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WIREs Comput Mol Sci
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Proteins in the gas phase

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Abstract Proteins are complex macromolecules that evolved over billions of years to be active in aqueous solution. Water is a key element that stabilizes their structure, and most structural studies on proteins have thus been carried out in aqueous environment. However, recent experimental approaches have opened the possibility to gain structural information on proteins from gas‐phase measurements. The obtained results revealed significant structural memory in proteins when transferred from water to the gas phase. However, after several years of experimental and theoretical research, the nature of the structural changes induced by vaporization, the exact characteristics of proteins in the gas phase, and the physicochemical forces stabilizing dehydrated proteins are still unclear. We will review here these issues using both experimental and theoretical sources of information. © 2012 John Wiley & Sons, Ltd. This article is categorized under: Molecular and Statistical Mechanics > Molecular Interactions

Charged droplet emission in electrospray. (a) Photographs showing droplet emission from the electrospray tip in the burst mode. (b) Close‐up on a charged filament that break up through varicose instabilities. Droplets of different sizes are produced. The beforelast droplet undergoes jet fission due to charge instability and produces secondary droplets.14 (c–g) Atomic configurations are taken from a molecular dynamics simulation of a 10 nm neutral droplet of NaI in formamide, following the application of a uniform external electric field of 0.625 V/nm. Na+ and I ions are shown as red and yellow spheres respectively, and formamide molecules are in blue.15 (Reproduced with permission from Ref 15. Copyright 2008, American Chemical Society.)

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Fast collapse of polar surface area upon transfer into vacuum (at 10 nanosecond) and close to complete recovery after rehydration (110 nanosecond). Structural changes occur within the first few nanoseconds after transfer to vacuum. (Reproduced with permission from Ref 35. Copyright 2009, Elsevier.)

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Sketch of the expected changes to a protein's free energy surface upon transfer from aqueous (blue) to the gas phase (magenta, yellow). Extended (partially (A) and completely (U) unfolded) and reversed structures become accessible in absence of the hydrophobic effect. Whether or not they become populated depends on the charge state magenta vs. yellow) and whether the unfolding barrier can be surmounted. Coulomb repulsion (ΔEQ) destabilizes compact structures [in vacuo native structure (IVNS), inside‐out] and drives highly charged ions into extended states, low‐charge ions may fold into the hypothetical inside‐out conformation; however, the folding pathway (‡) may be entropically blocked.

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Laser induced ablation from a solid crystalline support matrix (a) and from a liquid water beam (b). UV laser cause photothermal (a left) and photochemical (a middle, right) excitation, the latter leads to bond cleavage formation of ions and radicals. Computer simulations of the supercritical phase expansion observed in laser‐induced liquid beam desorption and ionization revealed transient water bridges that facilitate ion recombination (c). [Figure 3(a) reproduced with permission from Ref 38. Copyright 2007, Elsevier. Figure 3(b) reproduced with permission of Prof. Bernd Abel. Figure 3(c) reproduced with permission from Ref 45. Copyright 2009, Dr. Frank Wiederschein.]

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Final production of desolvated macromolecule ions in electrospray. (a, b) representative electrospray mass spectra of a folded protein (panel a: myoglobin at pH 7) and of an unfolded protein (panel b: myoglobin at pH 2). (c) Cartoon illustrating the charged residue mechanism (CRM). (d) Cartoon illustrating the ion evaporation mechanism (IEM). (e, f) Simulation data from molecular dynamics runs for (e) a folded/hydrophobic polymer and (f) an unfolded/hydrophobic polymer, each in 1000 water molecules and excess NH4+ as charge carriers. (Reprodcued with permission from Ref 28. Copyright 2012, American Chemical Society.)

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