Investigation of the dynamics of aqueous proline solutions using neutron scattering and molecular dynamics simulations

Collective dynamics and self-motions in the van der Waals liquid tetrahydrofuran from meso- to inter-molecular scales disentangled by neutron spectroscopy with polarization analysis

By using time-of-flight neutron spectroscopy with polarization analysis, we have separated coherent and incoherent contributions to the scattering of deuterated tetrahydrofuran in a wide scattering vector (Q)-range from meso- to inter-molecular length scales. The results are compared with those recently reported for water to address the influence of the nature of inter-molecular interactions (van der Waals vs hydrogen bond) on the dynamics. The phenomenology found is qualitatively similar in both systems. Both collective and self-scattering functions are satisfactorily described in terms of a convolution model that considers vibrations, diffusion, and a Q-independent mode. We observe a crossover in the structural relaxation from being dominated by the Q-independent mode at the mesoscale to being dominated by diffusion at inter-molecular length scales. The characteristic time of the Q-independent mode is the same for collective and self-motions and, contrary to water, faster and with a lower activation energy (≈1.4 Kcal/mol) than the structural relaxation time at inter-molecular length scales. This follows the macroscopic viscosity behavior. The collective diffusive time is well described by the de Gennes narrowing relation proposed for simple monoatomic liquids in a wide Q-range entering the intermediate length scales, in contraposition to the case of water.

Insect cross-tolerance to freezing and drought stress: role of metabolic rearrangement

The accumulation of trehalose has been suggested as a mechanism underlying insect cross-tolerance to cold/freezing and drought. Here we show that exposing diapausing larvae of the drosophilid fly, Chymomyza costata to dry conditions significantly stimulates their freeze tolerance. It does not, however, improve their tolerance to desiccation, nor does it significantly affect trehalose concentrations. Next, we use metabolomics to compare the complex alterations to intermediary metabolism pathways in response to three environmental factors with different ecological meanings: environmental drought (an environmental stressor causing mortality), decreasing ambient temperatures (an acclimation stimulus for improvement of cold hardiness), and short days (an environmental signal inducing diapause). We show that all three factors trigger qualitatively similar metabolic rearrangement and a similar phenotypic outcome-improved larval freeze tolerance. The similarities in metabolic response include (but are not restricted to) the accumulation of typical compatible solutes and the accumulation of energy-rich molecules (phosphagens). Based on these results, we suggest that transition to metabolic suppression (a state in which chemical energy demand is relatively low but need for stabilization of macromolecules is high) represents a common axis of metabolic pathway reorganization towards accumulation of non-toxic cytoprotective compounds, which in turn stimulates larval freeze tolerance.

A mixture of innate cryoprotectants is key for freeze tolerance and cryopreservation of a drosophilid fly larva

Insects that naturally tolerate internal freezing produce complex mixtures of multiple cryoprotectants (CPs). Better knowledge on composition of these mixtures, and on mechanisms of how the individual CPs interact, could inspire development of laboratory CP formulations optimized for cryopreservation of cells and other biological material. Here we identify and quantify (using high resolution mass spectrometry) a range of putative CPs in larval tissues of a subarctic fly, Chymomyza costata that survives long-term cryopreservation in liquid nitrogen. The CPs (proline, trehalose, glutamine, asparagine, glycine betaine, glycerophosphoethanolamine, glycerophosphocholine, and sarcosine) accumulate in hemolymph in a ratio of 313:108:55:26:6:4:2.9:0.5 mmol.L-1. Using calorimetry, we show that the artificial mixtures, mimicking the concentrations of major CPs' in hemolymph of freeze-tolerant larvae, suppress the melting point of water and significantly reduce the ice fraction. We demonstrate in a bioassay that mixtures of CPs administered through the diet act synergistically rather than additively to enable cryopreservation of otherwise freeze-sensitive larvae. Using MALDI-MSI, we show that during slow extracellular freezing trehalose becomes concentrated in partially dehydrated hemolymph where it stimulates transition to the amorphous glass phase. In contrast, proline moves to the boundary between extracellular ice and dehydrated hemolymph and tissues where it likely forms a layer of dense viscoelastic liquid. We propose that amorphous glass and viscoelastic liquids may protect macromolecules and cells from thermomechanical shocks associated with freezing and transfer into and out of liquid nitrogen.

Molecular Insights into Dipole Relaxation Processes in Water-Lysine Mixtures

Dielectric spectroscopy is a robust method to investigate relaxations of molecular dipoles. It is particularly useful for studies of biological solutions because of the potential of this method to cover a broad range of dynamical time scales typical for such systems. However, this technique does not provide any information about the nature of the molecular motions, which leads to a certain underemployment of dielectric spectroscopy for gaining microscopic understanding of material properties. For such detailed understanding, computer simulations are valuable tools because they can provide information about the nature of molecular motions observed by, for example, dielectric spectroscopy and to further complement them with structural information. In this work, we acquire information about the nature of dipole relaxation, in n-lysine solutions by means of molecular dynamics simulations. Our results indicate that the experimentally observed main relaxation process of n-lysine has different origins for the single monomer and the polypeptide chains. The relaxation of 1-lysine is due to the motions of whole molecules, whereas the experimentally observed relaxation of 3-lysine and 4-lysine is due to the motions of the residues, which, in turn, are promoted by water relaxation. Furthermore, we propose a new structural model of the lysine amino acids, which can quantitatively account for the experimental dielectric relaxation data. Hydrogen bonding and the structure of water are also discussed in terms of their influence on relaxation processes.

Three-Dimensional Enantiomeric Recognition of Optically Trapped Single Chiral Nanoparticles

We optically trap freestanding single metallic chiral nanoparticles using a standing-wave optical tweezer. We also incorporate within the trap a polarimetric setup that allows us to perform in situ chiral recognition of single enantiomers. This is done by measuring the S_{3} component of the Stokes vector of a light beam scattered off the trapped nanoparticle in the forward direction. This unique combination of optical trapping and chiral recognition, all implemented within a single setup, opens new perspectives towards the control, recognition, and manipulation of chiral objects at nanometer scales.