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Native electrospray ionization - ion mobility mass spectrometry (N-ESI/IM-MS) is a powerful approach for low-resolution structural studies of DNAs in the free state and in complex with ligands. Solvent vaporization is coupled with proton transfers from ammonium ions to the DNA resulting in a reduction of the DNA charge. Here we provide insight on these processes by classical MD and QM/MM free energy calculations on the (GpCpGpApApGpC) heptamer, for which a wealth of experiments is available. Our multiscale simulations, consistent with experimental data, reveal a highly complex scenario: the proton either sits on one of the molecules or is fully delocalized on both, depending on the level of hydration of the analytes and on size of the droplets formed during the electrospray experiments. This work complements our previous study on theintramolecularproton transfer on the same heptamer occurring after the processes studied here, and, together, provide a first molecular view of proton transfer in N-ESI/IM-MS.TOC GRAPHICS

Chloroquine (CQ) is a first-choice drug against malaria and autoimmune diseases. It has been co-administered with zinc against SARS-CoV-2 and soon dismissed because of safety issues. The structural features of Zn-CQ complexes and the effect of CQ on zinc distribution in cells are poorly known. In this study, state-of-the-art computations combined with experiments were leveraged to solve the structural determinants of zinc-CQ interactions in solution and the solid state. NMR, ESI-MS, and X-ray absorption and diffraction methods were combined with ab initio molecular dynamics calculations to address the kinetic lability of this complex. Within the physiological pH range, CQ binds Zn2+ through the quinoline ring nitrogen, forming [Zn(CQH)Clx(H2O)3-x](3+)-x (x = 0, 1, 2, and 3) tetrahedral complexes. The Zn(CQH)Cl3 species is stable at neutral pH and at high chloride concentrations typical of the extracellular medium, but metal coordination is lost at a moderately low pH as in the lysosomal lumen. The pentacoordinate complex [Zn(CQH)(H2O)4]3+ may exist in the absence of chloride. This in vitro/in silico approach can be extended to other metal-targeting drugs and bioinorganic systems.

This webinar presents two examples of research done within BioExcel that demonstrates how hybrid quantum/classical (QM/MM) simulations using GROMACS, CP2K and CPMD performed on high-performance computing (HPC) resources can be used to elucidate biomolecular properties and mechanisms not accessible through a purely classical description. As well as giving insight into particular systems of interest – fluorescent proteins and proton dynamics in bioanalytical mass spectrometry – the webinar serves as an illustration of how the QM/MM approach generally and the above software in particular can be used productively to tackle biomolecular problems. Fluorescent proteins are the backbone for high-resolution biological imaging, but designing suitable proteins for specific experimental conditions is difficult. The goal of our work inBioExcelis to unlock the predictive power of MD and QM/MM simulations for predicting and optimizing the properties of such systems. We have thus developed a protocol to automatically compute the relevant quantities of these proteins and their mutants based on established atomistic simulation methods. The protocol combines (i) force field MD simulations with GROMACS, (ii) PMX and free-energy calculations, and (iii) QM/MM calculations to predict the thermostability (protein folding and oligomerization affinity) and photochemical properties (absorption spectrum and emission spectrum) of fluorescent proteins. Mass spectrometry has become a powerful tool in bioanalytics and can predict structural data of biomolecules at low resolution. Indeed, mass spectrometry requires far less samples than any other higher solution structural biology technique. However, the interpretation of the data is non-trivial. Beside structural changes upon passage from solution into the gas phase, one of the main difficulties in interpreting the data is the presence of quantum phenomena associated with proton transfer. Atomistic simulation gives invaluable insights on these processes. Here, we studied such phenomena by the application of high-performance computing QM/MM software developed within BioExcel to provide important information for the interpretation of mass spectrometry data.

In this work we present a high-throughput approach to the computation of absorption UV-Vis spectra tailored to mutagenesis studies. The scheme makes use of a single molecular dynamics trajectory of a reference (non-mutated) species. The shifts in absorption energy caused by a residue mutation are evaluated by building an effective potential of the environment and computing a correction term based on perturbation theory. The sampling is only performed in the phase space of the initial protein. We analyze the robustness of the method by comparing different approximations for the effective potential, the sampling of mutant residue geometries and observing the impact in the prediction of both bathocromic and hypsochromic shifts. As a test subject, we consider a red fluorescent protein variant with potential biotechnological applications.

Electronic absorption spectra are oftentimes used to identify reaction intermediates or substrates/products in enzymatic systems, as long as absorption bands can be unequivocally assigned to the species being studied. The latter task is far from trivial given the transient nature of some states and the complexity of the surrounding environment around the active site. To identify unique spectral fingerprints, controlled experiments with model compounds have been used in the past, but even these can sometimes be unreliable. Circular dichroism (CD) and ultraviolet-visible spectra have been tools of choice in the study of the rich chemistry of thiamin diphosphate-dependent enzymes. In this study, we focus on the Zymomonas mobilis pyruvate decarboxylase, and mutant analogues thereof, as a prototypical representative of the thiamin diphosphate (ThDP) enzyme superfamily. Through the use of electronic structure methods, we analyze the nature of electronic excitations in the cofactor. We find that all the determining CD bands around the 280-340 nm spectral range correspond to charge-transfer excitations between the pyrimidine and thiazolium rings of ThDP, which, most likely, is a general property of related ThDP-dependent enzymes. While we can confirm the assignments of previously proposed bands to chemical states, our calculations further suggest that a hitherto unassigned band of enzyme-bound ThDP reports on the ionization state of the canonical glutamate that is required for cofactor activation. This finding expands the spectroscopic "library" of chemical states of ThDP enzymes, permitting a simultaneous assignment of both the cofactor ThDP and the activating glutamate. We anticipate this finding to be helpful for mechanistic analyses of related ThDP enzymes.

The underlying molecular mechanisms of cooperativity and allosteric regulation are well understood for many proteins, with haemoglobin and aspartate transcarbamoylase serving as prototypical examples1,2. The binding of effectors typically causes a structural transition of the protein that is propagated through signalling pathways to remote sites and involves marked changes on the tertiary and sometimes even the quaternary level1-5. However, the origin of these signals and the molecular mechanism of long-range signalling at an atomic level remain unclear5-8. The different spatial scales and timescales in signalling pathways render experimental observation challenging; in particular, the positions and movement of mobile protons cannot be visualized by current methods of structural analysis. Here we report the experimental observation of fluctuating low-barrier hydrogen bonds as switching elements in cooperativity pathways of multimeric enzymes. We have observed these low-barrier hydrogen bonds in ultra-high-resolution X-ray crystallographic structures of two multimeric enzymes, and have validated their assignment using computational calculations. Catalytic events at the active sites switch between low-barrier hydrogen bonds and ordinary hydrogen bonds in a circuit that consists of acidic side chains and water molecules, transmitting a signal through the collective repositioning of protons by behaving as an atomistic Newton's cradle. The resulting communication synchronizes catalysis in the oligomer. Our studies provide several lines of evidence and a working model for not only the existence of low-barrier hydrogen bonds in proteins, but also a connection to enzyme cooperativity. This finding suggests new principles of drug and enzyme design, in which sequences of residues can be purposefully included to enable long-range communication and thus the regulation of engineered biomolecules.