Dipole orientation of hydrated gas phase proteins

In the aerosolization of single proteins from solution, the proteins may be covered by a layer of water. This is relevant to consider in sample delivery for single particle imaging (SPI) with X-ray free-electron lasers. Previous studies suggest that the presence of a 3 Å water layer stabilizes the molecular structure and decreases structural heterogeneity which is important since it facilitates the structure determination in SPI. It has also been shown that SPI would benefit from the possibility of controlling the particle orientation in the interaction region. It has been proposed that such control would be possible by applying a DC electric field that interacts with the intrinsic dipole of the particle. This study investigates how SPI experiments, including dipole orientation, would be affected by the presence of a hydration layer covering the proteins. We investigated this by performing classical MD simulations of a globular protein in gas phase interacting with an external electric field. Two hydration levels were used: a fully desolvated molecule and one with a water layer corresponding to 3 Å covering the proteins surface. Our simulations show that a water layer enables the molecules to orient at lower field amplitudes, and on shorter time scales, as compared to the desolvated case. We also see a marginally larger stability of the molecular structure in the hydrated case at field strengths below 2 V nm^-1. The presence of a water layer, in combination with an electric field, also tend to stabilize the dipole axis significantly within the molecular structure.