Stopped-flow kinetic analysis using Hadamard transform time-of-flight mass spectrometry

RECENT ADVANCES IN INSTRUMENTAL APPROACHES TO TIME-OF-FLIGHT MASS SPECTROMETRY

Time-of-flight mass spectrometry (TOFMS) is one of the simplest and most powerful approaches for mass spectrometry. Realization of the advantages inherent in TOFMS requires innovation in the theory and practice of the technique. Instrumental developments, in turn, create new capabilities that enable applications in chemical measurement. This review focuses on the recent advances in TOFMS instrumentation. New strategies for ion acceleration, multiplexed detection, miniaturized TOFMS instruments, approaches to extend the length of ion flight, and novel ion detection technologies are reviewed. Techniques that change the basic paradigm of TOFMS by measuring m/z based on ion flight distance are considered, as are applications at the frontiers of instrumental performance. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Mechanistic understanding of catalysis by combining mass spectrometry and computation

The combination of mass spectrometry and computational chemistry has been proven to be powerful for exploring reaction mechanisms. The former provides information of reaction intermediates, while the latter gives detailed reaction energy profiles.

Mass spectrometry-based monitoring of millisecond protein-ligand binding dynamics using an automated microfluidic platform

Characterizing protein-ligand binding dynamics is crucial for understanding protein function and for developing new therapeutic agents. We present a novel microfluidic platform that features rapid mixing of protein and ligand solutions, variable incubation times, and an integrated electrospray ionization source for mass spectrometry-based monitoring of protein-ligand binding dynamics. This platform offers many advantages, including solution-based binding, label-free detection, automated operation, rapid mixing, and low sample consumption.

Dissociation kinetics of the streptavidin-biotin interaction measured using direct electrospray ionization mass spectrometry analysis

Dissociation rate constants (k (off)) for the model high affinity interaction between biotin (B) and the homotetramer of natural core streptavidin (S(4)) were measured at pH 7 and temperatures ranging from 15 to 45 °C using electrospray ionization mass spectrometry (ESI-MS). Two different approaches to data analysis were employed, one based on the initial rate of dissociation of the (S(4) + 4B) complex, the other involving nonlinear fitting of the time-dependent relative abundances of the (S(4) + iB) species. The two methods were found to yield k (off) values that are in good agreement, within a factor of two. The Arrhenius parameters for the dissociation of the biotin-streptavidin interaction in solution were established from the k (off) values determined by ESI-MS and compared with values measured using a radiolabeled biotin assay. Importantly, the dissociation activation energies determined by ESI-MS agree, within 1 kcal mol(-1), with the reported value. In addition to providing a quantitative measure of k (off), the results of the ESI-MS measurements revealed that the apparent cooperative distribution of (S(4) + iB) species observed at short reaction times is of kinetic origin and that sequential binding of B to S(4) occurs in a noncooperative fashion with the four ligand binding sites being kinetically and thermodynamically equivalent and independent.

Time-resolved mass spectrometry for monitoring millisecond time-scale solution-phase processes

Many chemical and biochemical reactions equilibrate within a few seconds of initiation under "native" conditions. To understand the microscopic processes underlying these reactions, the most direct approach is to monitor the reaction as equilibrium is established (i.e. the reaction kinetics). However, this requires the ability to characterize the reaction mixture on the millisecond time-scale. In this review, we survey the contributions of time-resolved mass spectrometry (TR-MS) to the characterization of millisecond time-scale (bio)chemical processes, with a focus on biochemical applications. Compared to conventional time-resolved techniques, which use optical detection, the primary advantage of TR-MS is the ability to detect virtually all reactive species simultaneously. This provides a singularly high detail account of the reaction without the need for a chromophoric change on turnover or artificial chromophoric probes. To provide millisecond time-resolution, TR-MS set-ups usually employ continuous-flow rapid mixers, corresponding either to a fixed "tee" that provides a single reaction time-point or an adjustable position mixer that allows continuous reaction monitoring. TR-MS has been used to monitor processes with rates up to 500 s(-1) and to provide a detailed account of complex reactions involving 10 co- populated species. This corresponds to significantly lower time-resolution than optical methods, which can measure rates in excess of 900 s(-1) under ideal conditions, but substantially more detail (optical studies are typically limited to one or two analytes). TR-MS has been implemented on a number of platforms, including capillary and microfluidic set-ups. Capillary-based implementations are straightforward to fabricate and provide the most efficient rapid mixing. Microfluidic implementations have also been devised to enable multi-step experimental workflows that incorporate TR-MS. As a general method for time-resolved measurements, the applications for TR-MS are wide ranging. TR-MS has been used to identify intermediates in organic reactions, reveal protein (un)folding mechanisms, monitor enzyme catalysis in the pre-steady-state and, in conjunction with hydrogen-deuterium exchange, characterize protein conformational dynamics. While not without limitations, TR-MS represents a powerful alternative for measuring solution phase processes on the millisecond time-scale and a new, promising approach for revealing mechanistic details in (bio)chemical reactions.