Daniel P. Weitekamp

Bio: Daniel P. Weitekamp is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Spin polarization & Nuclear magnetic resonance spectroscopy. The author has an hindex of 29, co-authored 87 publications receiving 4868 citations. Previous affiliations of Daniel P. Weitekamp include University of Groningen & University of California, Berkeley.

Parahydrogen and synthesis allow dramatically enhanced nuclear alignment

Abstract: The PASADENA effect is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen. When the parahydrogen mole fraction differs from the high-temperature limit of 1/4, this population difference constitutes a form of spin order which can be converted to magnetization observable by NMR. Large NMR signals are observed, if subsequent to the hydrogen addition, the two protons experience magnetic inequivalence and spin-spin coupling and if observation is made before spin-lattice relaxation restores the equilibrium spin order. The analogous effect for D2 is also possible. The kinetic mechanisms of the homogeneous hydrogenation catalysts which permit the realization of the PASADENA effect have been the target of the experimental applications. The enhancement of the NMR transitions has facilitated the determination of true molecular rate constants. Ordinarily, the activity of a catalyst is assessed by dividing the observed rate by the total catalyst concentration. However, the question as to whether most of the catalytic rate is due to a tiny fraction of active species or a large fraction with a relatively low molecular rate is not clearly addressed by such an analysis. This ambiguity is entirely avoided in the PASADENA studies, since only active catalyst molecules can contribute to the enhanced signals from which all kinetic inferences are made. The sensitivity enhancement has also led to the identification of a novel intermediate in the mechanism for the Rh(DIPHOS)+ catalyzed hydrogenation of styrene. The rate of conversion of this species into product and starting material has been studied using two-dimensional NMR. The dramatically improved sensitivity should make it possible to observe key catalytic intermediates which do not build up in sufficient quantity to allow detection by conventional NMR arising from Curie-Law magnetization. The study of surface sites which bind pairwise with H2 is also a potentially fruitful area for future experimental work. The ambient temperature NMR spectroscopy of surfaces is not often feasible due to sensitivity limitations. Simulations have been performed using typical shift and coupling parameters in an effort to characterize the enhanced lineshapes which can be expected. The inverse of the PASADENA effect has also been proposed, whereby the spin order of a molecule containing hydrogen is probed by measuring the branching ratio to ortho and para dihydrogen. This RAYMOND phenomenon (radiowave application yields modulated ortho number desorbed) has the potential for measuring precursor NMR with extraordinary sensitivity, since it finesses the need for detection of radiowaves.

Transformation of symmetrization order to nuclear-spin magnetization by chemical reaction and nuclear magnetic resonance.

Abstract: A method of obtaining very large nuclear-spin polarizations is proposed and illustrated by density-operator calculations. The prediction is that chemical reaction and rf irradiation can convert the scalar parahydrogen state into polarization of order unity on the nuclear spins of the products of molecular-hydrogen addition reactions. A means of extending the resultant sensitivity enhancement to other spins is proposed in which the transfer of order occurs through population differences not associated with magnetization.

Fourier transform multiple quantum nuclear magnetic resonance

Abstract: The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Universal Quantum Simulators

Abstract: Feynman's 1982 conjecture, that quantum computers can be programmed to simulate any local quantum system, is shown to be correct.