Quantitative determination of saccharide surfactants in protein samples by liquid chromatography coupled to electrospray ionization mass spectrometry

Quantification of Detergents Complexed with Membrane Proteins

Most membrane proteins studies require the use of detergents, but because of the lack of a general, accurate and rapid method to quantify them, many uncertainties remain that hamper proper functional and structural data analyses. To solve this problem, we propose a method based on matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) that allows quantification of pure or mixed detergents in complex with membrane proteins. We validated the method with a wide variety of detergents and membrane proteins. We automated the process, thereby allowing routine quantification for a broad spectrum of usage. As a first illustration, we show how to obtain information of the amount of detergent in complex with a membrane protein, essential for liposome or nanodiscs reconstitutions. Thanks to the method, we also show how to reliably and easily estimate the detergent corona diameter and select the smallest size, critical for favoring protein-protein contacts and triggering/promoting membrane protein crystallization, and to visualize the detergent belt for Cryo-EM studies.

Interfacing multistage mass spectrometry with liquid chromatography or ion mobility separation for synthetic polymer analysis

Synthetic polymers are naturally mixtures of homologs, even in pure form. More complexity is introduced by the presence of different comonomers, end groups and/or macromolecular architectures. The analysis of such systems is substantially facilitated by interfacing mass spectrometry (MS), which disperses based on mass, with an additional level of separation involving either interactive liquid chromatography (LC) or ion mobility (IM) spectrometry, both of which are readily coupled online with electrospray ionization and MS detection. IM-MS separates in the gas phase, post-ionization and, therefore, is ideally suitable for labile and reactive polymers. Its usefulness is illustrated with the characterization of non-covalent siloxane-saccharide complexes, metallosupramolecular assemblies and an air- and moisture-sensitive inorganic polymer, poly(dichlorophosphazene). Conversely, LC-MS which separates in solution phase, before ionization, is most effective for the analysis of polymeric mixtures whose components differ in polarity. Interactive LC conditions can be optimized to disperse by the content of hydrophobic units, as is demonstrated for amphiphilic polyether copolymers and sugar-based nonionic surfactant blends. Both LC-MS and IM-MS can be extended into a third dimension by tandem mass spectrometry (MS(2)) studies on select oligomers, in order to obtain insight into individual end groups and isomeric architectures, comonomer sequences and degree of substitution, for example, by hydrophobic functionalities.

Gel chromatography and analytical ultracentrifugation to determine the extent of detergent binding and aggregation, and Stokes radius of membrane proteins using sarcoplasmic reticulum Ca2+-ATPase as an example

For structural studies of integral membrane proteins, including their 3D crystallization, the judicious use of detergent for solubilization and purification is required. Detergent binding by the solubilized protein is an important parameter to determine the hydrodynamic properties in terms of size and aggregational (monomeric/oligo(proto)meric) state of the protein. Detergent binding can be measured by gel filtration chromatography under equilibrium conditions and after separation from mixed micelles of solubilized lipid and detergent. Using sarcoplasmic reticulum Ca(2+)-ATPase as an example, we demonstrate in this protocol complete procedures for measurement of detergent binding using (i) radiolabeled n-dodecyl-beta-D-maltoside (DM) or (ii) from measurements of the increase in refractive index due to the presence of bound detergent on the protein. The latter measurement can also be performed by sedimentation velocity (SV) analysis in the analytical ultracentrifuge which in addition allows determination of the sedimentation coefficient. In combination with estimation of Stokes radius by gel filtration calibration, the molecular mass and asymmetry of the solubilized protein can be calculated. In the proposed protocols, the gel chromatographic procedures require 1 d; SV experiments are performed just after size exclusion. The whole time for these experiments is 24 h. Data analysis of analytical ultracentrifugation requires a couple of days.

Method for Lipoprotein(a) Density Profiling by BiEDTA Differential Density Lipoprotein Ultracentrifugation

In this article, we demonstrate the analytical power of linking density gradient ultracentrifugation with affinity separations. Here we address some of the analytical challenges in the study of lipoprotein(a), (Lp(a)). The mean density distribution of Lp(a) was determined by a differential density lipoprotein profile (DDLP). For DDLP, the lipoprotein density distribution of a serum sample with elevated Lp(a) levels was determined by ultracentrifugation using a BiEDTA complex as a density gradient. Lp(a) was removed from a second aliquot of the same serum sample by carbohydrate affinity using wheat germ agglutinin (WGA). WGA was demonstrated to have high specificity for Lp(a) in a serum sample. This sample was ultracentrifuged to obtain a lipoprotein density distribution in the absence of Lp(a). A DDLP was obtained after subtracting the Lp(a)-depleted lipoprotein density profile from the untreated lipoprotein density profile. The DDLP methodology reported herein gives relevant information of the lipoproteins in serum such as density, isoform, and subclass characteristics. Lp(a) was quantitatively isolated from serum with a recovery efficiency of 82%. Lp(a) was purified by ultracentrifugation. Lp(a) retained its inherent density (1.086 g/mL) and immunoreactivity. The major outcome of this research was the effectiveness of using affinity separations coupled with density ultracentrifugation for the isolation of pure Lp(a) from serum and its isoform characterization based on density by DDLP.

Resolution of overlapped non-absorbing and absorbing solutes using either an absorption null-balance detection window or multivariate deconvolution applied to capillary electrophoresis of anionic surfactants

Non-absorbing alkyl ether sulfates (AES) can be separated using anthraquinone-2-carboxylic acid (AQCA) as a probe; however, absorbing alkyl benzene sulfonates (ABS), if present, interfere indirect detection of most AES oligomers. Overcoming of this interference, as well as the simultaneous characterisation and evaluation of AES, fatty acids and ABS, was accomplished by using a diode-array detector and the procedures here discussed. First, it was shown that ABS can be made undetectable by using a 9 nm wide and 227 nm centred charge-absorptivity null-balance detection window (NBDW), where its contribution to the absorbance cancels the dilution effects that its presence induces on the signal of the background chromophore (BGC). Two other procedures, not requiring any prior knowledge on the nature of the absorbing interference, were also addressed. In the first one, the NBDW procedure was emulated by software, by treating the time-wavelength data matrix stored during the experimental run, and in the second one, both the ABS and BGC spectra, and the concentration profiles of ABS and the non-absorbing solutes, were recovered by orthogonal projection approach (OPA) and alternating least squares (ALS). The OPA-ALS processing provided the deconvolved signals and the wavelengths required to implement the experimental and software-emulated NBDW procedures. A composite ABS spectrum and a mixed concentration profile of the non-absorbing solutes, that involves mutual ABS-BGC dilution effects are enclosed in the OPA-ALS straightforward solutions. The pure spectra and concentration profiles were finally retrieved by crossed orthogonalisation. For the NBDW procedures, the limits of detection (S/N = 3) for AES oligomers overlapped by 1500 microg ml(-1) ABS were of ca. 10 microM AES. Using decyl sulfate as internal standard, the relative standard deviation for AES in an ABS containing industrial sample was 4.5%. The procedures here described are useful to remove the interference produced by any absorbing solute when overlapped with indirectly detected solutes in both capillary electrophoresis (CE) and HPLC.