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Cross Polarization (cross + polarization)

Distribution by Scientific Domains

Chemistry	33%

Selected Abstracts

Effect of residual vanadyl ions on the spectroscopic analysis of humic acids: a multivariate approach

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2008
E. H. Novotny
Summary In a study of the vanadyl (VO2+)-humic acids system, the residual vanadyl ion suppressed fluorescence and specific electron paramagnetic resonance (EPR) and NMR signals. In the case of NMR, the proton rotating frame relaxation times (T1,H) indicate that this suppression is due to an inefficient H-C cross polarization, which is a consequence of a shortening of T1,H. Principal components analysis (PCA) facilitated the isolation of the effect of the VO2+ ion and indicated that the organic free radical signal was due to at least two paramagnetic centres and that the VO2+ ion preferentially suppressed the species whose electronic density is delocalized over O atoms (greater g -factor). Additionally, the newly obtained variables (principal components , PC) indicated that, as the result of the more intense tillage a relative increase occurred in the accumulation of: (i) recalcitrant structures; (ii) lignin and long-chain alkyl structures; and (iii) organic free radicals with smaller g -factors. [source]

Conformational heterogeneity of transmembrane residues after the Schiff base reprotonation of bacteriorhodopsin

FEBS JOURNAL, Issue 9 2005
15N CPMAS NMR of D85N/T170C membranes
bR, N-like and O-like intermediate states of [15N]methionine-labelled wild type and D85N/T170C bacteriorhodopsin were accumulated in native membranes by controlling the pH of the preparations. 15N cross polarization and magic angle sample spinning (CPMAS) NMR spectroscopy allowed resolution of seven out of nine resonances in the bR-state. It was possible to assign some of the observed resonances by using 13C/15N rotational echo double resonance (REDOR) NMR and Mn2+ quenching as well as D2O exchange, which helps to identify conformational changes after the bacteriorhodopsin Schiff base reprotonation. The significant differences in chemical shifts and linewidths detected for some of the resonances in N- and O-like samples indicate changes in conformation, structural heterogeneity or altered molecular dynamics in parts of the protein. [source]

Solid-state NMR Structure Determination

IUBMB LIFE, Issue 9 2003
Alison Drechsler
Abstract Biological applications of solid-state NMR (SS-NMR) have been predominantly in the area of model membrane systems. Increasingly the focus has been membrane peptides and proteins. SS-NMR is able to provide information about how the peptides or proteins interact with the lipids or other peptides/proteins in the membrane, their effect on the membrane and the location of the peptides or proteins relative to the membrane surface. Recent developments in biological SS-NMR have been made possible by improvements in labelling and NMR techniques. This review discusses aligned systems and magic angle spinning techniques, bilayers and bicelles, and measurement of chemical shift anisotropy and dipolar coupling. A number of specific experiments such as cross polarization, rotational resonance, REDOR, PISEMA, MAOSS and multidimensional experiments are described. In addition to 2H, 13C and 15N, recent solid-sate 1H, 19F and 17O NMR work is discussed. Several examples of the use of SS-NMR to determine the structure of membrane peptides and proteins are given. IUBMB Life, 55: 515-523, 2003 [source]

Quantitation of crystalline and amorphous forms of anhydrous neotame using 13C CPMAS NMR spectroscopy

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2005
Thomas J. Offerdahl
Abstract Although most drugs are formulated in the crystalline state, amorphous or other crystalline forms are often generated during the formulation process. The presence of other forms can dramatically affect the physical and chemical stability of the drug. The identification and quantitation of different forms of a drug is a significant analytical challenge, especially in a formulated product. The ability of solid-state 13C NMR spectroscopy with cross polarization (CP) and magic-angle spinning (MAS) to quantify the amounts of three of the multiple crystalline and amorphous forms of the artificial sweetener neotame is described. It was possible to quantify, in a mixture of two anhydrous polymorphic forms of neotame, the amount of each polymorph within 1,2%. In mixtures of amorphous and crystalline forms of neotame, the amorphous content could be determined within 5%. It was found that the crystalline standards that were used to prepare the mixtures were not pure crystalline forms, but rather a mixture of crystalline and amorphous forms. The effect of amorphous content in the crystalline standards on the overall quantitation of the two crystalline polymorphic forms is discussed. The importance of differences in relaxation parameters and CP efficiencies on quantifying mixtures of different forms using solid-state NMR spectroscopy is also addressed. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:2591,2605, 2005 [source]

Meandering probe fed patch antenna with high gain characteristic for circularly polarized application

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 5 2007
Hau-Wah Lai
Abstract A meandering probe-fed circularly polarized stacked patch antenna with truncated corners is studied. The antenna has a wide 3-dB axial ratio bandwidth of 12%. It exhibits a stable radiation pattern across the axial ratio bandwidth. The antenna has low cross polarization and high gain, which are ,14 dB and 10 dBi, respectively. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1095,1098, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22357 [source]

Uniplanar four-element Vivaldi array fed by a coplanar waveguide

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 1 2001
I. Linardou
Abstract This letter describes the design of a uniplanar parallel-series-fed linear array of Vivaldi antennas. Matching conditions have been optimized with an equivalent-circuit model. The array exhibits 12dBi gain in the substrate plane, 80% radiation efficiency, and low cross polarization over 5% bandwidth centered at 4.2GHz. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 60,62, 2001. [source]