention behaviour with mobile phases of <40% (v/v) ACN content with both stationary phases but with the BDC8 stationary phase providing longer retention. The PerfluoroC8, but not the BDC8, stationary phase also exhibited significant retention of these analytes under conditions typical of an aqueous normal phase (ANP) mode (i.e. with mobile phases of >80% (v/v) ACN content), with the analytes exhibiting overall U-shape retention dependencies on the ACN content of the mobile phase. Further, these stationary phases showed differences in their selectivity behaviour with regard to the n-alkyl chain lengths of the different analytes. These observations could not be explained in terms of pK _a , log P, molecular mass or linear solvation energy concepts. However, density functional theory (DFT) simulations provided a possible explanation for the observed selectivity trends, namely differences in the molecular geometries and structural organisation of the immobilised ligands of these two stationary phases under different solvational conditions. For mobile phase conditions favouring the RP mode, these DFT simulations revealed that interactions between adjacent BDC8 ligands occur, leading to a stationary phase with a more hydrophobic surface. Moreover, under mobile phase conditions favouring retention of the analytes in an ANP mode, these interactions of the bidentate-anchored C8 ligands resulted in hindered analyte access to potential ANP binding sites on the BDC8 stationary phase surface. With the PerfluoroC8 stationary phase, the DFT simulations revealed strong repulsion of individual perfluoroC8 ligand chains, with the perfluoroC8 ligands of this stationary phase existing in a more open brush-like state (and with a less hydrophobic surface) compared to the BDC8 ligands. These DFT simulation results anticipated the chromatographic findings that the phenylalkanols and phenylalkylamines had reduced retention in the RP mode with the PerfluoroC8 stationary phase. Moreover, the more open ligand structure of the PerfluoroC8 stationary phase enabled greater accessibility of the analytes to water solvated binding sites on the stationary phase surface under mobile phase conditions favouring an ANP retention mode, leading to retention of the analytes, particularly the smaller phenylalkylamines, via hydrogen bonding and electrostatic effects.

Advances in the development of molecularly imprinted polymers for the separation and analysis of proteins with liquid chromatography

This review documents recent advances in the design, synthesis, characterization, and application of molecularly imprinted polymers in the form of monoliths and particles/beads for the use in the separation and analysis of proteins with solid-phase extraction or liquid chromatography. The merits of three-dimensional molecular imprinting, whereby the molecular template is randomly embedded in the polymer, and two-dimensional imprinting, in which the template is confined to the surface, are described. Target protein binding can be achieved by either using the entire protein as a template or by using a protein substructure as template, that is, a peptide, as in the "epitope" approach. The intended approach and strategy then determine the choice of polymerization method. A synopsis has been provided on methods used for the physical, chemical, and functional characterizations and associated performance evaluations of molecularly imprinted and nonimprinted control polymers, involving a diverse range of analytical techniques commonly used for low and high molecular mass analytes. Examples of recent applications demonstrate that, due to the versatility of imprinting methods, molecularly imprinted monoliths or particles/beads can be adapted to protein extraction/depletion and separation procedures relevant to, for example, protein biomarker detection and quantification in biomedical diagnostics and targeted proteomics.

Molecularly imprinted polymer membranes and thin films for the separation and sensing of biomacromolecules

This review describes recent advances associated with the development of surface imprinting methods for the synthesis of polymeric membranes and thin films, which possess the capability to selectively and specifically recognize biomacromolecules, such as proteins and single- and double-stranded DNA, employing "epitope" or "whole molecule" approaches. Synthetic procedures to create different molecularly imprinted polymer membranes or thin films are discussed, including grafting/in situ polymerization, drop-, dip-, or spin-coating procedures, electropolymerization as well as micro-contact or stamp lithography imprinting methods. Highly sensitive techniques for surface characterization and analyte detection are described, encompassing luminescence and fluorescence spectroscopy, X-ray photoelectron spectroscopy, FTIR spectroscopy, surface-enhanced Raman spectroscopy, atomic force microscopy, quartz crystal microbalance analysis, cyclic voltammetry, and surface plasmon resonance. These developments are providing new avenues to produce bioelectronic sensors and new ways to explore through advanced separation science procedures complex phenomena associated with the origins of biorecognition in nature.

Open tubular-capillary electrochromatography: Developments and applications from 2013 to 2015

Open tubular CEC (OT-CEC) separates analyte mixtures by a combination of electrophoretic, electro-osmotic, and/or chromatographic effects. OT-CEC research is an active and growing field, with studies encompassing a wide range of investigations related to new strategies for chemical modification of the inner surface of the capillary, leading to the introduction of novel stationary phase coatings. This review has examined the literature on OT-CEC from 2013 to August 2015 and highlights the developments in the fabrication of highly selective stationary phases, based on materials that include cyclodextrin chiral selectors, graphene and graphene oxide, metal-organic frameworks, molecularly imprinted polymers, nanoparticles, nanolatex particles, nanocomposites, in situ generated polymers, block polymers, tentacle-type polymers, polyelectrolyte multilayers, polysaccharides, phospholipids, and proteins. This review, while considering the development of novel OT-CEC coating materials, specifically examines different immobilization or coating methodologies and approaches and also discusses the separation mechanisms that occur with these new materials. These OT-CEC coatings are intended mainly to separate low molecular weight molecules relevant to the pharmaceutical, agricultural, and food industries as well as for use in environmental monitoring.

Confinement of water droplets on rectangular micro/nano-arrayed surfaces

Micro-patterned surfaces with alternate hydrophilic and hydrophobic rectangular areas effectively confine water droplets down to attolitre volumes. The contact angle, volume, and geometry of the confined droplets as a function of the geometry and physico-chemical properties of the confining surfaces have been determined by phenomenological simulations, validated by atomic force microscopy measurements. The combination between experiments and simulations can be used for the purposeful design of arrays with surface-addressable hydrophobicity employed in digital microfluidics and high-throughput screening nanoarrays.

Investigations into the separation behaviour of perfluorinated C8 and undecanoic acid modified silica hydride stationary phases

In this study, the surface charge properties of perfluorinated C8 (PerfluoroC8) and undecanoic acid (UDA) modified silica hydride stationary phases have been investigated. The zeta potential values of these stationary phases were measured in aqueous/acetonitrile mobile phases of different pH, buffer concentrations and acetonitrile contents. The retention behaviour of several basic, acidic and neutral compounds were then examined with these two stationary phases, with U-shaped retention dependencies evident with regard to the organic solvent content of the mobile phase. Plots of the logarithmic retention factor versus buffer concentration revealed slopes ≥ -0.41 for both stationary phases, indicating the involvement of mixed mode retention mechanisms with contributions from both ionic and non-ionic interactions. Using a linear solvation energy relationship approach, the origins of these interactions under different mobile phase conditions were differentiated and quantified. The PerfluoroC8 stationary phase exhibited stronger retention for basic compounds under high acetonitrile content mobile phase conditions, whilst stronger retention was observed for all compounds with the UDA stationary phase under high aqueous content mobile phase conditions. The more negative zeta potentials of the UDA stationary phase correlated with higher total charge density, surface charge density and charge density at the beta plane (the outer plane of the double layer) compared to the PerfluoroC8 stationary phase. With mobile phases of low buffer concentrations, more negative zeta potential values were unexpectedly observed for the PerfluoroC8 stationary phase with slight increases in the C descriptor value, reflecting also the greater accessibility of the analytes to the stationary phase surface. Comparison of the retention behaviours on these phases with other types of silica hydride stationary phases has revealed different patterns of selectivity.

Studies on the binding sites of IgG2 monoclonal antibodies recognized by terpyridine-based affinity ligands

This investigation has examined the origin of the molecular recognition associated with the interaction of monoclonal IgG2's with terpyridine-based ligands immobilized onto agarose-derived chromatographic adsorbents. Isothermal titration calorimetric (ITC) methods have been employed to acquire thermodynamic data associated with the IgG2-ligand binding. These ITC investigations have documented that different enthalpic and entropic processes are involved depending on the nature of the chemical substituents in the core structure of the terpyridinyl moiety. In addition, molecular docking studies have been carried out with IgG2 structures with the objective to identify possible ligand binding sites and key interacting amino acid residues. These molecular docking experiments with the different terpyridine-based ligands have shown that all of the examined ligands can potentially undergo favorable interactions with a site located within the Fab region of the IgG2. However, another favorable binding site was also identified from the docking poses to exist within the Fc region of the IgG2 for some, but not all, of the ligands studied. These investigations have provided a basis to elucidate the unique binding properties and chromatographic behaviors shown by several substituted terpyridine ligands in their interaction with IgGs of different isotype. Copyright © 2016 John Wiley & Sons, Ltd.

On-line determination by small angle X-ray scattering of the shape of hen egg white lysozyme immediately following elution from a hydrophobic interaction chromatography column

This study documents the use of an integrated approach, involving on-line HIC interfaced with SAXS measurements, to monitor the conformational status of proteins immediately upon elution from a chromatographic column.

The Application of Template Selectophores for the Preparation of Molecularly Imprinted Polymers

Molecularly imprinted polymers are versatile materials with wide application scope for the detection, capture and separation of specific compounds present in complex feed stocks. A major challenge associated with their preparation has been the need to sacrifice one mole equivalent of the template molecule to generate the complementary polymer cavities that selectively bind the target molecule. Moreover, template molecules can often be difficult to synthesise, expensive or lack stability. In this study, we describe a new approach, directed at the use of synthetic selectophores, chosen as readily prepared and low cost structural analogues with recognition groups in similar three-dimensional arrangements as found in the target molecule. To validate the approach, a comparative study of selectophores related to the polyphenolic compound (E)-resveratrol has been undertaken using traditional and green chemical synthetic approaches. These molecular mimic compounds were employed as polymer templates and also as binding analytes to interrogate the recognition sites associated with the molecularly imprinted polymers. Importantly, the study confirms that the use of selectophores has the potential to confer practical advantages, including access to more efficient methods for selection and preparation of suitable template molecules with a broader range of molecular diversity, as well as delivering imprinted polymers capable of recognizing the target compound and structurally related products.

Analysis of peptides and protein digests by reversed phase high performance liquid chromatography-electrospray ionisation mass spectrometry using neutral pH elution conditions

In this study, the advantages of carrying out the analysis of peptides and tryptic digests of proteins under gradient elution conditions at pH 6.5 by reversed-phase liquid chromatography (RP-HPLC) and in-line electrospray ionisation mass spectrometry (ESI-MS) are documented. For these RP separations, a double endcapped, bidentate anchored n-octadecyl wide pore silica adsorbent was employed in a capillary column format. Compared to the corresponding analysis of the same peptides and protein tryptic digests using low pH elution conditions for their RP-HPLC separation, this alternative approach provides improved selectivity and more efficient separation of these analytes, thus allowing a more sensitive identification of proteins at different abundance levels, i.e. more tryptic peptides from the same protein could be confidently identified, enabling higher sequence coverage of the protein to be obtained. This approach was further evaluated with very complex tryptic digests derived from a human plasma protein sample using an online two-dimensional (2D) strong cation-exchange (SCX)-RP-HPLC-ESI-MS/MS system. Again, at pH 6.5, with mobile phases of different compositions, improved chromatographic selectivities were obtained, concomitant with more sensitive on-line electrospray ionisation tandem mass spectrometric (ESI-MS/MS) analysis. As a consequence, more plasma proteins could be confidently identified, highlighting the potential of these RP-HPLC methods with elution at pH 6.5 to extend further the scope of proteomic investigations.

Enrichment of (E)-resveratrol from peanut byproduct with molecularly imprinted polymers

Molecularly imprinted solid phase extraction (MISPE) has been employed to isolate and concentrate bioactive polyphenols from peanut press waste. To this end, a molecularly imprinted polymer (MIP) templated with the phytoalexin (E)-resveratrol has been prepared via self-assembly with the functional monomer 4-vinylpyridine (4VP) in a 1:3 molar ratio. Subsequent molecular interrogation of the MIP binding sites demonstrated preferential structural selectivity for (E)-resveratrol with respect to other structurally related naturally occurring compounds. This selectivity was subsequently exploited to achieve substantial sample cleanup of peanut press waste under aqueous conditions with significant enrichment of (E)-resveratrol (>60 fold) requiring minimal sample preparation.

Peptide mapping with mobile phases of intermediate pH value using capillary reversed-phase high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry

This investigation describes the separation of tryptic peptides by capillary reversed-phase high-performance liquid chromatography (RP-HPLC) with eluents in the intermediate pH range, followed by in-line electrospray ionisation tandem mass spectrometry (ESI-MS/MS) analysis. For these purposes, gradient elution procedures with an aqueous eluent containing 20 mM ammonium formate, and an increasing content of acetonitrile or methanol, were employed. Compared to the analysis of the same tryptic peptides under low-pH conditions with an ion-pairing reagent, the increase in the pH with the 20 mM ammonium formate mobile phase led to significant changes in both peptide retention to the reversed-phase column and the collision-induced dissociation at the MS/MS stage as a consequence of the changes in the physico-chemical properties of these peptides, such as their overall charge, polarity and relative hydrophobicity. Thus, improved selectivity for the peptide separation and favourable tandem mass spectrometry analysis could be obtained with eluents in this intermediate pH range. The number of tryptic peptides identified by the new approach for the proteins investigated were significantly higher than that obtained by the conventional low-pH methods. Moreover, analysis of protein digests at very low concentrations was also performed under both acidic and intermediate pH conditions and similar improvements in selectivity and MS/MS detection limits were observed, i.e. identification of more distinct peptides and higher sequence coverage of the protein was obtained when eluents of intermediate pH were employed. This study therefore highlights the potential of conducting peptide mapping in the intermediate pH range to achieve more reliable and sensitive protein identifications with capillary RP-HPLC-ESI-MS/MS.

Aqueous normal-phase retention of nucleotides on silica hydride columns

The use of silica hydride-based stationary phases for the retention and analysis of nucleotides has been investigated. Both reversed-phase columns with a hydride surface underneath as well as those with an unmodified or a minimally modified hydride material were tested. With these systems, an aqueous normal-phase mode was used with high organic content mobile phases in combination with an additive to control pH for the retention of the hydrophilic nucleotides. Isocratic and gradient elution formats have been used to optimize separations for mixtures containing up to seven components. All conditions developed are suitable for methods that utilize mass spectrometry detection.

Molecular identification of lyso-glycerophosphocholines as endogenous immunosuppressives in bovine and rat gonadal fluids

The ability of the gametes to escape detection by the immune system is vital to successful human reproduction. Furthermore, the observed capacity of the testis in some species to support tissue grafts without rejection (immunological privilege) indicates that spermatogenic cells are protected by local immunoregulatory mechanisms. One of these mechanisms involves targeting T cells for inactivation and destruction within the testicular environment. Although the fluids of the testis and ovary surrounding the developing gametes contain soluble factors that inhibit T cells, the identity of the molecule(s) responsible for this activity has been unknown. Using a specific T-cell proliferation assay to monitor bioactivity, these molecules were purified from bovine ovarian follicular fluid by methanol extraction and sequential reverse-phase HPLC (RP-HPLC). All purified active fractions coincided with the elution position on RP-HPLC of several small molecules ranging in size from 496 to 522 Da. The same molecules were localized to the immunosuppressive fractions of rat testicular interstitial fluid. The active molecules were identified, using capillary electrophoresis electrospray ionization mass spectroscopy, as lyso-glycerophosphocholines (lyso-GPCs), namely, 1-palmitoyl-sn-glycero-3-phosphocholine, 1-oleoyl-sn-glycero-3-phosphocholine, a 18:2a/lyso-GPC (putatively, 1-linoleoyl-sn-glycero-3-phosphocholine), and a 20:4a/lyso-GPC (putatively, 1-arachidonyl-sn-glycero-3-phosphocholine). Comparison of the bioactivity and mass spectroscopy profiles of two of the purified molecules with their synthetic standards confirmed the identification. These molecules inhibit T-cell proliferation in response to activation and induce apoptosis of these cells in a time- and dose-dependent manner. The emergence of gonadal lyso-GPCs as potential regulators of critical immune events opens up new avenues of inquiry into the origins of autoimmune infertility and more generally into mechanisms of peripheral immunoregulation and the development of novel immunosuppressives.

Open-Tubular Capillary Electrochromatography Coupled with Electrospray Ionization Mass Spectrometry for Peptide Analysis

In this study, the open-tubular electrochromatographic (OT-CEC) migration behavior of various peptides has been characterized using etched and chemically (n-octadecyl- and cholesterol-) modified capillaries, interfaced to an electrospray ionization mass spectrometer through a sheath liquid configuration. The stationary phases were fabricated by etching the inner wall of the fused-silica capillary and then chemically modifying the new surface through a silanization/hydrosilation reaction. Unlike some other OT-CEC stationary-phase preparation methods, leaching of the immobilized stationary phase and subsequent contamination of the electrospray ion source was largely avoided with this novel surface modification technology. The influence of the immobilized organic phases and those of the buffer electrolytes (pH, the type and content of organic solvent) on the retention and separation of the selected peptides was investigated. Significant peptide retention was found even at very low pH with both types of stationary phases, under conditions whereby the electrophoretic migration dominated the separation process. Due to the effective coverage of the etched surface by a silanization/hydrosilation reaction, adverse adsorption of charged analytes onto the capillary wall was minimized. As a result, very efficient and highly reproducible peptide separations were achieved over a broad pH range. Moreover, peptide-specific multizoning effects were observed. The origin of this novel phenomenon was explored. Compared to capillary electrophoresis electrospray ionization mass spectrometry system, much higher detection sensitivity could be obtained, since a larger amount of sample could be injected and stacked at the head of the open-tubular capillary column without deteriorating the separation performance. On the basis of these observations, these procedures have been adapted to allow the analysis of tryptic peptides generated from proteins.

Desalting of Peptides and Protein Mixtures by RP-HPLC Techniques

INTRODUCTIONThe RP-HPLC technique can be used to "desalt" peptide or protein samples derived from extraction procedures, from chemical reactions such as reductive alkylation in the presence of urea or guanidine hydrochloride, citraconylation, iodination, or cyanogen bromide cleavage, or recovered from other chromatographic separation. The peptide or protein solution is injected onto a small RP-HPLC column. An aqueous buffer is used to elute the salts while the peptides or proteins are concentrated at the top of the column. After elution of the salts, monitored by UV detection, the peptides or proteins are eluted with H(2)O-acetonitrile or H(2)O-isopropanol mobile phases.

Purification of Peptides from Solid-Phase Peptide Synthesis with RP-HPLC

INTRODUCTIONPurification of peptides derived from solid-phase peptide synthesis (SPPS) requires the removal of deletion peptides resulting from incomplete coupling/deprotection steps, from racemization or side-chain rearrangement, and from various chemical substances introduced during the deprotection or cleavage stages of an SPPS procedure. This protocol details the purification and analysis of many synthetic peptides of 2-65 amino acid residues. These peptides contain a number of ionizable or polar side chains, but do not contain secondary structural elements (such as β-sheets) that favor supramolecular assembly.