Sangdoo Ahn

Visualizing the dipolar field in solution NMR and MR imaging: three-dimensional structure simulations

Abstract We propose and demonstrate an efficient numerical approach which switches back and forth between real and Fourier spaces to handle dipolar field effects for heterogeneous three-dimensional structures. Applications to magnetic resonance imaging and improving field homogeneity are discussed.

Intermolecular zero‐quantum coherence imaging of the human brain

The first intermolecular zero‐quantum coherence (iZQC) MR images of the human brain at 4T are presented. To generate iZQC images, a modified echo‐planar imaging pulse sequence was used which included an additional 45° RF pulse and a correlation gradient. The observability and nonconventional contrast of human brain iZQC images at 4T is demonstrated. Axial images are presented for various pulse sequence parameters, and a zero‐quantum relaxation map is obtained. Magn Reson Med 43:627–632, 2000.

The boundary between liquidlike and solidlike behavior in magnetic resonance

Recent experimental work in two-dimensional solution NMR (nuclear magnetic resonance) has demonstrated anomalous cross-peaks and additional resonances due to dipolar couplings between distant nuclei. These spectra have been analyzed either classically, using Bloch equations which include a mean-field approximation to the demagnetizing field, or quantum mechanically, using a full density matrix picture which shows that the peaks correspond to intermolecular multiple-quantum coherences (iMQCs). Here we use a density matrix treatment to predict intensities in solution for dipolar effects conventionally seen in solids; we also explore in detail the fundamental differences between dipolar effects in solids and liquids. For example, even though polarization transfer via the dipolar Hamiltonian in solution is not possible, indirect detection with substantial signal enhancement is possible. We find that, even for high-γ nuclei such as 1H or 3He, solidlike dipolar effects are quite small unless the diffusion const...

Selective Colorimetric Sensor for Hg2+ Ions Using a Mixture of Thiourea Derivatives and Gold Nanoparticles Stabilized with Adenosine Triphosphate

The detection of Hg ions has attracted considerable attention in recent years because it is one of the most toxic elements on the planet and is a known environmental pollutant routinely released from coal-burning power plants, oceanic and volcanic emissions, gold mining, and solid-waste incineration. As a result, numerous optical detection methods for Hg ions have been developed based upon several strategies, including fluorogenic and chromogenic organic dyes, functional polymers, oligonucleotides, and proteins. In addition, nanomaterials, including carbon nanotubes, silica nanoparticles, quantum dots, and gold nanoparticles, have been recently studied as alternative detection methods for Hg ions. In particular, colorimetric chemosensors are attractive because they can be understood with the naked eye, in some cases at the point of use. Therefore, a lot of colorimetric sensors for Hg ions have been developed based upon chromogenic organic dyes and gold nanoparticles. Gold nanoparticles (AuNPs) are attractive scaffolds for the creation of a colorimetric sensing system of Hg ions owing to their optical properties. Gold nanoparticles are good chromophores because their extinction coefficients are 3–5 orders of magnitude higher than those of organic dye molecules. Gold nanoparticles also have unique distance-dependent optical properties that can be chemically programmed through the use of specific host compounds; DNA, etc. can induce a dramatic red-to-blue color change in the AuNPs. So far, several strategies for the detection of Hg ions have been developed, including chemically modified AuNPs, DNA functionalized AuNPs, and a mixture of oligonucleotide and unmodified AuNPs. Chemical modification methods utilize the interconnection of chemically modified AuNPs induced by the interaction between Hg ions and ligands on the AuNPs. The DNA-functionalized AuNP method and the mixture of oligonucleotides and AuNPs utilize thymine–Hg–thymine interactions; Hg ions can selectively bind in between two mismatched oligonucleotide thymines, and a measurable signal is transduced by the interaction. Unmodified AuNP-based colorimetric chemosensor methods are the simplest of these methods and can detect Hg ions in aqueous media with high sensitivity and selectivity. Most of unmodified AuNPbased colorimetric chemosensors have been utilized with the mixture of oligonucleotides and AuNPs and the high specificity of oligonucleotide interactions with Hg ions. Therefore, it is desirable to develop another approach for unmodified AuNP-based chemosensors for Hg ions. Herein, we describe a new type of unmodified AuNPbased chemosensor for Hg ions in aqueous media with high sensitivity and selectivity using a Hg-promoted desulfurization reaction of a thiourea derivative and the reactivity of thiourea with gold nanoparticles. Thiourea is readily transformed into urea by a Hg-promoted desulfurization reaction owing to the strong thiophilic affinity of Hg ions which, unlike urea, can bind readily with gold nanoparticles. In addition, many thiourea derivatives have been developed as optical chemodosimeters for Hg ions by utilizing optical changes associated with this reaction. To develop an unmodified AuNP-based chemosensor, we used adenosine-triphosphate (ATP)-stabilized AuNPs (sAuNPs) prepared by mixing citric-acid-stabilized AuNPs and ATP. The sAuNPs are more stable than citric-acid-stabilized AuNPs over a wider pH range even in high salt concentrations. Typically, sAuNPs can aggregate immediately when exposed to metal ligands, such as thiourea. Therefore, we expected thiourea derivatives to react with the sAuNPs and cause them to aggregate owing to surface changes in the sAuNPs that transform them from being hydrophilic (ATP) into being hydrophobic (thiourea derivative). This reaction results in a visible color change in the sAuNP solu[a] S. Kim, N. H. Lee, S. H. Seo, M. S. Eom, Prof. Dr. S. Ahn, Prof. Dr. M. S. Han Department of Chemistry, Chung-Ang University Seoul 156-756 (Republic of Korea) Fax: (+82) 2-825-4736 E-mail : mshan@cau.ac.kr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201000483.

Effects of intermolecular dipolar couplings in solution NMR in separated time intervals: the competition for coherence transfer pathways

Abstract We investigate the competition between intermolecular dipolar effects in multiple time intervals and develop a theoretical framework to understand coherence transfer when this competition is important. The quantum picture permits explicit evaluation of coherence transfer pathways, which have been verified by experimental data obtained from pulse sequences with multiple-quantum selective phase cycling. Implications for sequences in common use in biological applications are discussed.

Understanding third-order dipolar effects in solution nuclear magnetic resonance: Hahn echo decays and intermolecular triple-quantum coherences

Dipolar effects in solution nuclear magnetic resonance lead to additional peaks in two-dimensional experiments. These peaks, which have the experimental properties of intermolecular multiple-quantum coherences, have been used in a variety of applications. Most efforts have focused on intermolecular zero-quantum or double-quantum coherences, which originate in two-spin terms from the equilibrium density matrix. In this paper, we characterize the “third-order experiments” (Hahn echo decay and triple-quantum CRAZED, which both originate in the three-spin terms in the equilibrium density matrix) both theoretically and experimentally. For example, in the coupled-spin picture, Hahn echo decays in concentrated solutions arise initially from intermolecular, 3-spin, −1-quantum coherences, which are partially converted to 3-spin, +1-quantum coherences by the second pulse, and hence survive the 1:1 coherence transfer echo. Such terms require two dipolar couplings to become observable. We discuss the general properti...

Multiple spin echo generation by gradients of the radio frequency amplitude: Two-dimensional nutation spectroscopy and multiple rotary echoes

NMR (nuclear magnetic resonance) nutation is treated with respect to demagnetizing-field effects on the evolution of spin coherences. A two-dimensional NMR nutation spectroscopy scheme is suggested consisting of a single radio frequency (RF) pulse and a free-evolution period. The RF pulse amplitude as well as the external magnetic field are assumed to be subject to gradients in the same but otherwise arbitrary direction. Cross peaks are predicted as frequency domain counterparts of multiple echoes. It is suggested to analyze the cross peak shape in terms of distributions of internal gradients arising from magnetic susceptibility inhomogeneities in heterogeneous samples. Furthermore, a pulse scheme solely based on gradients of the RF amplitude is treated resulting in the prediction of multiple rotary echoes as counterparts to the conventional rotary echo. The origin again is evolution in the presence of spatially modulated longitudinal magnetization in the tilted rotating frame.

In vivo 1H-MRS hepatic lipid profiling in nonalcoholic fatty liver disease: An animal study at 9.4 T

The applicability of the in vivo proton magnetic resonance spectroscopy hepatic lipid profiling (MR‐HLP) technique in nonalcoholic fatty liver disease was investigated. Using magnetic resonance spectroscopy, the relative fractions of diunsaturated (fdi), monounsaturated (fmono), and saturated (fsat) fatty acids as well as total hepatic lipid content were estimated in the livers of 8 control and 23 CCl4‐treated rats at 9.4 T. The mean steatosis, necrosis, inflammation, and fibrosis scores of the treated group were all significantly higher than those of the control group (P < 0.01). There was a strong correlation between the histopathologic parameters and the MR‐HLP parameters (r = 0.775, P < 0.01) where both steatosis and fibrosis are positively correlated with fmono and negatively correlated with fdi. Both necrosis and inflammation, however, were not correlated with any of the MR‐HLP parameters. Hepatic lipid composition appears to be changed in association with the severity of steatosis and fibrosis in nonalcoholic fatty liver disease, and these changes can be depicted in vivo by using the MR‐HLP method at 9.4 T. Thus, while it may not likely be that MR‐HLP helps differentiate between steatohepatitis in its early stages and simple steatosis, these findings altogether are in support of potential applicability of in vivo MR‐HLP at high field in nonalcoholic fatty liver disease. Magn Reson Med 70:620–629, 2013.

Combined Analysis of Stable Isotope, 1H NMR, and Fatty Acid To Verify Sesame Oil Authenticity

The aim of this study was to verify the authenticity of sesame oils using combined analysis of stable isotope ratio, (1)H NMR spectroscopy, and fatty acid profiles of the oils. Analytical data were obtained from 35 samples of authentic sesame oils and 29 samples of adulterated sesame oils currently distributed in Korea. The orthogonal projection to latent structure discriminant analysis technique was used to select variables that most effectively verify the sesame oil authenticity. The variables include δ(13)C value, integration values of NMR peaks that signify the CH3 of n-3 fatty acids, CH2 between two C═C, protons from sesamin/sesamolin, and 18:1n-9, 18:3n-3, 18:2t, and 18:3t content values. The authenticity of 65 of 70 blind samples was correctly verified by applying the range of the eight variables found in the authentic sesame oil samples, suggesting that triple analysis is a useful approach to verify sesame oil authenticity.

Detection of iron-labeled single cells by MR imaging based on intermolecular double quantum coherences at 14 T.

To evaluate the efficiency and feasibility of intermolecular multiple quantum coherence (iMQC) magnetic resonance (MR) imaging for single cell detection, we obtained intermolecular double quantum coherence (iDQC) and conventional gradient echo (GE) images of macrophage cells labeled by contrast agents in gel. The iDQC images obtained with echo-planar readout visualized the labeled cells effectively and with a higher contrast than seen in conventional GE images, especially at low planar resolutions and with thick slices. This implies that iDQC imaging with contrast agents could be a good alternative to conventional MR imaging for detecting labeled single cells or cell tracking under favorable conditions.