The low-density/high-density liquid phase transition for model globular proteins

The effect of molecule size (excluded volume) and the range of interaction on the surface tension, phase diagram and nucleation properties of a model globular protein is investigated using a combinations of Monte Carlo simulations and finite temperature classical Density Functional Theory calculations. We use a parametrized potential that can vary smoothly from the standard Lennard-Jones interaction characteristic of simple fluids, to the ten Wolde-Frenkel model for the effective interaction of globular proteins in solution. We find that the large excluded volume characteristic of large macromolecules such as proteins is the dominant effect in determining the liquid-vapor surface tension and nucleation properties. The variation of the range of the potential only appears important in the case of small excluded volumes such as for simple fluids. The DFT calculations are then used to study homogeneous nucleation of the high-density phase from the low-density phase including the nucleation barriers, nucleation pathways and the rate. It is found that the nucleation barriers are typically only a few $k_{B}T$ and that the nucleation rates substantially higher than would be predicted by Classical Nucleation Theory.Comment: To appear in Langmui

Electron beam-induced electrostatic charging causes spectral changes of an insulating electrochromic material

Electrostatic charging of electrically insulating samples upon electron irradiation plays a large role in the analysis and processing of materials in biosciences and the semiconductor industry. We have analyzed spectral changes of purple membranes (PMs) containing bacteriorhodopsin, an electrochromic biological material, upon irradiation with electrons at an acceleration voltage of 30kV. We observed a bathochromic shift in the UV/VIS spectrum of PM films, due to internal electric fields generated by charges injected into the films. The experiments demonstrate that spectroscopic changes, accompanying electron beam-induced charge injection into electrochromic materials, enable quantitative analysis of electrostatic charging of insulators upon electron irradiation by optical spectroscopy