Heteronuclear Single Quantum Research Articles

A heteronuclear intermolecular single-quantum coherence scheme for high-resolution 2D J-resolved 1H NMR spectra in inhomogeneous magnetic fields

ABSTRACTBased on heteronuclear intermolecular single-quantum coherences between proton (1H) and quadrupolar nuclei (i.e. deuterium 2H), a three-dimensional nuclear magnetic resonance (NMR) pulse sequence is proposed for recovering high-resolution two-dimensional J-resolved NMR spectra from samples mixed with a deuterated solvent in the presence of large magnetic field inhomogeneities. Benefitting from excitation of spins via two different radio frequency (RF) transmit channels, this sequence is suitable for applications in randomly large inhomogeneous fields and the solvent suppression generally required in homonuclear intermolecular multiple-quantum coherence approaches is no longer necessary. Systematic theoretical analyses are given based on the distant dipolar field treatment. Experiment on a sample of corn oil in deuterated acetone and ethyl 3-bromopropionate and acetone dissolved in DMSO-d6 in a deshimmed field with severe inhomogeneous broadening is performed to show the feasibility and applicability of this sequence.

Molecular level characterization of diatom‐associated biopolymers that bind 234Th, 233Pa, 210Pb, and 7Be in seawater: A case study with Phaeodactylum tricornutum

AbstractIn order to investigate the importance of biogenic silica associated biopolymers on the scavenging of radionuclides, the diatom Phaeodactylum tricornutum was incubated together with the radionuclides 234Th, 233Pa, 210Pb, and 7Be during their growth phase. Normalized affinity coefficients were determined for the radionuclides bound with different organic compound classes (i.e., proteins, total carbohydrates, uronic acids) in extracellular (nonattached and attached exopolymeric substances), intracellular (ethylene diamine tetraacetic acid and sodium dodecyl sulfate extractable), and frustule embedded biopolymeric fractions (BF). Results indicated that radionuclides were mostly concentrated in frustule BF. Among three measured organic components, Uronic acids showed the strongest affinities to all tested radionuclides. Confirmed by spectrophotometry and two‐dimensional heteronuclear single quantum coherence‐nuclear magnetic resonance analyses, the frustule BF were mainly composed of carboxyl‐rich, aliphatic‐phosphoproteins, which were likely responsible for the strong binding of many of the radionuclides. Results from this study provide evidence for selective absorption of radionuclides with different kinds of diatom‐associated biopolymers acting in concert rather than as a single compound. This clearly indicates the importance of these diatom‐related biopolymers, especially frustule biopolymers, in the scavenging and fractionation of radionuclides used as particle tracers in the ocean.

Preferential Protonation and Methylation Site of Thiopyrimidine Derivatives in Solution: NMR Data

Protonation (alkylation) sites of several thiopyrimidine derivatives were directly determined by 1H-15N (1H-13C) heteronuclear single quantum coherence/heteronuclear multiple bond correlation methods, and it was found that in all compounds, protonation (methylation) occurred at the N1 nitrogen. GIAO DFT chemical shifts were in full agreement with the determined tautomeric structures. According to ab initio calculations, the stability of the different protonated forms and methylated derivatives was favored due to thermodynamic control and not kinetic control.

A new two-dimensional pulse sequence for T(2)* measurements of protons in (13)C isotopomers.

A new two-dimensional pulse sequence for T(2)* measurement of protons directly coupled to (13)C spins is proposed. The sequence measures the tranverse relaxation time of heteronuclear proton single-quantum coherence under conditions of free precession and is therefore well suited to evaluate relaxation losses of proton magnetization during preparation delays of heteronuclear pulse experiments in analytical NMR. The relevant part of the pulse sequence can be inserted as a "building block" into any direct or inverse detecting H,C correlation pulse sequence if proton spin-spin relaxation is to be investigated. In this contribution, the building block is inserted into a HETCOR as well as into a HMQC pulse sequence. Experimental results for the HETCOR-based sequence are given.