Interplatform comparison between three ion mobility techniques for human plasma lipid collision cross sections.

The implementation of ion mobility spectrometry (IMS) in liquid chromatography-high-resolution mass spectrometry (LC-HRMS) workflows has become a valuable tool for improving compound annotation in metabolomics analyses by increasing peak capacity and by adding a new molecular descriptor, the collision cross section (CCS). Although some studies reported high repeatability and reproducibility of CCS determination and only few studies reported good interplatform agreement for small molecules, standardized protocols are still missing due to the lack of reference CCS values and reference materials. We present a comparison of CCS values of approximatively one hundred lipid species either commercially available or extracted from human plasma. We used three different commercial ion mobility technologies from different laboratories, drift tube IMS (DTIMS), travelling wave IMS (TWIMS) and trapped IMS (TIMS), to evaluate both instrument repeatability and interlaboratory reproducibility. We showed that CCS discrepancies of 0.3% (average) could occur depending on the data processing software tools. Moreover, eleven CCS calibrants were evaluated yielding mean RSD below 2% for eight calibrants, ESI Low concentration tuning mix (Tune Mix) showing the lowest RSD (< 0.5%) in both ion modes. Tune Mix calibrated CCS from the three different IMS instruments proved to be well correlated and highly reproducible (R2 > 0.995 and mean RSD ≤ 1%). More than 90% of the lipid CCS had deviations of less than 1%, demonstrating high comparability between techniques, and the possibility to use the CCS as molecular descriptor. We highlighted the need of standardized procedures for calibration, data acquisition, and data processing. This work demonstrates that using harmonized analytical conditions are required for interplatform reproducibility for CCS determination of human plasma lipids.

Reversed HILIC Gradient: A Powerful Strategy for On-Line Comprehensive 2D-LC.

The aim of the present work is to evaluate the possibilities and limitations of reversed hydrophilic interaction chromatography (revHILIC) mode in liquid chromatography (LC). This chromatographic mode consists of combining a highly polar stationary phase (bare silica) with a gradient varying from very low (1-5%) to high (40%) acetonitrile content (reversed gradient compared to HILIC). The retention behavior of revHILIC was first compared with that of reversed-phase LC (RPLC) and HILIC using representative mixtures of peptides and pharmaceutical compounds. It appears that the achievable selectivity can be ranked in the order RPLC > revHILIC > HILIC with the two different samples. Next, two-dimensional liquid chromatography (2D-LC) conditions were evaluated by combining RPLC, revHILIC, or HILIC with RPLC in an on-line comprehensive (LC × LC) mode. evHILIC × RPLC not only showed impressive performance in terms of peak capacity and sensitivity, but also provided complementary selectivity compared to RPLC × RPLC and HILIC × RPLC. Indeed, both the elution order and the retention time range differ significantly between the three techniques. In conclusion, there is no doubt that revHILIC should be considered as a viable option for 2D-LC analysis of small molecules and also peptides.

On-line reversed-phase liquid chromatography x supercritical fluid chromatography coupled to high-resolution mass spectrometry: A powerful tool for the characterization of advanced biofuels.

Bio-oils obtained by thermochemical or biochemical conversion of biomass represent a promising source of energy to complement fossil fuels, in particular for maritime or air transport for which the use of hydrogen or electricity appears complicated. As these bio-oils are very rich in water and heteroatoms, additional treatments are necessary before they can be used as biofuel. In order to improve the efficiency of these treatments, it is important to have a thorough knowledge of the composition of the bio-oil. The characterization of bio-oils is difficult because they are very complex mixtures with thousands of compounds covering a very wide range of molecular weight and polarity. Due to the high degree of orthogonality between the two chromatographic dimensions, the on-line combination of reversed-phase liquid chromatography and supercritical fluid chromatography (on-line RPLC x SFC) can significantly improve the characterization of such complex matrices. The hyphenation was optimized by selecting, in SFC, the stationary phase, the co-solvent, the make-up solvent prior to high resolution mass spectrometry (HRMS) and the injection solvent. Additionally, a new interface configuration is described. Quality descriptors such as the occupation of the separation space, the peak shapes and the signal intensity were considered to determine the optimal conditions. The best results were obtained with bare silica, a co-solvent composed of acetonitrile and methanol (50/50, v/v), a make-up solvent composed of methanol (90%) and water (10%) with formic acid (0.1%), an addition of co-solvent through an additional pump for SFC separation in a 2.1 mm column, and an hydro-organic solvent as injection solvent. The optimized setup was used to analyze two microalgae bio-oils: the full bio-oil coming from hydrothermal liquefaction and Soxhlet extraction of microalgae, and the gasoline cut obtained after distillation of the full bio-oil. Results in on-line RPLC x SFC-qTOF were particularly interesting, with very good peak shapes and high reproducibility. Moreover, the high degree of orthogonality for microalgae bio-oils of RPLC and SFC was highlighted by the very large occupation of the separation space. Isomeric profiles of compound families could be obtained in RPLC x SFC-qTOF and many isomers not separated in SFC alone were separated in RPLC and vice versa, thus showing the complementarity of the two chromatographic techniques.

Theoretical and practical comparison of RPLC and RPLC × RPLC: how to consider dilution effects and sensitivity in addition to separation power?

The objective of this work was to provide an unbiased comparison of one-dimensional reversed-phase liquid chromatography (1D-RPLC) and comprehensive two-dimensional RPLC (RPLC × RPLC), through calculations and experimental verifications. For this purpose, various quality descriptors were evaluated, including peak capacity, analysis time, dilution factor, number of runs in the second dimension, and injection volume. The same strategy was applied to small pharmaceuticals and peptides. Whatever the analysis time between 30 and 200 min, short columns of only 30 × 2.1 mm packed with sub-2-µm particles should be selected in both dimensions of the 2D-LC setup to obtain the best compromise in terms of peak capacity and sensitivity. The peak capacity in RPLC × RPLC vs. RPLC was significantly improved for analysis times beyond 5 min. However, extra-column volume located after the second-dimension column was found to be particularly critical for peptides, and up to 50% lower peak capacity was observed with MS vs. UV detection. Contrary to common belief, higher dilution is not always observed in RPLC × RPLC. With adequate analytical conditions, better sensitivity (in theory fivefold and in practice three- to fivefold) could be achieved in RPLC × RPLC compared to 1D-RPLC, regardless of the analysis time.

Sense and nonsense of shifting gradients in on-line comprehensive reversed-phase LC × reversed-phase LC.

In on-line comprehensive two-dimensional liquid chromatography (LC × LC), the combination of similar chromatographic modes such as reversed-phase liquid chromatography × reversed-phase liquid chromatography (RPLC × RPLC) usually leads to the partial occupation of the available separation space. A possible solution to circumvent this issue may be to dynamically adjust the gradient elution in the second dimension (2D) throughout the LC × LC analysis. This allows the gradient elution to be tailored to the elution conditions of each fraction instead of using a conventional full gradient program in which the same gradient profile is repeated for each 2D-fraction. In this study, an online RPLC × RPLC method is optimized with shifting gradients in 2D. The logic behind implementing such programs in on-line LC × LC is explained. The optimized method consists of a combination of segmented and shifted gradient modes. It is shown that the retention space coverage can be increased by 50 % compared to a conventional full gradient program, leading to a significant increase in peak capacity (about 35 %). However, such an increase comes at the expense of larger peak widths in 2D and thus lower peak intensities. It is shown here that the use of shifting gradients raises another serious issue related to the repeatability of retention times between two successive 2D-separations.

A simple mathematical treatment for predicting linear solvent strength behavior in gradient elution: Application to biomolecules.

This paper describes an approach to rapidly and easily calculate the linear solvent strength parameters, namely log k0 and S, under reversed-phase liquid chromatography conditions. This approach, which requires two preliminary gradient experiments to determine the retention parameters, was applied to various representative compounds including small molecules, peptides, and proteins. The retention time prediction errors were compared to the ones obtained with a commercial HPLC modeling software, and a good correlation was found between the values. However, two important constraints have to be accounted for to maintain good predictions with this new approach: i) the retention factor at the initial composition of the preliminary gradient series have to be large enough (i.e., log ki above 2.1) and ii) the retention models have to be sufficiently linear. While these two conditions are not always met with small molecules or even peptides, the situation is different with large biomolecules. This is why our simple calculation method should be preferentially applied to calculate the linear solvent strength parameters of protein samples.

Development of a sub-hour on-line comprehensive cation exchange chromatography x RPLC method hyphenated to HRMS for the characterization of lysine-linked antibody-drug conjugates.

This study details the development of on-line two-dimensional liquid chromatography (2D-LC) methods combining cation-exchange chromatography (CEX) and reversed-phase liquid chromatography (RPLC) for the separation of the charge variants of a lysine-linked antibody-drug conjugate (ADC). This combination gives an excellent example of the potential benefits of 2D-LC approaches for the analysis of such complex protein formats. CEX is considered the reference technique for the separation of protein charge variants but its retention mechanism usually requires the use of a high concentration of non-volatile salts, which impedes its compatibility with MS detection. In this context, the use of an on-line 2D-LC-MS approach not only allows on-line desalting and indirect coupling of CEX with mass spectrometry (MS) detection but it also provides increased and complementary information within a single analysis. The first part of this study was devoted to the choice of stationary phases and the optimization of chromatographic conditions in both dimensions. Based on the results obtained in 1D-CEX with ultraviolet detection (UV) and 1D-RPLC with UV and high-resolution mass spectrometry (HRMS) detections, an on-line comprehensive two-dimensional liquid chromatography method combining CEX and RPLC was developed. The last part of this study was devoted to the identification of the separated species using HRMS detection and in the comparison of three ADC samples exposed to different durations of thermal stress.

A comprehensive study on the phenomenon of total breakthrough in liquid chromatography.

Differences in elution strength between the sample solvent and the mobile phase usually give rise to undesirable effects on the chromatographic separation, which may range from slight broadening to severe peak deformation or even splitting. In the most extreme case, the retention factor of the analyte at the head of the column is so small at the time of injection that part of the analyte goes through the column with very little interaction with the stationary phase and hence elutes very close to the column dead time. This phenomenon is known as breakthrough. Usually, during breakthrough, the retained peak displays a wide array of deformations and it is not rare that multiple retained peaks appear for a given injected analyte. However, under certain conditions, it has been demonstrated that these deleterious effects could fully disappear, leaving only one breakthrough peak and one symmetrical peak on the chromatogram. This so-called "total breakthrough" phenomenon was recently highlighted in the specific context of the 2D-LC separation of peptides but has yet to be explained. In the present paper, we describe the results of a comprehensive study aiming to better understand and define the conditions of emergence of both breakthrough and total breakthrough phenomena in liquid chromatography. The effects of a broad range of parameters, including the nature of the solute, the retention mechanism, the injection and elution conditions, the column temperature, and the injected sample concentration on the occurrence of both phenomena were investigated. While breakthrough was found to occur for all studied compounds, it appears that the presence of positive charges on the molecule is a prerequisite for observing a total breakthrough phenomenon. Among all the parameters investigated in this work, only the injection conditions and the analyte retention were found to be impactful on the onset of both phenomena. This finding allowed us to suggest one necessary and sufficient condition, relying on the injection of critical volumes to observe each respective phenomenon. These critical volumes only depend on the column dead volume and the retention factor of the analyte in the injection solvent.

Interlaboratory study of a supercritical fluid chromatography method for the determination of pharmaceutical impurities: Evaluation of multi-systems reproducibility.

Modern supercritical fluid chromatography (SFC) is now a well-established technique, especially in the field of pharmaceutical analysis. We recently demonstrated the transferability and the reproducibility of a SFC-UV method for pharmaceutical impurities by means of an inter-laboratory study. However, as this study involved only one brand of SFC instrumentation (Waters®), the present study extends the purpose to multi-instrumentation evaluation. Specifically, three instrument types, namely Agilent®, Shimadzu®, and Waters®, were included through 21 laboratories (n = 7 for each instrument). First, method transfer was performed to assess the separation quality and to set up the specific instrument parameters of Agilent® and Shimadzu® instruments. Second, the inter-laboratory study was performed following a protocol defined by the sending lab. Analytical results were examined regarding consistencies within- and between-laboratories criteria. Afterwards, the method reproducibility was estimated taking into account variances in replicates, between-days and between-laboratories. Reproducibility variance was larger than that observed during the first study involving only one single type of instrumentation. Indeed, we clearly observed an 'instrument type' effect. Moreover, the reproducibility variance was larger when considering all instruments than each type separately which can be attributed to the variability induced by the instrument configuration. Nevertheless, repeatability and reproducibility variances were found to be similar than those described for LC methods; i.e. reproducibility as %RSD was around 15 %. These results highlighted the robustness and the power of modern analytical SFC technologies to deliver accurate results for pharmaceutical quality control analysis.

Comparison of existing strategies for keeping symmetrical peaks in on-line Hydrophilic Interaction Liquid Chromatography x Reversed-Phase Liquid Chromatography despite solvent strength mismatch.

In two-dimensional liquid chromatography, the combination of hydrophilic interaction liquid chromatography (HILIC) and reversed-phase liquid chromatography (RPLC) is very attractive due to the complementarity of their separation mechanisms. On-line comprehensive HILIC x RPLC is well-known to give rise to a large retention space coverage when dealing with ionisable compounds. However, method development in on-line HILIC x RPLC is challenging due to the reversed solvent strength between both dimensions, which can greatly affect the peak shapes in the second RPLC dimension, and thus the separation quality and the method sensitivity. In the present contribution, we compared four strategies designed to avoid this problem: (1) flow splitting, which consists in reducing the injection volume in the second dimension (2D), (2) on-line dilution with a make-up flow and (3) on-line dilution with Active Solvent Modulation (ASM), which both consist in reducing the solvent strength of the injected fractions, and (4) Total Breakthrough Strategy, which we recently proposed. Unlike the three preceding strategies, this latter one consists in injecting large volumes of strong solvent in 2D. The performance of each strategy was evaluated for sub-hour separations of a tryptic digest in on-line HILIC x RPLC. In this work, we considered the critical case for which the same column internal diameters (i.e. 2.1 mm here) are used in both dimensions. Peak capacity, peak shapes and peak intensities were considered for this evaluation. The highest peak capacity could be achieved with Total Breakthrough Strategy while the lowest one with on-line dilution using ASM. Peak intensities were usually higher with on-line dilution approaches (make-up flow and ASM). However, despite the presence of breakthrough, peak intensities were approximately 7-fold higher with Total Breakthrough Strategy than with flow splitting.

Two-Dimensional Liquid Chromatography Coupled to High-Resolution Mass Spectrometry for the Analysis of ADCs.

From a structural point of view, the complete characterization of ADCs is a challenging task due to their high complexity. ADCs combine the heterogeneity of the initial antibody to the variability associated with the conjugation strategy, the manufacturing process, and the storage. Given the inherent complexity of these biomolecules, online comprehensive two-dimensional liquid chromatography (LC × LC) is an attractive technique to address the challenges associated with ADC characterization. Compared to conventional one-dimensional liquid chromatography techniques (1D-LC), LC × LC combines two different and complementary separation systems. In the context of ADC analysis, LC × LC has been proven to be a rapid and efficient analytical tool: (1) to provide a higher resolving power by increasing the overall peak capacity and thus allowing to gain more information within a single run and (2) to allow mass spectrometry (MS) coupling with some chromatographic techniques that are not MS-compatible and hence to facilitate the structural elucidation of ADCs. In this chapter, we present the coupling of different chromatographic techniques including hydrophobic interaction chromatography (HIC), reversed phase liquid chromatography (RPLC), size exclusion chromatography (SEC), ion exchange chromatography (IEX), and hydrophilic liquid chromatography (HILIC). The interest of HIC × SEC, SEC × SEC, HIC × RPLC, IEX × RPLC, RPLC × RPLC, and HILIC × RPLC, all hyphenated to high-resolution mass spectrometry (HRMS), is discussed in the context of the characterization of ADCs.

Pushing the limits of resolving power and analysis time in on-line comprehensive hydrophilic interaction x reversed phase liquid chromatography for the analysis of complex peptide samples.

In the present work, we have investigated the combination of hydrophilic interaction liquid chromatography (HILIC) and reversed phase liquid chromatography (RPLC) for the separation of peptides in on-line HILIC x RPLC. This combination usually leads to significant solvent strength mismatch, since a weak solvent in HILIC becomes a strong solvent in RPLC. This may result in band broadening, peak distortion, and breakthrough phenomena. Our focus was directed towards the reduction of band broadening and peak distortion. The conditions of the emergence of breakthrough could be investigated with high resolution mass spectrometry (HRMS) detection. The importance of both the injection volume and the difference in composition between injection and elution solvents was highlighted. Reported strategies to avoid bad peak shapes mostly rely either on flow splitting to limit the injection volume, or on on-line dilution. Here, we propose an alternative approach which consists in injecting large volumes in the second dimension. In this case, no flow-splitting nor dilution prior to the second dimension is required. Our results show that above a certain critical injected volume, depending on both the compound and the elution conditions, narrow and symmetrical peaks can be obtained, despite the persistence of breakthrough. As a result, the injected volume in the second dimension must be larger than the largest critical volume. This counter-intuitive approach was applied for the on-line HILIC x RPLC-UV-HRMS analysis of a complex tryptic digest sample. A peak capacity close to 1500 could be achieved in 30 min, which is two-fold higher than in RPLC x RPLC within the same analysis time.

A Novel Online Four-Dimensional SEC×SEC-IM×MS Methodology for Characterization of Monoclonal Antibody Size Variants.

The determination of size variants is a major critical quality attribute of a therapeutic monoclonal antibody (mAb that may affect the drug product safety, potency, and efficacy. Size variant characterization often relies on size-exclusion chromatography (SEC), which could be hampered by difficult identification of peaks. On the other hand, mass spectrometry (MS)-based techniques performed in nondenaturing conditions have proven to be valuable for mAb-related compound characterization. On the basis of the observation that limited SEC performance was observed in nondenaturing MS compatible ammonium acetate buffer compared with classical phosphate salts, a multidimensional analytical approach was proposed. It combines comprehensive online two-dimensional chromatography (SEC×SEC), with ion mobility and mass spectrometry (IM-MS) in nondenaturing conditions for the characterization of a variety of mAbs. We first exemplify the versatility of our approach for simultaneous detection, identification, and quantitation of adalimumab size variants. Benefits of the SEC×SEC-native IM×MS were further highlighted on forced degraded pembrolizumab and bevacizumab samples, for which the 4D setup was mandatory to obtain an extensive and unambiguous identification, and accurate quantitation of unexpected high/low molecular weight species (HMWS and LMWS). In this specific context, monomeric conformers were detected by IM-MS as HMWS or LMWS. Altogether, our results emphasize the power of comprehensive 2D LC×LC setups hyphenated to IM×MS in nondenaturing conditions with unprecedented performance including: (i) maintaining optimal SEC performance (under classical nonvolatile salt conditions), (ii) performing online native MS identification, and (iii) providing IM-MS conformational characterization of all separated size variants.

Comprehensive two dimensional liquid chromatography as analytical strategy for pharmaceutical analysis.

Comprehensive on-line two-dimensional liquid chromatography (LCxLC) is expected to generate impressive peak capacities, which makes it a method of choice for the analysis of complex samples such as pharmaceuticals. A comparative study of different sets of chromatographic conditions including stationary phase, pH additive and organic modifier was carried out with two real pharmaceutical samples in order to find out the best analytical conditions for implementation of one or several generic on-line LCxLC separations. Our choice was based on the evaluation of both degree of orthogonality and practical sample peak capacity under linear gradient conditions. The potential of 190 combinations of chromatographic systems was compared. A set of 3 RPLCxRPLC configurations was found to be very attractive for both samples and in good agreement with the findings of a previous study carried out with 17 model compounds, thereby supporting the idea of using generic LCxLC conditions in the pharmaceutical area. The three selected 2D-systems were implemented for the on-line RPLCxRPLC-UV/MS analysis of two pharmaceutical samples. It was shown, for each sample, that these 2D-systems were able to generate an effective peak capacity close to 1000 in less than 50min. For each sample, baseline separation was obtained for every known compound and furthermore a large number of unknown impurities could also be separated and identified. Finally, in the proposed conditions, the total number of compounds detected was significantly improved from one RPLC separation to one RPLCxRPLC separation. Only a small additional gain was observed by performing a second RPLCxRPLC separation or even a third one.

An Online Four-Dimensional HIC×SEC-IM×MS Methodology for Proof-of-Concept Characterization of Antibody Drug Conjugates.

There are currently two main techniques allowing the analytical characterization of interchain cysteine-linked antibody drug conjugates (ADCs) under native conditions, namely, hydrophobic interaction chromatography (HIC) and native mass spectrometry (MS). HIC is a chromatographic technique allowing the evaluation of drug load profile and calculation of average drug-to-antibody ratio (DAR) in quality control laboratories. Native MS offers structural insights into multiple ADC critical quality attributes, thanks to accurate mass measurement. However, both techniques can lead to misinterpretations or incomplete characterization when used as standalone methods. Online coupling of both techniques can thus potentially be of great interest, but the presence of large amounts of nonvolatile salts in HIC mobile phases makes it not easily directly compatible with native MS. Here, we present an innovative multidimensional analytical approach combining comprehensive online two-dimensional (2D)-chromatography that consists of HIC and size-exclusion chromatography (SEC), to ion mobility and mass spectrometry (IM-MS) for performing analytical characterization of ADCs under nondenaturing conditions. This setup enabled comprehensive and streamlined characterization of both native and forced degraded ADC samples. The proposed 4D methodology might be more generally adapted for online all-in-one HIC×SEC-IM×MS analysis of single proteins or analysis of protein complexes in nondenaturing conditions.

Theoretical and experimental comparison of one dimensional versus on-line comprehensive two dimensional liquid chromatography for optimized sub-hour separations of complex peptide samples.

This study was devoted to the search for conditions leading to highly efficient sub-hour separations of complex peptide samples with the objective of coupling to mass spectrometry. In this context, conditions for one dimensional reversed phase liquid chromatography (1D-RPLC) were optimized on the basis of a kinetic approach while conditions for on-line comprehensive two-dimensional liquid chromatography using reversed phase in both dimensions (on-line RPLCxRPLC) were optimized on the basis of a Pareto-optimal approach. Maximizing the peak capacity while minimizing the dilution factor for different analysis times (down to 5min) were the two objectives under consideration. For gradient times between 5 and 60min, 15cm was found to be the best column length in RPLC with sub-2µm particles under 800bar as system pressure. In RPLCxRPLC, for less than one hour as first dimension gradient time, the sampling rate was found to be a key parameter in addition to conventional parameters including column dimension, particle size, flow-rate and gradient conditions in both dimensions. It was shown that the optimum sampling rate was as low as one fraction per peak for very short gradient times (i.e. below 10min). The quality descriptors obtained under optimized RPLCxRPLC conditions were compared to those obtained under optimized RPLC conditions. Our experimental results for peptides, obtained with state of the art instrumentation, showed that RPLCxRPLC could outperform 1D-RPLC for gradient times longer than 5min. In 60min, the same peak intensity (same dilution) was observed with both techniques but with a 3-fold lower injected amount in RPLCxRPLC. A significant increase of the signal-to-noise ratio mainly due to a strong noise reduction was observed in RPLCxRPLC-MS compared to the one in 1D-RPLC-MS making RPLCxRPLC-MS a promising technique for peptide identification in complex matrices.

Multiresidue analysis of aromatic organochlorines in soil by gas chromatography-mass spectrometry and QuEChERS extraction based on water/dichloromethane partitioning. Comparison with accelerated solvent extraction.

A novel multiresidue method was developed for the simultaneous analysis of 34 organochlorines, including chlorobenzenes, chlorophenols, chlorinated hydrocarbons and chlorinated olefins, in soil by GC-MS, using a QuEChERS-based extraction. The conventional QuEChERS method was optimised and, for the first time, the use of a non miscible-water solvent was required. The method was compared to ASE extraction, versatile technique widely used for the soils' extraction and QuEChERS-based method was shown to be the most efficient in terms of recoveries, simplicity and rapidity. For ASE, recoveries between 42% and 85% were obtained for the majority of the compounds. However, due to the high pressure, all volatile compounds were lost. In opposite, QuEChERS extraction allowed detection and quantification of all the compounds with recoveries between 60% and 100%. Moreover, no additional clean up by dispersive SPE on PSA was necessary, which allowed reducing the cost of the analysis. Performance of the method was assessed. The method was linear over the range of concentration of 10-5000 µg kg(-1). Precision, expressed as intra-day precision and inter-day variation was verified at three concentrations. Limits of detection were from 2 to 50 µg kg(-1) and limits of quantification from 7 to 170 µg kg(-1) for the majority of the compounds (chlorobenzenes and chlorinated hydrocarbons and olefins), except for chlorophenols. The method was further applied to different soils coming from a contaminated industrial site, where a new environmental remediation process, using phytoremediation, was tested. The results showed that the method could be applied to any kind of soils (mineral or organic) and was appropriate to very volatile compounds which were not available with conventional technique.

Rapid transmembrane diffusion of ceramide and dihydroceramide spin-labelled analogues in the liquid ordered phase.

In order to study the basic physical phenomena underlying complex lipid transbilayer movement in biological membranes, we have measured the transmembrane diffusion of spin-labelled analogues of sphingolipids in phosphatidylcholine (PC) large unilamellar vesicles in the absence or presence of cholesterol, going from a fluid ( liquid disordered) l(d), phase to a more viscous, liquid ordered (l(o)), phase. We have found cholesterol to reduce the transverse diffusion of glucosylceramide (GlcCer) and galactosylceramide (GalCer) in a concentration-dependent manner. However, surprisingly, we could neither detect any influence of cholesterol on the rapid flip-flop of ceramide nor on the flip-flop of dihydroceramide, for which the tau(1/2) of flip-flop remains in the order of 1 minute at 20 degrees C in the presence of cholesterol. As a consequence of rapid flip-flop of ceramide in both the l(o) and the l(d) phase, ceramide is likely to distribute between the two monolayers of a membrane, and could in principle partition into segregated domains in each side of the plasma membrane of eukaryotic cells.

The use of amphipols as universal molecular adapters to immobilize membrane proteins onto solid supports.

Because of the importance of their physiological functions, cell membranes represent critical targets in biological research. Membrane proteins, which make up approximately 1/3 of the proteome, interact with a wide range of small ligands and macromolecular partners as well as with foreign molecules such as synthetic drugs, antibodies, toxins, or surface recognition proteins of pathogenic organisms. Whether it is for the sake of basic biomedical or pharmacological research, it is of great interest to develop tools facilitating the study of these interactions. Surface-based in vitro assays are appealing because they require minimum quantities of reagents, and they are suitable for multiplexing and high-throughput screening. We introduce here a general method for immobilizing functional, unmodified integral membrane proteins onto solid supports, thanks to amphipathic polymers called "amphipols." The key point of this approach is that functionalized amphipols can be used as universal adapters to associate any membrane protein to virtually any kind of support while stabilizing its native state. The generality and versatility of this strategy is demonstrated by using 5 different target proteins, 2 types of supports (chips and beads), 2 types of ligands (antibodies and a snake toxin), and 2 detection methods (surface plasmon resonance and fluorescence microscopy).