1,721,100 research outputs found
Matrix-Assisted NMR
No full textDuring the last decades, the interest of chemistry toward increasingly sophisticated processes has grown exponentially. As a consequence, the evolution of the systems under investigation has been necessarily paired with the development of modern methodologies capable of handling the enormous amount of data stemming from samples of great complexity. Among the many examples in the literature, one of the biggest ongoing challenges is the analysis of mixtures, from reaction crude extracts to biological fluids like blood and urine. Indeed, chromatography has been - and still remains - one of the primary methods adopted to reduce the complexity of a multi-analyte system. Nonetheless, one intrinsic problem of the chromatographic approach is its inability to identify unknown molecules, and hyphenated techniques (mostly based on mass spectroscopy) have been developed just to overcome this stumbling block.
On the other hand, Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most powerful techniques for the investigation of organic compounds. NMR exploits an intrinsic property exhibited by some atomic nuclei -- the spin -- to acquire chemical and structural information through well-established experimental protocols, known as pulse sequences. In particular, solution-state NMR can boast a vast ensemble of procedures aimed at collecting detailed data about through bond connectivities (COSY, TOCSY, HSQC,...) or through space proximities (NOESY, ROESY,...). All these information are nothing less than fundamental for the structure determination of unknown compounds.
Even tough this makes NMR spectroscopy largely appealing, the acquisition of such extensive information ultimately translates into detecting many signals at once, so that spectra interpretation can become a very challenging task. This is especially true when observing 1H resonances, which display a small dispersion in the frequency domain (about 12 ppm) and spectral crowding becomes consequently a serious problem.
Not surprisingly, the situation becomes almost unmanageable when NMR is applied to the assay of mixtures, where the superposition of signals stemming from different species is virtually assured. Certainly, multidimensional NMR techniques can be useful for the interpretation of crowded single-molecule spectra, but they rapidly loose all their advantages as the number of components in the sample increases. As for chromatography, the advent of hybrid techniques like LC-NMR, where LC stands for Liquid Chromatography, has partly circumvented the aforementioned difficulties, yet at the cost of an expensive and dedicated instrumentation.
In the context of mixture analysis, matrix-assisted NMR methodologies stand as an alternative to the various hyphenated techniques. They rely on the combination of NMR spectroscopy and an external agent added to the sample, which can be either a molecular or macromolecular species, or even a mesoscopic matrix. The aim of such matrices is to differentiate the signals of the various components, favouring their detection and characterisation.
The present work is divided into three independent parts. The first two are dedicated to different subjects of matrix-assisted NMR. In particular, Part I is aimed at the understanding of the physical phenomena underlying signal broadening when a solid, stationary phase is used in Matrix-Assisted Diffusometry (MAD) NMR measurements. Part II focuses on nanoparticle-assisted NMR chemosensing, a technique where monolayer-protected gold nanoparticles are exploited to transfer magnetization to selected classes of analytes by means of the Nuclear Overhauser Effect. In this second part, different nanoparticle-assisted methodologies are presented and analysed, alongside with some strategies aimed at the enhancement of the sensitivity.
Part III concerns the complete 1H-NMR characterisation of the atomically precise Au38(SBut)24 gold nanocluster, which can be considered as a prototypical nanoparticle. The Au38 core features four different symmetry-unique and equally populated binding sites for the grafting of the ligands that constitute the coating monolayer. Each binding site shows a distinct pattern of resonances, so that the overall 1H-NMR spectrum of the cluster is the result of the superposition of four independent subspectra. In this case, the full characterisation of the spectrum has been achieved through a combined NMR-MD (Molecular Dynamics) analysis
The 46th EUCHEM conference on stereochemistry (Budie;rgenstock Conference 2011), brunnen, May 1-6, 2011
No full textCombinatorial chemistry: covalent versus dynamic approach
No full textA review on the concept of dynamic combinatorial chem. (DCC) and illustration in which areas of DCC has been applied and to which extend success has been achieved. In. Topics covered include chem. bond formation under thermodn. control, applications of DCC, DCC for detecting weak interactions, and systems chem
Dynamic Covalent Capture: A sensitive tool for detecting molecular interactions
No full textMolecular recognition is at the centre of many areas of chemistry. Examples are analytical chemistry (analyte-sensor), catalysis (transition state-catalyst), medicinal chemistry (drug-biotarget) and advanced materials chemistry (building block A-building block B). Methodology that allows the rapid and precise detection of molecular recognition events is essential in all these fields. Traditionally, molecular recognition has been studied based on a rational design approach involving many iterative optimisation loops, which makes it an energy- and time consuming process. Additionally, it requires detailed knowledge about the target and the recognition process itself, information which is not always available. Currently, combinatorial methods are increasingly being used for detecting molecular recognition events, allowing the simultaneous screening of a vast amount of chemical compounds enabling a much larger part of chemical space to be explored. Dynamic covalent capture extends on the combinatorial approach for detecting molecular recognition events, but at a higher sensitivity level compared to conventional methodologies and with the novelty of self-selection by the target. The essential point of dynamic covalent capture is that a molecular recognition event is followed by the formation of a reversible covalent bond between the two molecule
Catalytic self-assembled monolayers on gold nanoparticles
No full textThis review describes the attractiveness of catalytic self-assembled monolayers (SAMs) on gold
nanoparticles as catalytic systems. The hybrid inorganic–organic catalytic systems combine the
advantages of homogeneous and heterogeneous catalysis (higher activity and catalyst recycling,
respectively). The high fidelity process of SAM formation on gold nanoparticles, together with the
possibility of making mixed SAMs composed of different thiols, provides an unprecedented route
to stable, complex catalytic systems. Insertion of catalysts in a mixed monolayer can improve the
catalytic performances, due to catalyst orientation, changes in the local chemical environment, or
through the steering effect of neighbouring thiols. Alternatively, insertion of catalytic units in a
monolayer may be an essential prerequisite in the case when catalysis requires cooperation
between two catalytic units (for instance two metal ions). Finally, the multivalent nature of these
systems is an important feature especially in the case when the substrate contains multiple reactive
sites. Catalytic SAMs on gold nanoparticles also find applications beyond the field of catalysis,
for instance in diagnostics and nanotechnology
Dynamic covalent capture of hydrazides by a phosphonate-target immobilized on resin
Get PDFA protocol is described that permits the self-selection of hydrazides from a small library by a phosphonate-target immobilized on resin. Hydrazides are captured by a neighbouring aldehyde group through reversible hydrazone bond formation. Stabilizing intramolecular interactions between the phosphonate-target and functional groups of the hydrazides drive the selection process. The phosphonate-target is introduced onto commercially available Tentagel resin through straightforward synthetic steps. The functionalized resin could be conveniently characterized by HR-MAS NMR spectroscopy using a recently developed transverse relaxation filter that eliminates the strong phase defects commonly observed with CPMG sequences. In addition, a protocol was developed to quantitatively remove the captured hydrazides from resin in order to analyse their composition by LC/MS. Kinetic experiments were used to study hydrazone formation and exchange on resin yielding similar results to those obtained previously in solution. Competition experiments showed that the system reaches thermodynamic equilibrium if multiple hydrazides are added to the resin. Finally, competition experiments showed that the immobilized phosphonate-target indeed amplifies the capture of those hydrazides able to develop stabilizing interactions with the target. Importantly, the obtained amplification profile was nearly identical to the ones obtained previously in solution studies. Notably, the observed amplification factors for the self-selected hydrazides were higher, which was attributed to steric effects imposed by the resin
Controlling supramolecular complex formation on the surface of a monolayer-protected gold nanoparticle in water
Get PDFA combination of hydrophobic and electrostatic interactions
drives the self-assembly of a large number of small molecules on the surface of a monolayer-protected gold nanoparticle. The hydrophobic interactions originate
from the insertion of an aromatic unit in the hydrophobic part of the monolayer.This is evidenced by a shift in the emission wavelength of the fluorogenic probe upon binding. Up to around 35 small molecules can be simultaneously bound to the monolayer surface at micromolar concentrations in water. It is shown that an
understanding of the supramolecular interactions that drive complex formation on the monolayer surface provides unprecedented control over the supramolecular chemistry occurring on the surface. By taking advantage of the
different kinds of noncovalent interactions present in different probes, it is possibile to displace one type of surface-bound molecule from a heteromeric surface selectively. Finally, it is also possible to catch and release one type of surface-bound molecule selectively
Reversible Control over the Valency of a Nanoparticle-Based Supramolecular System
No full textThe reversible "catch-and-release" of small molecules from the surface of monolayer-protected gold nanoparticles is described. The valency of the system (i.e., the number of molecules bound to the surface) can be controlled through the addition and removal of metal ions from the monolayer. Both the change in valency and the release rate of the molecules are strongly pH-dependent. The release rate can be regulated by altering the ratio of metal ions in the monolayer
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