Dynamics of Biological Macromolecules by Neutron Scattering

The spatial and temporal ranges accessible using the technique of quasi-elastic neutron scattering (QENS) are ideally matched to the atomic and molecular vibrational displacements, correlation lengths and diffusive motions encountered in highly complex biological systems. The QENS method has been successfully applied to a diverse range of bio-molecular problems which encompass, for example, proteins, membranes, lipids, nucleic acids and saccharides. In this section, the basic principles of quasi-elastic neutron scattering pertinent to the study of dynamic processes in biological molecules are presented. An overview of the neutron instrumentation required for such studies is given as are experimental results which highlight the ideas outlined.

We first focus on the role of the instrumental resolution in Elastic Incoherent Neutron Scattering (EINS) where the connection between the Self Distribution Function (SDF) and the measured EINS intensity profile is highlighted. Second we show how the SDF procedure allows both the total and the partial Mean Square Displacement (MSD) evaluation, through the total and the partial SDFs. Finally, we compare the SDF and the Gaussian procedures, by applying the two approaches to EINS data collected, by the IN13 backscattering spectrometer (ILL, Grenoble), on aqueous mixtures of two homologous disaccharides, i.e. sucrose and trehalose, and on myoglobin.

Biological membranes are complex multicomponent systems whose dynamics and structure provide their physiological function. Many parameters interplay to determine the membrane flexibility; among them, lipid composition, lipid-protein interaction, hydration, temperature etc.

We provide here a tentative overview of recent successful neutron scattering experiments on different oriented model membranes, with the aim to demonstrate the many unique advantages that elastic and quasi-elastic neutron scattering offer for the investigation of membrane dynamics.

In this review we report on some experimental studies on the dynamics of Myoglobin in a confined geometry, obtained by encapsulation in a porous silica matrix, at low hydration levels. After formation through the solgel method, the samples were left aging/drying in order to reach a condition where only one or two water layers surround the proteins. In order to put in evidence the specific effect of confinement in the silica host, we compared this system with another one (i.e. hydrated powder) where proteins are confined by other proteins. Using elastic neutron scattering we investigate the temperature dependence of the mean square displacements of non-exchangeable hydrogen atoms of sol-gel encapsulated Myoglobin. In order to clarify the effect of hydration the study was extended to samples at 0.2, 0.3 and 0.5 [gr water]/[gr protein] fractions and comparison was made with Myoglobin powders at the same average hydration and with a dry powder sample. Comparison between the data relative to the different samples indicates that geometrical confinement within the matrix plays a crucial role in protein dynamics and conformational stability, the effect of sol-gel encapsulation being essentially a reduction of collective protein motions likely related to the slowing down of solvent confined diffusion. A dielectric spectroscopy investigation on the same systems helped us to clarify the effect of encapsulation on protein/solvent dynamics. In agreement with elastic neutron scattering, although in a much slower time scale, dielectric spectroscopy indicates a suppression of cooperative relaxation inside the gel, together with a clear dependence of relaxation rates on the hydration degree.

We report on experimental and simulative insights on saccharide-based bioprotection, obtained through the study of proteins embedded in amorphous saccharide matrices. The data presented come from a complementary set of techniques (FTIR, MD simulations and SAXS), which provides a description of the bioprotection mechanism from the atomistic to the macroscopic level. The results concur to draw a picture in which bioprotection by saccharides can be explained in terms of a tight anchorage of the protein surface to a stiff matrix, via extended hydrogen-bond networks, whose properties are defined by all its components, and are strongly dependent on the water content. In particular, they show how carbohydrates having similar hydrogenbonding capabilities exhibit different efficiency in preserving biostructures.

Sugar-lipid complexes are nowadays extensively applied in biology, medicine and food science. In particular, sugars play an important role in maintaining cell viability in stressful environmental conditions. A detailed understanding of lipid-saccharide-solvent interactions can be achieved by a combined use of advanced microscopic structural and dynamical investigation techniques. In this review the effect of saccharide content on the gel to liquid-crystalline phase transition and on the multilayer structure of lipid membranes, as well as on the aggregation properties of liposomes in colloidal systems, is discussed.

The advantages of vibrational spectroscopy by the use of inelastic neutron scattering (INS) spectroscopy are illustrated for biomacromolecules and their interaction with water. The complementarity with other vibrational spectroscopic techniques is demonstrated and the synergy with calculations is stressed.

A synergistic approach is described combining computer simulation with experiment in the interpretation of small angle neutron scattering (SANS) and inelastic scattering experiments on the structure and dynamics of proteins and other biopolymers. Simulation models can be tested by calculating neutron scattering structure factors and comparing the results directly with experimental scattering profiles. If the scattering profiles agree the simulations can be used to provide a detailed decomposition and interpretation of the experiments, and if not, the models can be rationally adjusted. Comparison with neutron experiment can be made at the level of the scattering functions or, less directly, of structural and dynamical quantities derived from them. This methodology is discussed in the context of the protein glass transition, protein-solvent dynamical coupling, the density of the hydration shell of proteins and the structural analysis of lignocellulosic biomass.

Full text available.

Full text available.