A silica-poly(methy1 methacrylate) (PMMA) composite was prepared by condensation polymerization of a 2 nm shell of the silane coupling agent 3-(trimethoxysilyl)propyl methacrylate (TPM) on the surface of 10.5 nm diameter sol-gel colloidal silica particles followed by free-radical Folymerization of a 50 wt % dispersion of the TPM-silica in methyl methacrylate. Cross-polarization combined with magic angle spinning and high-power decoupling (CPIMAS) and single-pulse 29Si NMR s p x t r a together with quasi-adiabatic cross-polarization (QACP) 13C NMR spectra provided quantitative analyses of the structural components of the parent silica, the TPM-silica, and the composite. The parent silica contained one ethoxy group and eleven hydroxy groups per ten silicon atoms. The TPM-silica containcd one residual methoxy group per TPM group and no residual hydroxy groups. Polymerization with M.MA consumed 85% of the methacrylate groups of the TPM. Time constants T l p~ for proton spin-lattice, relaxation in the rotating frame detected via 13C and 29Si CPMAS spectra showed rapid spin diffusion bstween all CH protons in the samples, but not between the CH protons and the OH protons that cross-polarize 29Si atoms in the parent silica. Time constants Tlpc for carbon spin-lattice relaxation in the rotating frame showed that the TPM-silica has substantial motion at kilohertz frequencies leading to fast relaxation, whereas the PMMA composite is more rigid and the ethoxy groups in the parent silica arc more mobile.Measurements of lH-lH dipolar transverse relaxation times via 13C and 29Si detection showed decreasing strengths of homonuclear dipolar interactions due to increasing molecular motion in the order composite > TPM-silica > OH groups in parent silica.
Diffusion dynamics of guest molecules in nanopores has been studied intensively because diffusion is center on a number of research fields such as separation, drug delivery, chemical reactions, and sensing. In the present work, we report an experimental investigation of the self-diffusion of water inside carbon nanotube (CNT) channels using a pulsed field gradient (PFG) NMR method. The dispersion of CNTs homogeneously in water and cooling to temperatures below the melting point of bulk water allow us to probe the translational motion of confined water molecules. The results demonstrate that the self-diffusion coefficient of water in CNTs is highly dependent on the diffusion time and CNT diameter. In particular, the diffusivity of water in double-walled carbon nanotubes (DWNTs) with an average inner diameter of 2.3 ± 0.3 nm is twice that in multiwalled carbon nanotubes (MWNTs) with an average inner diameter of 6.7 ± 0.8 nm in the temperature range of 263-223 K. In addition, the effective self-diffusion coefficient in DWNTs is 1 order of magnitude higher than that reported for mesoporous silica materials with a similar pore size. The faster diffusivity of water in CNTs could be attributed to the ordered hydrogen bonds formed between water molecules within the confined channels of CNTs and the weak interaction between water and the CNT walls.
One of the functions of MutY from Escherchia coli is removal of adenine mispaired with 7,8-dihydro-8-oxoguanine (8-oxoG), a common lesion in oxidatively damaged DNA. MutY is composed of two domains: the larger N-terminal domain (p26) contains the catalytic properties of the enzyme while the C-terminal domain (p13) affects substrate recognition and enzyme turnover. On the basis of sequence analyses, it has been recently suggested that the C-terminal domain is distantly related to MutT, a dNTPase which hydrolyzes 8-oxo-dGTP [Noll et al. (1999) Biochemistry 38, 6374-6379]. We have studied the solution structure of the C-terminal domain of MutY by NMR and find striking similarity with the reported solution structure of MutT. Despite low sequence identity between the two proteins, they have similar secondary structure and topology. The C-terminal domain of MutY is composed of two alpha-helices and five beta-strands. The NOESY data indicate that the protein has two beta-sheets. MutT is also a mixed alpha/beta protein with two helices and two beta-sheets composed of five strands. The secondary structure elements are similarly arranged in the two proteins.
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