Kayori Shimada

Precise measurement of the self-diffusion coefficient for poly(ethylene glycol) in aqueous solution using uniform oligomers

Uniform poly(ethylene glycol) (PEG) oligomers, with a degree of polymerization n=1-40, were separated by preparative supercritical fluid chromatography from commercial monodispersed samples. Diffusion coefficients, D, for separated uniform PEG oligomers were measured in dilute solutions of deuterium oxide (D(2)O) at 30 degrees C, using pulsed-field gradient nuclear magnetic resonance. The measured D for each molecular weight was extrapolated to infinite dilution. Diffusion coefficients obtained at infinite dilution follow the scaling behavior of Zimm-type diffusion, even in the lower molecular weight range. Molecular-dynamics simulations for PEG in H(2)O also showed this scaling behavior, and reproduced close hydrodynamic interactions between PEG and water. These findings suggest that diffusion of PEG in water is dominated by hydrodynamic interaction over a wide molecular weight range, including at low molecular weights around 1000.

Stable spherical hollow particles composed of bola-form amides via non-covalent interactions

Dipeptide-based bola-form amides, which self-assemble into fibrous structures under usual conditions, formed stable micrometer-sized hollow spheres directed by hydrophilic interfaces in aqueous solution. The chemical structure of the bola-form amide and the surface properties of the substrate proved to significantly affect the self-assembly process. Bis(N-α-amide-L-valyl-L-valine)1,n-alkane dicarboxylate (n = 10: (Val)2C10, 12: (Val)2C12) and bis(N-α-amide-L-isoleucyl-L-isoleucine)1,n-alkane dicarboxylate (n = 10: (i-Leu)2C10) produced hollow spheres, whereas (Val)2Cn (n = 7–9, 11) and bis(N-α-amide-L-valyl-L-methionine)C10 ((Val/Met)2C10) formed no spheres. Static light scattering measurements revealed that the rod-like micelles of (Val)2C10, (Val)2C12 and (i-Leu)2C10 were converted to the hollow spheres via vesicle-like intermediates. The vesicle-like intermediates gathered together to form the spherical hollow particles with the aid of the surface of hydrophilic glass substrates. On the other hand, (Val)2Cn (n = 7–9, 11) and (Val/Met)2C10 directly self-assembled into fibrous structures from rod-like micelles without passing through the vesicle-like intermediates. The carbon number of the spacer, bulkiness of the head groups and surface properties of the substrate played critical roles in determining the self-assembly. FT-IR, XRD and DSC measurements revealed that the packing of the bola-form amides in the hollow spheres differed from that in the fibrous assembly. Molecules in the spheres were more tightly packed, as in the crystalline state, than those in fibrous structures.

Effects of oxygen atom in the side chain on physical and optical properties of dodecapentoxypentasilane

Abstract The effects of the alkoxy side chain on the conformation of oligosilane have been studied for perpentoxypentasilane. The UV-absorption maxima shifted extremely to a shorter wavelength than that of peralkyloligosilane. Molecular dynamics and ab initio MO calculations showed that the excitation energy of the twisted structure for peroxyloligosilane (a model of perpentoxypentasilane) was smaller than that of permethyloligosilane, because of the interaction between the n orbital of the oxygen atom and the σ orbital of the Si–Si bond. The Coulomb repulsion energies between the oxygen atoms greatly affect the stability of the twisted conformation of the silicon backbone.