Abstract
Understanding the water sorption behavior of protein powders is important in applications such as the preservation of protein-based pharmaceuticals. Most globular proteins exhibit a characteristic sigmoidal water adsorption isotherm at ambient conditions. However, it is not well understood how water sorption behavior is influenced by intrinsic factors that are related to structural properties of proteins. We investigate computationally how structural constraints on proteins influence the water sorption isotherms of amorphous protein powders. Specifically, we study the effects of non-local disulfide linkages and backbone connectivity using pheromone ER-23 and lysozyme as model proteins. We find that non-local disulfide linkages can significantly restrict structural changes during hydration and dehydration, and this in turn greatly reduces the extent of hysteresis between the adsorption and desorption branches. Upon removing the backbone connectivity by breaking all peptide bonds in lysozyme, we find that the hysteresis shifts towards the lower humidity regime, and the water uptake capacity is significantly enhanced. We attribute these changes to the higher aggregation propensity of the constraint-free amino acids in dehydrated condition, and the formation of a spanning water network at high hydration levels.
Highlights
Acquiring fundamental understanding of protein-water interactions is important in numerous practical applications involving protein-based products, such as pharmaceuticals and biomaterials[1,2,3,4,5,6]
We chose pheromone ER-2327, a cysteine-rich signaling protein from the ciliated protozoan Euplotes raikovi, as our model system to study the effects of disulfide linkages on water sorption behavior
The structure and sequence of pheromone ER-23 are shown in Fig. 1a,b, respectively
Summary
Acquiring fundamental understanding of protein-water interactions is important in numerous practical applications involving protein-based products, such as pharmaceuticals and biomaterials[1,2,3,4,5,6]. One way of studying protein-water interactions systematically for a range of hydration levels of practical interest is through water sorption experiments[1] The goal of such experiments is to understand the equilibrium relationship between the moisture content of a protein powder or protein-based matrix and the relative humidity of the surrounding vapor at a given temperature and pressure. The water sorption isotherms of a variety of protein-based products have been measured across a range of conditions, in order to understand the effects of process factors that could potentially affect protein-water interactions[4,5,6, 9,10,11]. Recent advances in simulation methods[23], have enabled the efficient simulation of water sorption isotherms of flexible protein matrices using an exclusively MD-based approach
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