Thursday, January 08, 2009

J. Phys Chem B Vol 112, Issue 51

Dynamic Solvophobic Effect and Its Cooperativity in the Hydrogen-bonding Liquids Studied by Dielectric and Nuclear Magnetic Resonance Relaxation

Tsuyoshi Yamaguchi*, Hiroki Furuhashi, Tatsuro Matsuoka and Shinobu Koda

J. Phys. Chem. B, 2008, 112 (51), pp 16633–16641
Abstract: The reorientational relaxation of solvent molecules in the mixture of nonpolar solutes and hydrogen-bonding liquids including water, alcohols, and amides are studied by dielectric and 2H-nuclear magnetic resonance (NMR) spin−lattice relaxations. The retardation of the reorientational motion of the solvent by weak solute−solvent interaction is observed in all the solvent systems. On the other hand, no clear correlation between the strength of the solute−solvent interaction and the slowing down of the solvent motion is found in N,N-dimethylacetamide, which suggests the importance of the hydrogen bonding in the dynamic solvophobic effect. The cooperativity of the reorientational relaxation is investigated by the comparison between the collective relaxation measured by the dielectric spectroscopy and the single-molecular reorientation determined by NMR. The modification of the dielectric relaxation time caused by the dissolution of the solute is larger than that of the single-molecular reorientational relaxation time in all the solvents studied here. The effect of the static correlation between the dipole moments of different molecules is calculated from the static dielectric constant, and the effect of the dynamic correlation is estimated. The difference in the effects of the solutes on the collective and single-molecular reorientational relaxation is mainly ascribed to the dynamic cooperativity in the cases of water and alcohols, which is consistent with the picture on the dynamic solvophobicity derived by our previous theoretical analysis (Yamaguchi, T.; Matsuoka, T.; Koda, S. J. Chem. Phys. 2004, 120, 7590). On the other hand, the static correlation plays the principal role in the case of N-methylformamide.

Richard Weber*, Sabyasachi Sen§, Randall E. Youngman, Robert T. Hart and Chris J. Benmore

J. Phys. Chem. B, 2008, 112 (51), pp 16726–16733
DOI: 10.1021/jp807964u

Abstract: The structure of binary aluminosilicate glasses containing 60−67 mol % Al2O3 were investigated using high-resolution 27Al NMR and X-ray and neutron diffraction. The glasses were made by aerodynamic levitation of molten oxides. The 67% alumina composition required a cooling rate of ∼1600 °C s1− to form glass from submillimeter sized samples. NMR results show that the glasses contain aluminum in 4-, 5-, and 6-fold coordination in the approximate ratio 4:5:1. The average Al coordination increases from 4.57 to 4.73 as the fraction of octahedral Al increases with alumina content. The diffraction results on the 67% composition are consistent with a disordered Al framework with Al ions in a range of coordination environments that are substantially different from those found in the equilibrium crystalline phases. Analysis of the neutron and X-ray structure factors yields an average bond angle of 125 ± 4° between an Al ion and the adjoining cation via a bridging oxygen. We propose that the structure of the glass is a “transition state” between the alumina-rich liquid and the equilibrium mullite phase that are dominated by 4- and 6-coordinated aluminum ions, respectively.

Ultrahigh Resolution Characterization of Domain Motions and Correlations by Multialignment and Multireference Residual Dipolar Coupling NMR

Charles K. Fisher, Qi Zhang, Andrew Stelzer and Hashim M. Al-Hashimi*
J. Phys. Chem. B, 2008, 112 (51), pp 16815–16822
DOI: 10.1021/jp806188j

Abstract: Nuclear magnetic resonance (NMR) residual dipolar couplings (RDCs) provide a unique opportunity for spatially characterizing complex motions in biomolecules with time scale sensitivity extending up to milliseconds. Up to five motionally averaged Wigner rotation elements, ⟨D0k2(αβ)⟩, can be determined experimentally using RDCs measured in five linearly independent alignment conditions and applied to define motions of axially symmetric bond vectors. Here, we show that up to 25 motionally averaged Wigner rotation elements, ⟨Dmk2(αβγ)⟩, can be determined experimentally from multialignment RDCs and used to characterize rigid-body motions of chiral domains. The 25 ⟨Dmk2(αβγ)⟩ elements form a basis set that allows one to measure motions of a domain relative to an isotropic distribution of reference frames anchored on a second domain (and vice versa), thus expanding the 3D spatial resolution with which motions can be characterized. The 25 ⟨Dmk2(αβγ)⟩ elements can also be used to fit an ensemble consisting of up to eight equally or six unequally populated states. For more than two domains, changing the identity of the domain governing alignment allows access to new information regarding the correlated nature of the domain fluctuations. Example simulations are provided that validate the theoretical derivation and illustrate the high spatial resolution with which rigid-body domain motions can be characterized using multialignment and multireference RDCs. Our results further motivate the development of experimental approaches for both modulating alignment and anchoring it on specifically targeted domains.

Structure, Orientation, and Dynamics of the C-Terminal Hexapeptide of LRAP Determined Using Solid-State NMR

Wendy J. Shaw* and Kim Ferris
J. Phys. Chem. B, 2008, 112 (51), pp 16975–16981
DOI: 10.1021/jp808012g

Abstract: Amelogenin is the predominant protein found during enamel development and has been shown to be essential to proper enamel formation. Leucine-rich amelogenin peptide (LRAP) is a naturally occurring splice variant that preserves the charged N- and C-termini of full length amelogenin, regions thought to be crucial in interacting with hydroxaypatite. Particularly, the highly charged C-terminal hexapeptide (KREEVD) is thought to be the region most intimately interacting with hydroxyapatite (HAP). The structure of this charged region was investigated, along with the proximity to the surface and the mobility of two of the residues. The structure was found to be consistent with a random coil or more extended structure, as has been seen for more internalized residues in the C-terminus. The backbone K54(13C′), V58(13C′), and V58(15N) were all found to be close to the surface of HAP, ∼ 6.0 Å from the nearest 31P atom, suggesting a strong interaction and emphasizing the importance of these residues in interacting with HAP. However, both ends of the hexapeptide at residues K54 and V58 experience significant mobility under hydrated conditions, implying that another portion of the protein helps to stabilize the strong LRAP-HAP interaction. Interestingly, the backbone of the C-terminal third of the protein is consistently 6.0 Å from the HAP surface, providing a model in this region of the protein lying flat on the surface with no three-dimensional folding. The combination of these features, that is, a random coil structure, a significant mobility, and a lack of three-dimensional folding in this region of the protein, may have an important functional role, possibly allowing maximum crystal inhibition at low protein concentrations.

No comments: