Structural stability of electrosprayed proteins: temperature and hydration effects

Abstract

Electrospray ionization is a gentle method for sample delivery, routinely used in gas-phase studies of proteins. It is crucial for structural investigations that the protein structure is preserved, and a good understanding of how structure is affected by the transition to the gas phase is needed for the tuning of experiments to meet that requirement. Small amounts of residual solvent have been shown to protect the protein, but temperature is important too, although it is not well understood how the latter affects structural details. Using molecular dynamics we have simulated four sparingly hydrated globular proteins (Trp-cage; Ctf, a C-terminal fragment of a bacterial ribosomal protein; ubiquitin; and lysozyme) in vacuum starting at temperatures ranging from 225 K to 425 K. For three of the proteins, our simulations show that a water layer corresponding to 3 A preserves the protein structure in vacuum, up to starting temperatures of 425 K. Only Ctf shows minor secondary structural changes at lower starting temperatures. The structural conservation stems mainly from interactions with the surrounding water. Temperature scales in simulations are not directly translatable into experiments, but the wide temperature range in which we find the proteins to be stable is reassuring for the success of future single particle imaging experiments. The water molecules aggregate in clusters and form patterns on the protein surface, maintaining a reproducible hydrogen bonding network. The simulations were performed mainly using OPLS-AA/L, with cross checks using AMBER03 and GROMOS96 53a6. Only minor differences between the results from the three different force fields were observed.

Do Charge State Signatures Guarantee Protein Conformations?

“…Early studies of small proteins [11,41] and peptides [42,43] have revealed candidate structures for these gas-phase species that have been informative both in terms of folding and the effects of dehydration. In vacuo MD simulations of four globular proteins revealed residual water molecules, aggregating on the protein surface, which assist the preservation of folded structure [44], a result in accord with earlier studies [11]. Limited conformational changes were observed upon dehydration; these included the inward-folding of hydrophilic side chains, which tended to form new hydrogen bonds.…”

Molecular Simulation-Based Structural Prediction of Protein Complexes in Mass Spectrometry: The Human Insulin Dimer

“…Computational approaches such as ours or those by other groups [24], [56], [75], may therefore be instrumental to understand how desolvation affects the structure and stability of other protein complexes. Such simulations may establish whether the present findings can be generalized.…”

Organic molecules on the surface of water droplets – an energetic perspective

“…We have previously studied process of "drying" proteins in electrospray 68,69 and the encapsulation of proteins in lipid aggregates [70][71][72] , the Konermann group have focused on the related process of electrospray charge state distribution 73,74 and more recently on the effect of protein conformation on drying in electrospray 75 . Existing models of the electrospray process, in particular the charge residue model 76,77 assume a biomolecule will be completely immersed in solvent, and it has even been suggested that this feature could be exploited for protecting biomolecules in droplets in X-ray free electron lasers [78][79][80] .…”