Which electrospray-based ionization method best reflects protein-ligand interactions found in solution? A comparison of ESI, nanoESI, and ESSI for the determination of dissociation constants with mass spectrometry

We present a comparison of three different electrospray-based ionization techniques for the investigation of noncovalent complexes with mass spectrometry. The features and characteristics of standard electrospray ionization (ESI), chip-based nanoESI, and electrosonic spray ionization (ESSI) mounted onto a hybrid quadrupole time-of-flight mass spectrometer were compared in their performance to determine the dissociation constant (K D) of the model system hen egg white lysozyme (HEWL) binding to N,N′,N″-triacetylchitotriose (NAG3). The best K D value compared with solution data were found for ESSI, 19.4 ± 3.6 µM. Then, we determined the K Ds of the two nucleotide binding sites of adenylate kinase (AK), where we obtained K Ds of 2.2 ± 0.8 µM for the first and 19.5 ± 8.0 µM for the second binding site using ESSI. We found a weak charge state dependence of the K D for both protein-ligand systems, where for all ionization techniques the K D value decreases with increasing charge state. We demonstrate that ESSI is very gentle and insensitive to instrumental parameters, and the K D obtained is in good agreement with solution phase results from the literature. In addition, we tried to determine the K D for the lymphocyte-specific kinase LCK binding to a kinase inhibitor using nanoESI due to the very low amount of sample available. In this case, we found K D values with a strong charge state dependence, which were in no case close to literature values for solution phas

Shrinking droplets in electrospray ionization and their influence on chemical equilibria

We investigated how chemical equilibria are affected by the electrospray process, using simultaneous in situ measurements by laser-induced fluorescence (LIF) and phase Doppler anemometry (PDA). The motivation for this study was the increasing number of publications in which electrospray ionization mass spectrometry is used for binding constant determination. The PDA was used to monitor droplet size and velocity, whereas LIF was used to monitor fluorescent analytes within the electrospray droplets. Using acetonitrile as solvent, we found an average initial droplet diameter of 10 µm in the electrospray. The PDA allowed us to follow the evolution of these droplets down to a size of 1 µm. Rhodamine B-sulfonylchloride was used as a fluorescent analyte within the electrospray. By spatially resolved LIF it was possible to probe the dimerization equilibrium of this dye. Measurements at different spray positions showed no influence of the decreasing droplet size on the monomer-dimer equilibrium. However, with the fluorescent dye pair DCM and oxazine 1 it was shown that a concentration increase does occur within electrosprayed droplets, using fluorescence resonance energy transfer as a probe for the average pair distanc

Identification of Endocrine-Disrupting Compounds Using Nanoelectrospray Ionization Mass Spectrometry

A method using chip-based nanoelectrospray mass spectrometry (nanoESI-MS) is described to detect noncovalent ligand binding to the human estrogen receptor alpha ligand-binding domain (hER? LBD). This system represents an important environmental interest, because a wide variety of molecules, known as endocrine-disrupting compounds (EDCs), can bind to the estrogen receptor (ER) and induce adverse health effects in wildlife and humans. An efficient analytical method is therefore required to identify EDCs and characterize their solution-phase binding affinity and character (i.e. agonist or antagonist). Using proper experimental conditions, the nanoESI-MS approach allowed the detection of specific ligand interactions with hER? LBD. The best approach to evaluate solution-binding affinity by nanoESI-MS was to perform competitive binding experiments with 17?-estradiol (E2) as a reference ligand. Among the ligands tested, the relative binding affinity for hER? LBD measured by nanoESI-MS was 4-hydroxytamoxifen ? diethylstilbestrol > E2 ? genistein ? bisphenol A, consistent with the order of the binding affinities in solution. To discern agonist from antagonist, we used the specificity of a coactivator peptide for agonist-bound receptor. A specific coactivator-hER? LBD complex was detected only in the presence of an agonist ligand. Therefore, the specificity of nanoESI-MS combined with its speed (1 min/ligand), low sample consumption (90 pmol protein/ligand), and its sensitivity for ligand (30 ng/ml) demonstrates that this method is promising for the identification and characterization of suspected ER ligands in a high-throughput manner

Shrinking Droplets in Electrospray Ionization and Their Influence on Chemical Equilibria

ISSN:1879-1123ISSN:1044-030

Estrogen receptor–ligand complexes measured by chip-based nanoelectrospray mass spectrometry: An approach for the screening of endocrine disruptors

In the present report, a method based on chip-based nanoelectrospray mass spectrometry (nanoESI-MS) is described to detect noncovalent ligand binding to the human estrogen receptor α ligand-binding domain (hERα LBD). This system represents an important environmental interest, because a wide variety of molecules, known as endocrine disruptors, can bind to the estrogen receptor (ER) and induce adverse health effects in wildlife and humans. Using proper experimental conditions, the nanoESI-MS approach allowed for the detection of specific ligand interactions with hERα LBD. The relative gas-phase stability of selected hERα LBD–ligand complexes did not mirror the binding affinity in solution, a result that demonstrates the prominent role of hydrophobic contacts for stabilizing ER–ligand complexes in solution. The best approach to evaluate relative solution-binding affinity by nanoESI-MS was to perform competitive binding experiments with 17β-estradiol (E2) used as a reference ligand. Among the ligands tested, the relative binding affinity for hERα LBD measured by nanoESI-MS was 4-hydroxtamoxifen ≈ diethylstilbestrol > E2 >> genistein >> bisphenol A, consistent with the order of the binding affinities in solution. The limited reproducibility of the bound to free protein ratio measured by nanoESI-MS for this system only allowed the binding constants (Kd) to be estimated (low nanomolar range for E2). The specificity of nanoESI-MS combined with its speed (1 min/ligand), low sample consumption (90 pmol protein/ligand), and its sensitivity for ligand (30 ng/mL) demonstrates that this technique is a promising method for screening suspected endocrine disrupting compounds and to qualitatively evaluate their binding affinity

FLEUR

FLEUR is an all-electron DFT code based on the full-potential linearized augmented plane-wave method (FLAPW). It is mainly developed at the Forschungsentrum Jülich, Germany and available for the materials research community