We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are ...stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd2+ is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.
Surface Chemistry of Colloidal PbSe Nanocrystals Moreels, Iwan; Fritzinger, Bernd; Martins, José C ...
Journal of the American Chemical Society,
11/2008, Volume:
130, Issue:
45
Journal Article
Peer reviewed
Solution nuclear magnetic resonance spectroscopy (NMR) is used to identify and quantify the organic capping of colloidal PbSe nanocrystals (Q-PbSe). We find that the capping consists primarily of ...tightly bound oleic acid ligands. Only a minor part of the ligand shell (0−5% with respect to the number of oleic acid ligands) is composed of tri-n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concentration. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and organic ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidation leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.
Nonstructural protein 5B (NS5B) is essential for hepatitis C virus (HCV) replication as it carries the viral RNA-dependent RNA polymerase enzymatic activity. HCV replication occurs in a ...membrane-associated multiprotein complex in which HCV NS5A and host cyclophilin A (CypA) have been shown to be present together with the viral polymerase. We used NMR spectroscopy to perform a per residue level characterization of the molecular interactions between the unfolded domains 2 and 3 of NS5A (NS5A-D2 and NS5A-D3), CypA, and NS5BΔ21. We show that three regions of NS5A-D2 (residues 250–262 (region A), 274–287 (region B), and 306–333 (region C)) interact with NS5BΔ21, whereas NS5A-D3 does not. We show that both NS5BΔ21 and CypA share a common binding site on NS5A that contains residues Pro-306 to Glu-323. No direct molecular interaction has been detected by NMR spectroscopy between HCV NS5BΔ21 and host CypA. We show that cyclosporine A added to a sample containing NS5BΔ21, NS5A-D2, and CypA specifically inhibits the interaction between CypA and NS5A-D2 without altering the one between NS5A-D2 and NS5BΔ21. A high quality heteronuclear NMR spectrum of HCV NS5BΔ21 has been obtained and was used to characterize the binding site on the polymerase of NS5A-D2. Moreover these data highlight the potential of using NMR of NS5BΔ21 as a powerful tool to characterize in solution the interactions of the HCV polymerase with all kinds of molecules (proteins, inhibitors, RNA). This work brings new insights into the comprehension of the molecular interplay between NS5B, NS5A, and CypA, three essentials proteins for HCV replication.
HCV replication requires the interaction of the viral polymerase NS5B with both viral and host proteins.
We performed the molecular characterization of the interactions between HCV NS5B, NS5A, and host CypA.
HCV NS5B and host CypA share a binding site on HCV NS5A.
The NS5A-D2 site, which interacts with both the HCV polymerase NS5B and the host CypA, might regulate the HCV replication.
Tuberculosis remains one of the most important causes of global mortality and morbidity, and the molecular mechanisms of the pathogenesis are still incompletely understood. Only few virulence factors ...of the causative agent Mycobacterium tuberculosis are known. One of them is the heparin-binding haemagglutinin (HBHA), an important adhesin for epithelial cells and an extrapulmonary dissemination factor. HBHA mediates mycobacterial adherence to epithelial cells via the interactions of its C-terminal, lysine rich repeat domain with sulfated glycoconjugates on the surface of epithelial cells.
Using defined heparin sulfate (HS) analogs, we determined the minimal heparin fragment length for HBHA binding and structural adaptations of the HBHA heparin-binding domain (HBD) upon binding to heparin. The NMR studies show significant shifts of all residues in the HBD upon interaction with heparin, with stronger shifts in the last repeats compared to the upstream repeats, and indicated that the HS fragments with 14 sugar units cover the entire C-terminal lysine-rich domain of HBHA. The differential implication of the repeats is determined by the relative position of prolines and lysines within each repeat, and may contribute to binding specificity. GAG binding induces a non-homogeneous structural rearrangement in the HBD, with stabilization of a nascent α-helix only in the last penta-repeats.
Mycobacterial HBHA undergoes structural adaptation upon interaction with GAGs, which is likely involved in binding specificities of the adhesin, and mycobacterial pathogens may use HBD polymorphisms for host or organ specificity. Further studies will aim at decoding the complementarity between HBD repeats and HS sequence.
Recently, solution NMR-based approaches have been developed that represent useful new tools for the in situ characterization of the capping ligand in colloidal nanocrystal dispersions. So far, this ...development has focused mainly on tightly bound ligands (no exchange) or ligands in slow exchange with the nanocrystal surface. In such systems, the ligand can be identified and its amount and interaction quantified via 1D 1H NMR, 1H−13C HSQC, and DOSY spectra. Here, we explore the case where capping ligands are in fast exchange with the nanocrystal surface. Using dodecylamine-stabilized CdTe (Q-CdTe|DDA) and octylamine-stabilized ZnO (Q-ZnO|OctA) nanoparticles, we first show that the NMR methods developed so far fail to evidence the bound ligand when the effect of the latter on the exchange-averaged parameters is marginalized by an excess of free ligand. Next, transfer NOE spectroscopy, a well-established technique in biomolecular NMR, is introduced to demonstrate and characterize the interaction of a ligand with the nanocrystal surface. Using Q-PbSe nanocrystals capped with oleic acids as a reference system, we show that bound and free ligands have strongly different NOE spectra wherein only bound ligands develop strong and negative NOEs. For the Q-CdTe|DDA system, transfer NOE spectra show a similar rapid appearance of strong, negative NOEs, thereby unambiguously demonstrating that DDA molecules spend time at the nanocrystal surface. In the case of Q-ZnO|OctA, where a more complex mixture is analyzed, transfer NOE spectroscopy allows distinguishing capping from noncapping molecules, thereby demonstrating the screening potential offered by this technique for colloidal quantum dot dispersions.
In the present work, high-resolution magic angle spinning (hr-MAS) NMR spectroscopy is applied as a straightforward nondestructive technique to quantify unreacted methacrylamide functionalities in ...cross-linked gelatin hydrogels. By adjusting several cross-linking parameters including the ultraviolet (UV) irradiation time and the photo-initiator concentration, the cross-linking degree can be easily varied. Remarkably, under all experimental conditions typically applied for hydrogel development, no more than 40% of the methacrylamide moieties present have reacted. The hr-MAS based approach to determine the cross-linking efficiency is shown to provide an innovative and more convenient alternative to the well-established classical techniques. In addition, the results obtained are in good correlation with mechanical analysis data.
The hydrodynamic radius of PAMAM dendrimers as a function of molar mass is investigated by diffusion NMR. As a characteristic length, the hydrodynamic radius is calculated by Stokes‐Einstein ...equation. Poly(amidoamine) (PAMAM) dendrimers of generations 0 to 7 of two different terminal groups (NH2, COONa) have been investigated. This dependence of the hydrodynamic radius from molar mass is compared with a scaling model and statistical model. A scaling exponent of 3.7 has been found in both cases. The scaling exponent found exceeds the dimension of the embedding space, thus the possible growth of dendrimers of this structure is limited.
Hydrodynamic radius of PAMAM‐NH2 □, PAMAM‐COONa ○ as a function of molar mass.
The application of Saturation Transfer Difference (STD) NMR spectroscopy for the characterization of dispersant particle interactions is introduced. STD NMR has hitherto been applied, with great ...success, to the characterization of ligand−protein interactions and is currently a standard tool in biomolecular NMR spectroscopy. Nevertheless, the STD NMR technique has so far not yet crossed the boundaries of the biomolecular field. Here, we demonstrate that in spite of clear differences between a protein binding site and the surface of a pigment nanoparticle, the latter can also be subjected to STD NMR analysis, allowing us to detect (screen for) binding ligands, discriminate ligand from nonligand, and obtain information on the binding epitope. The approach should be generally applicable as long as the nanoparticle is comprised of a dense network of hydrogens, implicating almost all organic molecular nanocrystals. Thus it provides a novel investigative tool for the study of dispersions that is highly complementary to existing ones.
Due to its low aqueous solubility paclitaxel is currently formulated in a Cremophor EL
®/ethanol mixture. However, the vehicle of this formulation causes several side-effects. Our objective was to ...formulate a tensioactive-free and solvent-free paclitaxel solution, which can be used for a hyperthermic intraperitoneal chemoperfusion procedure (HIPEC). The potential of chemically modified β-cyclodextrins to form complexes with paclitaxel was investigated as a means to increase the aqueous solubility of paclitaxel. Methylated β-CDs (randomly methylated and 2,6-dimethylated) showed the best ability to solubilise paclitaxel compared to sulfobutyl-ether- and hydroxypropyl-β-CD. The minimal ratio of paclitaxel versus randomly methylated-β-cyclodextrin (RAME-β-CD) yielding 100% inclusion efficiency was 1/20 (mol/mol). Paclitaxel/RAME-β-CD inclusion complexes prepared via freeze drying were stable for at least 6 months when stored at 4
°C. A 5
mg/ml paclitaxel solution was formulated using paclitaxel/RAME-β-CD-complexes. Upon dilution of these solutions, no precipitation was seen. After 24
h storage at room temperature or 2
h at HIPEC conditions (41.5
°C) the 1/40 (mol/mol) ratio showed the highest stability at paclitaxel concentrations of 0.1 and 0.5
mg/ml. When hydroxypropyl methylcellulose (HPMC) was added to the reconstitution medium, the stability significantly increased, offering the opportunity to reduce the amount of RAME-β-CDs in the formulation.
Cover:
The figure shows the molar‐mass dependence of the hydrodynamic radius for a series of poly(amidoamine) dendrimers investigated by diffusion NMR, sketched at the bottom. The scaling exponent ...describing the use of space in the dendrimers growth is derived and it is found that, for the dendrimers with a tetrafunctional core, it exceeds the dimension of the embedding space, thus implying that the growth of the molecule is limited. Further details can be found in the Full Paper by B. Fritzinger and U. Scheler
*
on page 1288.
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