Biomedical subjects

Niklas Hedin

Publications and source records attributed to Niklas Hedin.

5 recordsLinked to original sources

PubMed2006

Selective NMR measurements of homonuclear scalar couplings in isotopically enriched solids.

Scalar (J) couplings in solid-state NMR spectroscopy are sensitive to covalent through-bond interactions that make them informative structural probes for a wide range of complex materials. Until now, however, they have been generally unsuitable for use in isotopically enriched solids, such as proteins or many inorganic solids, because of the complications presented by multiple coupled but nonisolated spins. Such difficulties are overcome by incorporating a z-filter that results in a robust method for measuring pure J-coupling modulations between selected pairs of nuclei in an isotopically enriched spin system. The reliability of the new experimental approach is established by using numerical simulations and tested on fully (13)C-labeled polycrystalline L-alanine. It is furthermore shown to be applicable to partially enriched systems, when used in combination with a selective double-quantum (DQ) filter, as demonstrated for the measurement of (2)J((29)Si-O-(29)Si) couplings in a 50% (29)Si-enriched surfactant-templated layered silicate lacking long-range 3D crystallinity. J-coupling constants are obtained with sufficient accuracy to distinguish between different (29)Si-O-(29)Si pairs, shedding insight on the local structure of the silicate framework. The new experiment is appropriate for fully or partially enriched liquid or solid samples.

Journal Article↗

PubMed2006

Sensitivity considerations in polarization transfer and filtering using dipole-dipole couplings: implications for biomineral systems.

The robustness and sensitivities of different polarization-transfer methods that exploit heteronuclear dipole-dipole couplings are compared for a series of heterogeneous solid systems, including polycrystalline tetrakis(trimethylsilyl)silane (TKS), adamantane, a physical mixture of doubly (13)C,(15)N-enriched and singly (13)C-enriched polycrystalline glycine, and a powder sample of siliceous marine diatoms, Thalossiosira pseudonana. The methods were analyzed according to their respective frequency-matching spectra or resultant signal intensities. For a series of (13)C{(1)H} cross-polarization experiments, adiabatic passage Hartmann-Hahn cross-polarization (APHH-CP) was shown to have several advantages over other methods, including Hartmann-Hahn cross-polarization (HHCP), variable-amplitude cross-polarization (VACP), and ramped-amplitude cross-polarization (RACP). For X-Y systems, such as (13)C{(15)N}, high and comparable sensitivities were obtained by using APHH-CP with Lee-Goldburg decoupling or by using the transferred-echo double resonance (TEDOR) experiment. The findings were applied to multinuclear (1)H, (13)C, (15)N, and (29)Si CP MAS characterization of a powder diatom sample, a challenging inorganic-organic hybrid solid that places high demands on NMR signal sensitivity.

Biopolymers↗

PubMed2004

Interactions of charged porphyrins with nonionic triblock copolymer hosts in aqueous solutions.

The extent and locus of solubilization of guest and self-assembling surfactant host molecules in aqueous solutions are influenced by a variety of hydrophobic and hydrophilic interactions, as well as by more specific interactions between the various species present. By using a combination of two-dimensional heteronuclear 13C[1H] NMR correlation experiments with pulsed-gradient NMR diffusion and proton cross-relaxation measurements, the locations and distributions of porphyrin guest molecules have been established unambiguously with respect to the hydrophobic and hydrophilic moieties of a triblock copolymer species in solution. The interactions of tetra(4-sulfonatophenyl)porphyrin with the poly(propylene oxide) (PPO) and the poly(ethylene oxide) (PEO) segments of amphiphilic PEO-PPO-PEO triblock copolymer species have been measured as functions of solution conditions, including temperature and pH. The porphyrin/PEO-PPO-PEO interactions are established to be selective and adjustable according to the different temperature-dependent hydrophilicities or hydrophobicities of the PEO and PPO triblock copolymer components. Furthermore, such interactions influence the self-assembly properties of the block-copolymer amphiphiles in solution by stabilizing molecular porphyrin/PEO-PPO-PEO complexes well above the critical micellization temperature of the triblock copolymer species under otherwise identical conditions.

Micelles↗

PubMed2004

Structure of a surfactant-templated silicate framework in the absence of 3d crystallinity.

The structure of a novel molecularly ordered two-dimensional (2D) silicate framework in a surfactant-templated mesophase has been established by using a combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction, and quantum chemical and empirical force-field modeling. These materials are unusual in their combination of headgroup-directed 2D crystalline framework ordering, zeolite-like ring structures within the layers, and long-range mesoscopic organization without three-dimensional (3D) atomic periodicity. The absence of registry between the silicate sheets, resulting from the liquidlike disorder of the alkyl surfactant chains, has presented significant challenges to the determination of framework structures in these and similar materials lacking 3D crystalline order. Double-quantum (29)Si NMR correlation experiments establish the interactions and connectivities between distinct intra-sheet silicon sites from which the structure of the molecularly ordered inorganic framework is determined.

Journal Article↗

PubMed2003

Nanostructural features of demosponge biosilica.

Recent interest in the optical and mechanical properties of silica structures made by living sponges, and the possibility of harnessing these mechanisms for the synthesis of advanced materials and devices, motivate our investigation of the nanoscale structure of these remarkable biomaterials. Scanning electron and atomic force microscopic (SEM and AFM) analyses of the annular substructure of demosponge biosilica spicules reveals that the deposited material is nanoparticulate, with a mean particle diameter of 74+/-13 nm. The nanoparticles are deposited in alternating layers with characteristic etchant reactivities. Further analyses of longitudinally fractured spicules indicate that each deposited layer is approximately monoparticulate in thickness and exhibits extensive long range ordering, revealing an unanticipated level of nanoscale structural complexity. NMR data obtained from differentially heated spicule samples suggest that the etch sensitivity exhibited by these annular domains may be related to variation in the degree of silica condensation, rather than variability in the inclusion of organics. In addition, AFM phase imaging in conjunction with results obtained from HF and alkaline etching experiments suggest that at various stages in spicule biosynthesis, regions of unusually low silica condensation are deposited, indicating a possible interruption in normal spicule formation. While this discovery of nanoparticulate silica aggregation in demosponge skeletal elements is likely to reflect the intrinsic kinetic tendency of silica to form such particles during polycondensation, the heirarchical organization of these nanoparticles is biologically unique.

Animals↗