Solid-state Nuclear Magnetic Resonance Techniques Research Articles

Recent advances in solid-state nuclear magnetic resonance techniques to quantify biomolecular dynamics.

The emergence of solid-state NMR (SSNMR) techniques over the last decade has greatly expanded the range of accessible biomolecules amenable to structural study.1-2 These methods were developed and first applied to a variety of microcrystalline model proteins including BPTI,3 SH3,4-5 ubiquitin,6-7 kaliotoxin,8 and GB1.9-11 These studies demonstrated that individual 13C, 15N and 1H sites throughout the entire protein could be uniquely resolved and assigned for purposes of high-resolution structure determination. Whereas traditional solid-state NMR approaches required site-specific isotopic labeling, the new developments enable entire proteins to be examined by resolving signals in multiple dimensions. These capabilities also permit the measurement of dynamic parameters through correlation spectroscopy, which increases the throughput, sensitivity, reliability and reproducibility of such measurements. A major advantage of SSNMR is that a large range of sample conditions and types can be examined, including many that are not accessible to solution NMR methods, such as membrane proteins and fibrils in addition to microcrystals. Recent examples include fibrils of the HET-s prion,12 high molecular weight oligomers of αB crystallin,13 the tetrameric assembly of the M2 peptide with bound amantadine drug in physiologically relevant bilayers,14 the trimeric membrane protein YadA,15 the photoreceptor proteorhodopsin,16 fibrils of the core domain of transthyretin,17 and beta-amyloid plaques derived from the brains of Alzheimer's disease patients.18 Such studies have opened up new avenues for exploration of fundamental biophysics and biochemistry, and yielded insights into clinically relevant events in membranes and aggregated states not accessible to solution NMR or X-ray crystallography. Moreover, SSNMR experiments are applicable over a larger range of temperatures than can be accessed by solution NMR19 and thereby enable a continuum of experimental conditions from physiological temperatures to the cryogenic temperatures at which most crystal structures are solved. Another advantage of NMR methods in general is that they are non-perturbing and do not require covalent modification of the protein. Thus, the dynamic information procured from such experiments has the potential to accurately represent the native protein behavior. This review focuses on developments over the last five years, regarding methods and applications of dynamics studies by solid-state NMR over timescales ranging from nanoseconds to days.

ChemInform Abstract: Progress in Development and Application of Solid‐State NMR for Solid Acid Catalysis

Solid acid catalysts have been widely used in petrochemical industry and their catalytic activities are normally dictated by their acidities. Unlike conventional acidity characterization techniques such as titration, infrared, or temperature-programmed desorption, detailed acid features of solid acids, such as type, distribution, concentration, and strength of acid sites may be attained by advanced methods involving pertinent probe molecules and state-of-the-art solid-state nuclear magnetic resonance (SSNMR) techniques, i.e. double resonance and two-dimensional correlation spectroscopies. In addition, in situ solid-state NMR method is capable of probing the guest/host properties of the reactant at the active centers of the catalysts as well as the intermediate species formed during conversion. It provides direct experimental evidence for exploring the mechanism of catalytic reaction. In this report, the fundamental theory and the recent developments in solid-state NMR are reviewed with specific focus on relevant applications in structure and acidity characterization of solid acid catalysts and catalytic mechanisms. (C) 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Solid-State NMR Analysis of a Boron-Containing Pharmaceutical Hydrochloride Salt

A novel crystalline form of the boron-containing antibacterial drug (S)-3-(aminomethyl)-7-(3-hydroxypropoxy)benzo[c] [1,2]oxaborol-1(3H)-ol hydrochloride is studied by solid-state nuclear magnetic resonance (SSNMR) and single-crystal X-ray diffraction techniques. After determination of the crystal structure by X-ray diffraction, SSNMR spectroscopy of this form is performed to obtain structural information using experimental approaches based on dipolar correlation, chemical shift analysis, and quadrupolar interaction analysis. 1H SSNMR experiments at 16.4 T using magic-angle spinning (MAS) and homonuclear dipolar decoupling, 2D SSNMR experiments based on (1)H–(13)C and (1)H–(11)B dipolar heteronuclear correlation, and density functional theory (DFT) calculations are combined in a novel approach to obtain a nearly complete assignment of the (1)H spectrum of this crystalline phase. (11)B and (35)Cl chemical shift and quadrupolar parameters are obtained using the analysis of MAS spectra and are found to be accurately reproduced using DFT calculations. NMR chemical shielding and electric field gradient parameters obtained using these methods are related to hydrogen-bonding trends in the crystal structure. The results illustrate the increasing capability of SSNMR techniques involving (1)H, (11)B, and (35)Cl SSNMR in the analysis of the crystal structure of a pharmaceutical compound containing covalently bonded boron.

Synthesis and characterization of fluorinated carbon nanotubes for lithium primary batteries with high power density

The synthesis and characterization of fluorinated carbon nanotubes have been carried out under an inert gas containing fluorine. All of the samples have been characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (13C and 19F ss-NMR) and transmission electron microscopy (TEM) techniques. The comparison of the effects of various experimental parameters on the structure of fluorinated materials allows the disclosure of the fluorination mechanism. It is shown that fluorine was intercalated into the outer part of the carbon nanotubes initially where graphene layers were coaxial within a distance of 0.60 nm. In contrast, the inner part of the carbon nanotubes was not intercalated. The electrochemical performance such as discharge capacity as a cathode for a primary lithium battery has also been investigated. The samples with a F/C ratio of 0.75 exhibited the best performance, namely high energy and power densities. The highest specific energy density and specific power density were 1147 Wh kg−1 and 8998 W kg−1, respectively, at a current density of 4 A g−1.

Decay of spin coherences in one-dimensional spin systems

Strategies to protect multi-qubit states against decoherence are difficult to formulate because of their complex many-body dynamics. A better knowledge of the decay dynamics would help in the construction of dynamical decoupling control schemes. Here we use solid-state nuclear magnetic resonance techniques to experimentally investigate decay of coherent multi-spin states in linear spin chains. Leveraging on the quasi-one-dimension geometry of fluorapatite crystal spin systems, we can gain a deeper insight on the multi-spin states created by the coherent evolution, and their subsequent decay, than it is possible in three-dimensional (3D) systems. We are then able to formulate an analytical model that captures the key features of the decay. We can thus compare the decoherence behavior for different initial states of the spin chain and link their decay rate to the state characteristics, in particular their coherence and long-range correlation among spins. Our experimental and theoretical study shows that the spin chains undergo a rich dynamics, with a slower decay rate than for the 3D case, and thus might be more amenable to decoupling techniques.

Study on Thermal Degradation of Phenolic Resin

The thermal degradation of phenolic formaldehyde resins (PF) were studied using thermo gravimetric analysis (TGA). The evolved volatiles during thermal degradation of PF were examined by mass spectrometry (MS). The structure changes of thermal degradation of synthetical phenolic resin (PF1.2) and commercial resin (CPF) were investigated by solid-state 13C nuclear magnetic resonance (13C-NMR) techniques. The experimental results show that the degradation process of PF could be separated into three stages. In the first stage, ether bonds and unreacted terminal hydroxymethyl groups of the cured resin degraded. Methylene bridges decomposed into methyl groups then yielded both phenol and cresol homolog in the second stage. In the third stage, cyclodehydration and forming char of phenolic hydroxyl occurred. Our study showed that the main thermal degradation of PF in the second stage is the decomposition reaction of the methylene bridges instead of auto-oxidation of the methylene bridges reported in previous works.

Molecular characterization of organic matter in Canadian Arctic paleosols for paleoecological applications

The preservation of soil organic matter (SOM) in loess–paleosol sequences from the Klondike goldfields in the Canadian Arctic has recorded several Quaternary glacial–interglacial cycles. Both non-targeted and targeted molecular organic geochemical techniques are employed to provide an additional approach to confirm and expand on existing reconstructions of late Quaternary environments in this region. Solid-state nuclear magnetic resonance (NMR) techniques performed on 31 paleosol samples formed during marine isotope stage (MIS) 2 and MIS 4 showed similar SOM characteristics. However solution-state NMR of the base-soluble extracts detected a large amount of vascular plant inputs and preservation. Biomarker analysis revealed a large predominance of constituents from higher plants such as long chain n-alkyl lipids, hydroxyl n-alkyl lipids, sterols and terpenoids. Vegetation source proxies based on the distribution of n-alkanes were inconclusive, however when combined with additional targeted extractions of hydrolysable lipids and lignin-derived phenols, it was revealed that the major vegetative sources originated from woody and non-woody angiosperms. In addition, a novel thiazole–sulfur biomolecular structure in a paleosol from the Goldbottom Creek, 14C dated to 25.3kyr before present (BP), was observed by solution-state NMR. This type of molecule has not been previously detected in base-soluble extracts of SOM. Although the origin and preservation of this structure is unknown, a mechanism based on microbial biomolecular intermediates is proposed. The spectra also contained large resonances attributed to plant cuticle functional groups providing further evidence of plant-derived material preservation. Collectively, this comprehensive dataset agrees with paleoecological hypotheses derived from physical morphology and plant macrofossils suggesting that these molecular methods may be applied as an alternative technique for Arctic paleoclimate studies as well as identification of molecular constituents not previously identified in modern soils.

Coexistence of Helical Morphologies in Columnar Stacks of Star-Shaped Discotic Hydrazones

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone derivatives with dendritic groups attached to their outer peripheries, containing six, eight, and ten carbons of linear alkoxy chains. On the basis of two-dimensional wide angle X-ray scattering (2DWAXS), the elevated temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh) organization with nontilted hydrazone discs for all three compounds. With WAXS, advanced solid-state nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the compounds with six and ten carbons of achiral alkoxy side chains were further subjected to studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains. This combined approach led to models of coexisting helical columnar stacking morphologies for both systems with two different tilt/pitch angles between successive hydrazone molecules. The differences in tilt/pitch angles between the two compounds illustrate that the columns with short alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity, while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone. The formation of different packing morphologies in the crystalline phase of a columnar LC has rarely been reported so far, which suggests the possibility of complex stacking structures of similar organic LC systems, utilizing small molecules as potential materials for applications in organic electronics.

Highly Mobile Ions: Low-Temperature NMR Directly Probes Extremely Fast Li+ Hopping in Argyrodite-Type Li6PS5Br

The development of safe and long-lasting all-solid-state lithium-ion batteries needs electrolytes with exceptionally good transport properties. Here, we report on the combination of several solid-state nuclear magnetic resonance (NMR) techniques which have been used to precisely probe short-range as well as long-range Li+ dynamics in Li6PS5Br from an atomic-scale point of view. NMR data clearly reveal an extraordinary high Li diffusivity. This manifests in so-called diffusion-induced spin–lattice relaxation NMR rate peaks showing up at temperatures as low as 260 K. From a quantitative point of view, at ambient temperature the Li jump rate is of the order of 109 s–1 which corresponds to a Li+ conductivity in order of 10–3 to 10–2 S/cm, thus, indicating “liquid-like” Li+ diffusion behavior in Li6PS5Br.

Progress in development and application of solid-state NMR for solid acid catalysis

Solid acid catalysts have been widely used in petrochemical industry and their catalytic activities are normally dictated by their acidities. Unlike conventional acidity characterization techniques such as titration, infrared, or temperature-programmed desorption, detailed acid features of solid acids, such as type, distribution, concentration, and strength of acid sites may be attained by advanced methods involving pertinent probe molecules and state-of-the-art solid-state nuclear magnetic resonance (SSNMR) techniques, i.e. double resonance and two-dimensional correlation spectroscopies. In addition, in situ solid-state NMR method is capable of probing the guest/host properties of the reactant at the active centers of the catalysts as well as the intermediate species formed during conversion. It provides direct experimental evidence for exploring the mechanism of catalytic reaction. In this report, the fundamental theory and the recent developments in solid-state NMR are reviewed with specific focus on relevant applications in structure and acidity characterization of solid acid catalysts and catalytic mechanisms.

Characterization and Catalytic Performance in n-Hexane Cracking of HEU-1 Zeolites Dealuminated Using Hydrochloric Acid and Hydrothermal Treatments

A series of modified HEU-1 zeolites were prepared using hydrochloric acid treatment, hydrothermal treatment, and a combination of these. The parent and modified HEU-1 zeolites were characterized by X-ray diffraction, N2 adsorption-desorption isotherms, X-ray fluorescence, temperature-programmed desorption of ammonia, Fourier-transform infrared spectroscopy of adsorbed pyridine, and 27Al solid-state magic-angle spinning nuclear magnetic resonance techniques. The catalytic cracking performances of the modified HEU-1 zeolites were evaluated on a fixed-bed microreactor at 625 °C, atmospheric pressure, and an n-hexane weight hourly space velocity of 2.0 h−1. The results showed that extra-framework alumina could be removed by directly treating with hydrochloric acid, and framework alumina could be removed by hydrothermal treatment. Acidity characterization showed that the weak acid sites of HEU-1 zeolites could be adjusted using acid treatment, and the number of strong acid sites could be reduced using hydrothermal treatment. The framework nSi:nAl ratio was improved by using a combination of hydrothermal treatment and acid leaching. The acidities of the HEU-1 zeolites were adjusted and both the mesopore volume and external surface area were enhanced. In n-hexane cracking reactions, the dealuminated HEU-1 zeolite obtained by hydrothermal treatment for 4 h at 600 °C and then washing with hydrochloric acid exhibited the highest propylene production as a result of suppression of the hydrogen transfer reaction. At 625 °C, the selectivity for propylene on HEU-1(HT4-HCl) increased to 35.2% from 21.9%, and the lifetime of HEU-1(HT4-HCl) was also prolonged from not more than 10 to 40 h. 采用 HCl 溶液和/或水热处理 HEU-1 分子筛, 利用 X 射线衍射、N2 吸附-脱附、X 射线荧光光谱、NH3 程序升温脱附、吡啶吸附傅里叶变换红外光谱和铝固体核磁共振等对脱铝处理的 HEU-1 分子筛进行了表征, 并考察了脱铝 HEU-1 分子筛上正己烷的催化裂解性能. 结果表明, 采用 HCl 溶液处理 HEU-1 分子筛, 能有效脱除非骨架铝, 可调变样品的弱酸中心; 而水热处理 HEU-1 分子筛, 能有效脱除骨架铝, 可调变样品的强酸中心. 水热处理和 HCl 溶液处理有效结合可以达到同时调变 HEU-1 分子筛的酸类型和酸密度的目的. 脱铝 HEU-1 分子筛上正己烷催化裂解反应结果表明, 氢转移反应被明显抑制, 丙烯选择性提高. 600 °C 水热处理后再用 HCl 溶液处理的 HEU-1 分子筛于 625 °C 反应时丙烯选择性由未改性的 21.9% 提高到 35.2%, 活性稳定性也由不足 10 h 提高到 40 h.

Pillaring cetyltrimethylammonium-magadiite: a stepwise method to mesoporous materials with controlled pores sizes and distribution

The reaction of layered silicates like magadiite with the cetyltrimethylammonium cation (CTA) was studied using different CTA/Na molar ratios. The layered materials have been synthesized by traditional methods and the intercalation of the lamellar material followed the conventional ion exchange procedure by controlled introduction of different amounts of surfactants as spacers in the lamellar structure. After the introduction of the surfactant molecules, two more steps are necessary to obtain the Pillared Interlayered Clays (PILCs): (i) the introduction of the pillarizing agent which is responsible for the growth of interlamellar pillars and (ii) the calcination process. Characterizations were carried out by different techniques to probe the effects of interlayer amounts of surfactant. X-ray diffraction showed that small amounts of surfactant are enough to increase the interlayer space to the maximum attainable by this process. The evolution of the structural order was probed through solid-state nuclear magnetic resonance (SSNMR) techniques. Finally, surface analyses indicated variations in the shape of mesopores at different CTA loadings.

Deciphering the inert atmosphere degradation patterns in hybrid silicones

Structurally similar silicone ceramers were synthesized by sol–gel processing of organic silanes. The chemical structures of the hardened ceramer materials were investigated by studying the gaseous products produced during elevated temperature heating. Fourier transformation infra-red (FTIR) spectroscopy and thermogravimetric analysis (TGA) were utilized to study the degradation phenomenon in the ceramers. The pyrolyzed residues were analyzed by Raman and 29Si solid-state nuclear magnetic resonance (SSNMR) and X-ray diffraction (XRD) techniques. The surfaces of the residues were analyzed with a scanning electron microscope (SEM). The results from the characterization techniques were utilized to investigate the degradation mechanisms of the ceramer materials.

Solid-state 1H-NMR relaxation behavior for ultra-high-molecular-weight polyethylene reactor powders with different morphologies

The relaxation behaviors of several commercial ultra-high-molecular-weight polyethylene (UHMW-PE) reactor powders were compared. The surface and internal morphologies of these powders were characterized by scanning and transmission electron microscopy observations. On the basis of these morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. Molecular motions were detected by solid-state proton nuclear magnetic resonance (1H-NMR) techniques. The temperature dependence of the molecular motion characterizes the structural differences between these powders. The intermediate region between crystal/amorphous phases first relaxes during gradual heating from room temperature for all reactor powders examined in this study. Such a release of constraint for boundary chains induces structural reorganization beyond the critical temperature before melting. This trend was also confirmed by an increase in crystallinity beyond the critical temperature. In contrast, the results of differential calorimetry (DSC) analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. A relationship between the morphological and 1H-NMR relaxation characteristics was interpreted for several reactor powders. The relaxation behaviors for several commercial ultra-high molecular weight polyethylene reactor powders were compared. On the basis of the morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. The results of differential scanning calorimetry analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. In contrast, molecular motions detected by solid-state proton nuclear magnetic resonance techniques characterize the structural differences between these powders.

Structural characterization of silk-polyurethane composite material for biomaterials using solid-state NMR

In this paper, we performed solid-state nuclear magnetic resonance (NMR) measurements, including T1H and T1ρ, of silk fibroin (SF)/polyurethane (PU) composites to examine their possible use as a material for artificial vascular grafts. In the development of new artificial vascular grafts made from SF/PU, it is important to examine the miscibility of the composites and their molecular dynamics, because these properties are intimately involved in the resulting physical properties of the resulting vascular graft. The T1H measurements showed that the domain size of the SF/PU=1:1 composite is smaller than the domain size of the 1:10 and 1:2 composites, indicating that the molecular miscibility between SF and PU are partially in close proximity, particularly in the SF/PU=1:1 composite. Additionally, we observed that the molecular motion of the soft segment of PU in the SF/PU composites becomes slow, suggesting that the soft segment of PU interacts with SF to some extent. These analyses provided basic structural information for the development of silk-based artificial vascular grafts using PU. In this study, we investigated the molecular structure of the composite material of silk fibroin and polyurethane as coating agent of vascular grafts by using solid-state nuclear magnetic resonance (NMR) techniques. To elucidate the structure and dynamics, 13C cross-polarization (CP)/magic angle spinning (MAS) NMR studies including NMR relaxation experiments were performed. These observations showed that there is a partial interacted portion between soft segment of PU and SF, in particular, the domain size of PU:SU=1:1 composite was smaller than that of the other composites. This study gave basic information on development of new silk-like materials for vascular grafts.

The stability of organic solvents and carbon electrode in nonaqueous Li-O2 batteries

The effects of six types of aprotic organic solvents on the discharge performance and discharge products in Li-O2 batteries are systematically investigated. A large amount of Li2O2 is identified in the air electrodes discharged in glyme-based electrolytes, while only a small amount of Li2O2 is detected in the air electrodes discharged in the electrolytes of nitrile, ionic liquid, phosphate, and sulfoxide. Li2CO3 and LiF are also found as byproducts whose compositions are related to the solvents. Li2CO3 is produced from oxidation and decomposition of the solvent, not from the oxidation of the carbon-based air electrode, as revealed by using a 13C-labeled carbon electrode and the solid-state 13C-magic angle spinning nuclear magnetic resonance technique. LiF in the discharge products can be attributed to the attack of superoxide radical anions to the Teflon binder and/or the F-containing imide salt. The formation of these byproducts will significantly reduce the Coulombic efficiency and cycle life of the Li-air batteries. Among the studied solvents, dibutyl diglyme is the suitable solvent for Li-O2 batteries based on its overall properties. However, better electrolytes that can ensure the formation of Li2O2 but minimize other reaction products need to be further investigated for long cycling rechargeable Li-air batteries.

Chemical structure changes in kerogen from bituminous coal in response to dike intrusions as investigated by advanced solid-state 13C NMR spectroscopy

Coal maturation due to igneous intrusions offers the ideal opportunity to study the chemical-structural evolution of rapidly heated coal. Variations in the chemical structures of kerogens from high volatile bituminous coal near two igneous dike intrusions in the Illinois Basin were characterized in detail by advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. With decreasing distance to dike contacts and increasing thermal maturity (vitrinite reflectance Ro from 0.62% to 5.03%), the new insights into the chemical structural changes in coal kerogens include (i) the C(CH2)C groups removed faster than CCH3 groups, suggesting that the major cleavage of aliphatics should not occur at aryl rings, (ii) a progressive decrease of the relative abundance of protonated aromatic carbons, possibly due to replacement of aromatic hydrogens by aryl groups via cross-linking, and (iii) an increase in the aromatic cluster size, in addition to the well established changes of the elimination of aliphatics and aromatic C-O groups and an increase in aromaticity. For the first time the growth of aromatic cluster sizes with increasing maturity was quantitatively estimated. Another novel finding is that coal kerogen (kerogen #22) directly at the contact with the large dike retained considerable aliphatic components with olefinic and COO groups. These aliphatic components could have been entrapped and failed to diffuse out rapidly enough because intense and very fast heating rendered this kerogen partially fluidized. Our advanced solid-state NMR techniques provided deep insights into the structural changes in kerogen from bituminous coal in response to dike intrusions which cannot be achievable using routine 13C cross polarization/magic angle spinning technique.

Determination of Phase Content in Multiphase Polymers by Solid-State NMR Techniques

Solid-state nuclear magnetic resonance (NMR) spectroscopy has emerged as a relatively facile technique for the characterization of multi-component polymer systems. In particular, it has emerged to be a useful technique for probing the molecular structure, conformation and dynamics of polymer chains at interfaces between phases in various types of multi-component polymer systems including nanomaterials. The usefulness of solid-state NMR stems from its ability to non-destructively probe not only the bulk of the polymer, but moreover its ability to selectively probe the interface or interphase. As such, the technique has been extensively exploited in the study of multi-component polymer systems. To achieve 13C spectral resolution in the solid-state magic angle spinning (MAS), dipolar decoupling and cross-polarization are applied which enables the study of individual carbon atoms directly with excellent resolution and sensitivity. Some examples of applications of this technique to the study of multiphase aliphatic polyesters are reviewed herein.

New Insights into Frustrated Lewis Pairs: Structural Investigations of Intramolecular Phosphane–Borane Adducts by Using Modern Solid-State NMR Techniques and DFT Calculations

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.

Amyloid-Derived Peptide Forms Self-Assembled Monolayers on Gold Nanoparticle with a Curvature-Dependent β-Sheet Structure

Using a combination of Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (SSNMR) techniques, the secondary structure of peptides anchored on gold nanoparticles of different sizes is investigated. The structure of the well-studied CALNN-capped nanoparticles is compared to the structure of nanoparticles capped with a new cysteine-terminated peptide, CFGAILSS. The design of that peptide is derived from the minimal amyloidogenic sequence FGAIL of the human islet polypeptide amylin. We demonstrate that CFGAILSS forms extended fibrils in solution. When constrained at a nanoparticle surface, CFGAILSS adopts a secondary structure markedly different from CALNN. Taking into account the surface selection rules, the FTIR spectra of CFGAILSS-capped gold nanoparticles indicate the formation of β-sheets which are more prominent for 25 nm diameter nanoparticles than for 5 nm nanoparticles. No intermolecular (13)C-(13)C dipolar coupling is detected with rotational resonance SSNMR for CALNN-capped nanoparticles, while CALNN is in a random coil configuration. Coupling is detected for CFGAILSS-capped gold nanoparticles, however, consistent with an intermolecular (13)C-(13)C distance of 5.0 ± 0.3 Å, in agreement with intermolecular hydrogen bonding in a parallel β-sheet structure.