Low temperature heat capacity of water clusters

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

The mechanism by which arthropods (e.g., spiders and many insects) can produce silk fibres from an aqueous protein (fibroin) solution has remained elusive, despite much scientific investigation. In this work, we used several techniques to explore the role of a hydration shell bound to the fibroin in native silk feedstock (NSF) from Bombyx mori silkworms. Small angle X-ray and dynamic light scattering (SAXS and DLS) revealed a coil size (radius of gyration or hydrodynamic radius) around 12 nm, providing considerable scope for hydration. Aggregation in dilute aqueous solution was observed above 65 °C, matching the gelation temperature of more concentrated solutions and suggesting that the strength of interaction with the solvent (i.e., water) was the dominant factor. Infrared (IR) spectroscopy indicated decreasing hydration as the temperature was raised, with similar changes in hydration following gelation by freezing or heating. It was found that the solubility of fibroin in water or aqueous salt solutions could be described well by a relatively simple thermodynamic model for the stability of the protein hydration shell, which suggests that the affected water is enthalpically favoured but entropically penalised, due to its reduced (vibrational or translational) dynamics. Moreover, while the majority of this investigation used fibroin from B. mori, comparisons with published work on silk proteins from other silkworms and spiders, globular proteins and peptide model systems suggest that our findings may be of much wider significance.

Phase transitions in free water nanoparticles. Theoretical modeling of [H2O]48 and [H2O]118

Classical parallel-tempering Monte Carlo simulations of [H2O]48 and [H2O]118 have been performed in the isothermal-isobaric ensemble and a two-dimensional multiple-histogram method has been used to calculate the heat capacity of the two clusters. A semiempirical procedure is proposed for the inclusion of quantum effects and transformed heat capacity profiles are compared with state-of-the-art experimental data [C. Hock et al., Phys. Rev. Lett., 2009, 103, 073401]. A very good agreement is achieved. A detailed analysis of the simulation data is provided to gain an insight into the nature of the phase change which takes place in the two clusters at T ≈ 100 K.