Ga2O3 photocatalyzed on-line tagging of cysteine to facilitate peptide mass fingerprinting

beta-Ga2O3 is a wide-band-gap semiconductor having strong oxidation ability under light irradiation. Herein, the steel target plates modified with beta-Ga2O3 nanoparticles have been developed to carry out in-source photo-catalytic oxidative reactions for online peptide tagging during laser desorption/ ionization mass spectrometry (LDI-MS) analysis. Under UV laser irradiation, beta-Ga2O3 can catalyze the photo-oxidation of 2-methoxyhydroquinone added to a sample mixture to 2-methoxy benzoquinone that can further react with the thiol groups of cysteine residues by Michael addition reaction. The tagging process leads to appearance of pairs of peaks with an m/z shift of 138.1Th. This online labelling strategy is demonstrated to be sensitive and efficient with a detection-limit at femtomole level. Using the strategy, the information on cysteine content in peptides can be obtained together with peptide mass, therefore constraining the database searching for an advanced identification of cysteine-containing proteins from protein mixtures. The current peptide online tagging method can be important for specific analysis of cysteine-containing proteins especially the low-abundant ones that cannot be completely isolated from other high-abundant non-cysteine-proteins.

Miniaturized analytical systems for mass spectrometry-based protein studies

Mélanie Abonnenc

Current proteomic strategies depend strongly on the development of analytical methodologies and instrumentation. In parallel to the development of mass spectrometry (MS) - based proteomic workflows, microfluidic devices emerged in this field as a flexible tool for rapid and sensitive protein studies. In this context, the present work focuses on the development of miniaturized analytical systems for protein studies, especially by electrospray ionization mass spectrometric detection. Several approaches have been proposed to complement the mass spectrometric analysis of tryptic peptides using chemical tagging to isolate specific subclasses of proteins by affinity baits or for quantitation purposes. Optimization of a chemical tagging reaction in a sandwich mixer-reactor that consists of mixing two solutions providing the reactants in a channel, one central laminar flow being sandwiched between two outer flow solutions, was first investigated by finite-element method. The numerical simulations have highlighted the importance of positioning the reactant presenting the lowest diffusion coefficient close to the sidewall, as well as decreasing the outer flow velocities, to enhance a chemical reaction because of the parabolic flow profile across the microchannel. This optimization is even more relevant for consecutive reactions. As infusion-based electrospray microchips are known to present great advantages in terms of sensitivity compared to standard ionization source, an ESI emitter microchip based on polymer photo-ablation was designed. First, the hyphenation of the chip in a LC-MS workflow was successfully achieved and evaluated by comparison with a standard pneumatically-assisted ESI source. To perform enhanced on-chip post-column derivatization of peptides, a substitute design to the sandwich mixer-reactor was explored to improve reaction in a microchip before ESI – MS analysis. The electrospray micromixer chip includes a passive mixing unit to perturbate the flow along the microchannel. The mixing efficiency was demonstrated by fluorescence imaging, and on-chip chemical derivatization of peptides and kinetic studies before mass spectrometry were achieved. The on-line LC-MS derivatization of cysteinyl peptides was shown to provide a more confident and accurate protein identification by adding information on the peptidic sequence. Within the development of electrospray microchips for mass spectrometry, many efforts have been put forward in the hyphenation of functional units. Immobilization of functionalized stationary phase in microfluidic systems is widely used to achieve protein or peptide isolation, sample cleaning, separation or reaction. In this context, magnetic beads have proven to be quite useful compared to other methods, such as immobilized packed beads or monoliths, because of their high and reversible magnetization, and various surface functionalization. A polymer microchip including a magnetic track array was designed to focus the magnetic field generated by permanent magnets towards precise locations along the microchannel. A multi-plug magnetic bead capture was obtained and a significant increase of bead capture efficiency was demonstrated both numerically and experimentally, which is beneficial for affinity separation applications, as example. The research in microfluidic front-end devices for mass spectrometry is actually oriented to the development of multi-spray interfaces for high-throughput analysis. An electrospray microchip with a multi-track array was developed. The capability of the device to screen alternatively up to six samples and to perform relative quantification was assessed. Soft ionization techniques have revolutionized the analysis of proteins by mass spectrometry. In contrast with the major content of the thesis dealing with microfluidic systems for protein analysis, the last chapter presents a novel ambient ionization method, so-called membrane-desorption electrospray ionization (M-DESI). The principle is similar to the desorption electrospray ionization (DESI) method. But, in the M-DESI approach the sample is desorbed directly from a mesh membrane positioned vertically and in-axis between the ESI emitter and the mass spectrometer inlet. Analysis of peptides and proteins was demonstrated as a proof-of-concept, offering great perspectives for detection after separation on membrane.

EPFL2009

Mass Spectrometry Based Analysis and Bioanalysis

Hongyan Bi

MS (mass spectrometry) has been developed as an important analytical tool to study large molecules including polymer macromolecules and proteins. Its instrumentation technique including ionization methods, mass analyzer and detector systems has been greatly improved from last century to present. Bio-molecules and organic macromolecules tend to be fragile and fragment when they are ionized by conventional ionization methods. ESI (electrospray ionization) and MALDI (matrix assisted laser desorption/ionization) are two relatively tender ionization methods and suitable for the ionization of big molecules. The study on mechanism surround these two ionization methods is a hot issue. For MALDI, one wants to figure out what exactly happens when the laser irradiates the analyte, where in-plume reaction occurs. As an example, the photo-triggered reaction can induce the fragmentation of peptides or proteins. Proteolysis is a key step for proteomic study especially bottom-up proteomic strategy. Porous materials have been developed in last decades. The catalysis of porous materials to enzymatic reaction is a hot issue, and has been demonstrated in much published works. But the mechanism of how different porous materials affect the reaction is not completely clear yet. We compared several different porous material influences for the reaction efficiencies. A kinetic model was proposed to explain and foresee the proteolysis. The model was verified by integrated CE (capillary electrophoresis)/MS detection system, where a custom-designed CE-MS interface was utilized to collect the fraction of digests for off-line MALDI TOF (time-of-flight) MS analysis. From an instrumental point, we then introduce Al-foil as disposable and once-used MALDI-target substitute, by which the sample preparation for MALDI TOF MS analysis is greatly simplified. The regeneration procedure of typical target in each run and attrition of target-plate can be neglected after holding this Al-foil on a commercially available target plate. This substrate provides comparable detection limits (LOD) of protein and digests (peptides) as the typical stainless steel MALDI target plate. The pliability of Al-foil makes it easy to be printed. While nano-scale semiconductor materials such as TiO2 have been widely used for dye-sensitized solar cell study. Following the MALDI target holder study, TiO2 nanoparticles were prepared as optimized ink and fabricated on the Al-foil via screen- and rotogravure- printing procedures. The woven mesh and engraved image carrier can be designed and controlled for different print pattern. The given apparatus here can provide two different TiO2 functionalized Al-foil. The functionalized foil then was then taken for the analysis of matrix free LDI (laser desorption ionization) and the study of protein phosphorylation, which is one of the most common and important protein post-translational modifications that occur in animal cells. Since the typical organic matrix interference always makes noise signals in low mass/charge on a mass spectrum, especially below 1000 or 700 Th, matrix free LDI is introduced to study the low mass molecules or small molecules. In this context, besides TiO2 nanoparticles, we also investigate CdSe semiconductor quantum dots, and electrochemically polymerized film of an aromatic diamine for the LDI analysis. A standard peptide was applied for the feasibility study of LDI based on these substrates. Semiconductor quantum dots are also hot target molecules for the dye-sensitized solar cell. By comparing the photocurrents on different quantum dots fabricated electrodes, the electron transfer process inside was simulated to understand the kinetics, which is also expected to guide the LDI study including the mechanism involved.

EPFL2010

Polymer nanospray interfaces for on-line electrochemically induced modifications

Tatiana Rohner

The aim of this work has been to design, and fabricate a miniaturized electrospray-type interface for mass spectrometry analysis, which allows the nebulization of solutions by applying a high potential difference between the source and the mass spectrometer. The fabrication of the nanospray emitter is based on UV-laser photoablation of polymer substrates. Thick-film channel flow electrodes can be integrated in the microfluidic system, allowing the application of a high voltage to generate electrospray from the open outlet of the microchannel. The polymer-based interface first presented is characterized by a flat edge interface. However, a V-shaped nozzle has been found to present higher stability and ease of use. This design also allows the generation of spray by applying high voltage through an electrode located in a solution reservoir, thereby inducing an electroosmotic pumping. Taking advantage of the intrinsic electrolytic behavior of electrospray, a specific electrochemically induced tagging of free cysteine residues located in proteins can be performed on-line. A p-hydroquinone buffer is used to perform a continuous infusion of protein solution. When in contact with the high-voltage microelectrode, p-hydroquinone undergoes oxidation, thereby producing protonated p-benzoquinone. The latter reacts with free cysteine residues, following a Michael-type addition. The modification of the protein can be followed thanks to MS analyses. A complete study of the mechanisms involved in the electrochemically induced tagging with hydroquinone has been carried out by cyclic voltammetry and digital simulation. The process of the tagging consists of an ECE mechanism, since the formed adduct is stabilized in the reduced state, and is thus free to be oxidized. The rate constant of the chemical reaction has been extracted, by performing cyclic voltammetry studies and modeling the whole system by digital simulations. Finite element simulations have also permitted to highlight several mechanistic aspects of the electrotagging. The nanospray interface has been modeled as a channel-flow electrode cell. After validation of the model with respect to the experimental results, kinetics and mass transport conditions have been investigated. The flow profile, as well as the kinetic rate constant, has been shown to be of great importance in the electrochemically induced tagging. The use of a sacrificial electrode as metallic ion source to generate electrospray has also been investigated. Transition metal electrodes, i.e. copper, zinc, nickel and iron, as well as silver, were used to get on-line complexation with model peptides. It has been demonstrated that the use of in-reservoir sacrificial electrode was an efficient method to generate metal ions in order to form complexes with peptides, without the addition of metallic salts. These studies have demonstrated that polymer nanospray interfaces can be advantageously used as analytical devices, together with their primary function, i.e. ionization sources for protein analyses by mass spectrometry.

EPFL2003