Sylvia E. McLain

Orientational correlations in liquid acetone and dimethyl sulfoxide: A comparative study

The structure of acetone and dimethyl sulfoxide in the liquid state is investigated using a combination of neutron diffraction measurements and empirical potential structure refinement (EPSR) modeling. By extracting the orientational correlations from the EPSR model, the alignment of dipoles in both fluids is identified. At short distances the dipoles or neighboring molecules are found to be in antiparallel configurations, but further out the molecules tend to be aligned predominately as head to tail in the manner of dipolar ordering. The distribution of these orientations in space around a central molecule is strongly influenced by the underlying symmetry of the central molecule. In both liquids there is evidence for weak methyl hydrogen to oxygen intermolecular contacts, though these probably do not constitute hydrogen bonds as such.

On the hydration of the phosphocholine headgroup in aqueous solution

The hydration of the phosphocholine headgroup in 1,2-dipropionyl-sn-glycero-3-phosphocholine (C(3)-PC) in solution has been determined by using neutron diffraction enhanced with isotopic substitution in combination with computer simulation techniques. The atomic scale hydration structure around this head group shows that both the -N(CH(3))(3) and -CH(2) portions of the choline headgroup are strongly associated with water, through a unique hydrogen bonding regime, where specifically a hydrogen bond from the C-H group to water and a strong association between the water oxygen and N(+) atom in solution have both been observed. In addition, both PO(4) oxygens (P=O) and C=O oxygens are oversaturated when compared to bulk water in that the average number of hydrogen bonds from water to both X=O oxygens is about 2.5 for each group. That water binds strongly to the glycerol groups and is suggestive that water may bind to these groups when phosophotidylcholine is embedded in a membrane bilayer.

Investigations on the structure of dimethyl sulfoxide and acetone in aqueous solution

Aqueous solutions of dimethyl sulfoxide (DMSO) and acetone have been investigated using neutron diffraction augmented with isotopic substitution and empirical potential structure refinement computer simulations. Each solute has been measured at two concentrations-1:20 and 1:2 solute:water mole ratios. At both concentrations for each solute, the tetrahedral hydrogen bonding network of water is largely unperturbed, though the total water molecule coordination number is reduced in the higher 1:2 concentrations. With higher concentrations of acetone, water tends to segregate into clusters, while in higher concentrations of DMSO the present study reconfirms that the structure of the liquid is dominated by DMSO-water interactions. This result may have implications for the highly nonideal behavior observed in the thermodynamic functions for 1:2 DMSO-water solutions.

Atomic pair distribution function analysis of materials containing crystalline and amorphous phases

Abstract The atomic pair distribution function (PDF) approach has been used to study the local structure of liquids, glasses and disordered crystalline materials. In this paper, we demonstrate the use of the PDF method to investigate systems containing a crystalline and an amorphous structural phase. We present two examples: Bulk metallic glass with crystalline reinforcements and Fontainebleau sandstone, where an unexpected glassy phase was discovered. In this paper we also discuss the refinement methods used in detail.

Charge-based interactions between peptides observed as the dominant force for association in aqueous solution.

The process by which proteins fold in solution into their biologically functional forms is still not well understood despite intense research. The association of hydrophobic amino acid side chains in proteins—the hydrophobic effect— is frequently invoked to be the fundamental driving force behind protein folding in vivo. However, there is little direct experimental evidence that supports this assertion, and protein assembly purely from hydrophobic association gives an incomplete picture of the folding process. While many fully folded protein cores contain associated hydrophobic residues, ion pairs or salt bridges are important in stabilizing protein structures, and a proportion of proteins have ion pairs buried in their core. Moreover, the presence of a hydrophobic core does not necessarily implicate hydrophobic forces as the primary driving force of folding. To gain further understanding of the relative roles of hydrophobic and hydrophilic interactions in the process of protein formation, we determined the structure in aqueous solution of three dipeptide fragments containing both hydrophobic and hydrophilic portions exposed to the surrounding water solvent by using a combination of neutron diffraction and computer simulation techniques. The series of peptides investigated consisted of glycyl-l-alanine, glycyl-l-proline, and l-alanyl-l-proline (Figure 1). The hydrophobicity of these dipeptides increases across the series; glycine has the

Amphipathic solvation of indole: implications for the role of tryptophan in membrane proteins.

The microscopic structure of the tryptophan side chain, indole, in an amphiphilic environment has been investigated using a combination of neutron diffraction measurements and simulations in solution. The results show that indole is preferentially solvated by hydrogen bonding interactions between water and alcohol -OH groups rather than the interaction being dominated by indole-methyl interactions. This has implications for understanding how tryptophan interacts with the amphipathic membrane environment to anchor proteins into membranes, where the results here suggest that the benzene ring of tryptophan interacts directly with the interfacial water at the membrane surface rather than being buried into the hydrophobic regions of the membrane bilayer.

Short-Range Interactions of Concentrated Proline in Aqueous Solution

Molecular interactions for proline in a highly concentrated aqueous solution (up to 1:5 proline:water molecular ratio) have been investigated using a variety of experimental and computational techniques. Rather than the solution containing either small crystallites or large aggregates of proline, three-dimensional structural analysis reveals the presence of proline-proline dimers. These dimers appear to be formed by cyclic electrostatic interactions between CO2(-) and NH2(+) groups on neighboring proline molecules, which causes the ring motifs of proline to be roughly parallel to one another. In addition, water appears to aggregate around the electrostatic groups of the proline-proline dimers where it may in fact bridge these groups on different molecules. The observed short-range interactions for proline in solution may explain its function as a hydrotrope in vivo in which this observed dimerization might allow proline molecules to generate small pockets of a hydrophobic environment that can associate with nonpolar motifs of other molecules in solution. The results presented here emphasize the need for careful three-dimensional analysis to assess the short-range order of highly concentrated solutions.

Water-Peptide Site-Specific Interactions: A Structural Study on the Hydration of Glutathione

Water-peptide interactions play an important role in determining peptide structure and function. Nevertheless, a microscopic description of these interactions is still incomplete. In this study we have investigated at the atomic scale length the interaction between water and the tripeptide glutathione. The rationale behind this work, based on the combination between a neutron diffraction experiment and a computer simulation, is twofold. It extends previous studies on amino acids, addressing issues such as the perturbation of the water network brought by a larger biomolecule in solution. In addition, and more importantly, it seeks a possible link between the atomic length scale description of the glutathione-water interaction with the specific biological functionality of glutathione, an important intracellular antioxidant. Results indicate a rather weak hydrogen bond between the thiol (-SH) group of cysteine and its first neighbor water molecule. This -SH group serves as a proton donor, is responsible for the biological activity of glutathione, and it is involved in the formation of glutathione disulfide, the oxidized form of glutathione. Moreover, the hydration shell of the chemically identical carboxylate group on the glutamic acid residue and on the glycine residue shows an intriguing different spatial location of water molecules and coordination numbers around the two CO2(-) groups.

Structural Evidence for Inter-Residue Hydrogen Bonding Observed for Cellobiose in Aqueous Solution

The structure of the disaccharide cellulose subunit cellobiose (4-O-β-D-glucopyranosyl-D-glucose) in solution has been determined via neutron diffraction with isotopic substitution (NDIS), computer modeling and nuclear magnetic resonance (NMR) spectroscopic studies. This study shows direct evidence for an intramolecular hydrogen bond between the reducing ring HO3 hydroxyl group and the non-reducing ring oxygen (O5′) that has been previously predicted by computation and NMR analysis. Moreover, this work shows that hydrogen bonding to the non-reducing ring O5′ oxygen is shared between water and the HO3 hydroxyl group with an average of 50% occupancy by each hydrogen-bond donor. The glycosidic torsion angles φH and ψH from the neutron diffraction-based model show a fairly tight distribution of angles around approximately 22° and −40°, respectively, in solution, consistent with the NMR measurements. Similarly, the hydroxymethyl torsional angles for both reducing and non-reducing rings are broadly consistent with the NMR measurements in this study, as well as with those from previous measurements for cellobiose in solution.

Water structure around dipeptides in aqueous solutions.

The bulk water structure around small peptide fragments—glycyl-l-alanine, glycyl-l-proline and l-alanyl-l-proline—has been determined by a combination of neutron diffraction with isotopic substitution and empirical potential structural refinement techniques. The addition of each of the dipeptides to water gives rise to decreased water–water coordination in the surrounding water solvent. Additionally, both the Ow–Ow radial distribution functions and the water–water spatial density functions in all of the solutions indicate an electrostrictive effect in the second water coordination shell of the bulk water network. This effect is not observed in similar experiments on the amino acid l-proline alone in solution, which is one component of two of the peptides measured here.