Phase evolution of Na2O-Al2O3-SiO2-H2O gels in synthetic aluminosilicate binders

This study demonstrates the production of stoichiometrically controlled alkali-aluminosilicate gels ('geopolymers') via alkali-activation of high-purity synthetic amorphous aluminosilicate powders. This method provides for the first time a process by which the chemistry of aluminosilicate-based cementitious materials may be accurately simulated by pure synthetic systems, allowing elucidation of physicochemical phenomena controlling alkali-aluminosilicate gel formation which has until now been impeded by the inability to isolate and control key variables. Phase evolution and nanostructural development of these materials are examined using advanced characterisation techniques, including solid state MAS NMR spectroscopy probing (29)Si, (27)Al and (23)Na nuclei. Gel stoichiometry and the reaction kinetics which control phase evolution are shown to be strongly dependent on the chemical composition of the reaction mix, while the main reaction product is a Na2O-Al2O3-SiO2-H2O type gel comprised of aluminium and silicon tetrahedra linked via oxygen bridges, with sodium taking on a charge balancing function. The alkali-aluminosilicate gels produced in this study constitute a chemically simplified model system which provides a novel research tool for the study of phase evolution and microstructural development in these systems. Novel insight of physicochemical phenomena governing geopolymer gel formation suggests that intricate control over time-dependent geopolymer physical properties can be attained through a careful precursor mix design. Chemical composition of the main N-A-S-H type gel reaction product as well as the reaction kinetics governing its formation are closely related to the Si/Al ratio of the precursor, with increased Al content leading to an increased rate of reaction and a decreased Si/Al ratio in the N-A-S-H type gel. This has significant implications for geopolymer mix design for industrial applications.

Solid-state NMR structure characterization of a 13CO-Labeled Ir(I) complex with a P,N-donor ligand including ultrafast MAS methods

The structural characterization of a (13)CO-labeled Ir(I) complex bearing an P,N-donor ligand (1-[2-(diphenylphosphino)ethyl]pyrazole), [Ir(PyP)((13)CO)Cl] is demonstrated using a series of tailored solid-state NMR techniques based on ultrafast (60 kHz) Magic Angle Spinning (MAS), which facilitates correlations with narrow proton line-widths. Our 1D (1)H MAS and 2D (13)C and (31)P CP-MAS NMR spectra provided structural information similar to that obtained using NMR spectroscopy in solution. We employed high-resolution 2D solid-state correlation spectroscopy ((1)H-(13)C HETCOR, (1)H-(31)P correlation) to characterize the networks of dipolar couplings between protons and carbon/phosphorus. (1)H-(1)H SQ-SQ correlation spectra showed the dipolar contacts between all protons in a similar fashion to its solution counterpart, NOESY. The use of the (1)H single quantum/double quantum experiments made it possible to observe the dipolar-coupling contacts between immediately adjacent protons. Additionally, internuclear (13)CO-(31)P distance measurements were performed using REDOR. The combination of all of these techniques made it possible to obtain comprehensive structural information on the molecule [Ir(PyP)((13)CO)Cl] in the solid state, which is in excellent agreement with the single crystal X-ray structure of the complex, and demonstrates the enormous value of ultrafast MAS NMR techniques for a broad range of future applications.

Context dependence of protein misfolding and structural strains in neurodegenerative diseases

Vast arrays of structural forms are accessible to simple amyloid peptides and environmental conditions can direct assembly into single phases. These insights are now being applied to the aggregation of the Aβ peptide of Alzheimer's disease and the identification of causative phases. We extend use of the imaging agent Pittsburgh compound B to discriminate among Aβ phases and begin to define conditions of relevance to the disease state. Also, we specifically highlight the development of methods for defining the structures of these more complex phases.

A practical implementation of de-Pake-ing via weighted Fourier transformation

We provide an NMRPipe macro to meet an increasing need in membrane biophysics for facile de-Pake-ing of axially symmetric deuterium, and to an extent phosphorous, static lineshapes. The macro implements the development of McCabe & Wassall (1997), and is run as a simple replacement for the usual Fourier transform step in an NMRPipe processing procedure.

The lipid network

Natural cell membranes are composed of a remarkable variety of lipids, which provide specific biophysical properties to support membrane protein function. An improved understanding of this complexity of membrane composition may also allow the design of membrane active drugs. Crafting a relevant model of a cell membrane with controlled composition is becoming an art, with the ability to reveal the molecular mechanisms of biological processes and lead to better treatment of pathologies. By matching physiological observations from in vivo experiments to high-resolution information, more easily obtained from in vitro studies, complex interactions at the lipid interface are determined. The role of the lipid network in biological membranes is, therefore, the subject of increasing attention.

Copper Modulation of Amyloid Beta 42 Interactions with Model Membranes

Growing evidence supports that interactions of the amyloid-β peptide Aβ(1–42) with neuronal cell membranes and copper are involved in Alzheimer’s disease pathogenesis. We report using solid-state NMR that the peptide significantly perturbed the phosphate and upper acyl chain region of bilayers comprising brain total lipid extract to cause domain segregation. Deep headgroup perturbations were also realized for palmitoyloleoylphospatidylcholine–cholesterol model systems; however, incorporating 10 % palmitoyloleoylphosphatidylserine or the ganglioside GM1 resulted in a more peripheral interaction. Cu2+ at a 1 : 7 molar ratio to peptide caused deeper penetration into model systems, but partially attenuated interactions with brain total lipid extract. Thioflavin T assay showed that bilayers affected amyloid formation in a mode dependant on lipid content, and was further modulated by addition of Cu2+. Our data support that ternary interactions between Cu2+, lipids and Aβ(1–42) may have significance in Alzheimer’s disease, and challenge the validity of model bilayers as substitutes for natural systems.

Self-assembly of peptides into spherical nanoparticles for delivery of hydrophilic moieties to the cytosol

We report a novel class of self-assembling peptide nanoparticles formed by mixing aqueous solutions of K(16) peptide and a 20 amino acid peptide of net charge -5 (GLFEALLELLESLWELLLEA). Particle formation is salt-dependent and yields perfectly spherical nanoparticles of approximately 120 to approximately 800 nm diameter, depending on buffer composition and temperature, with a stoichiometry of approximately 1:2.5 for the cationic and anionic peptides. The anionic peptide forms an alpha-helix in aqueous solution, has all five glutamates on one side of the helix, and exists entirely as a discrete oligomer of 9-10 peptides. A rigid oligomer with 45-50 negative charges almost certainly represents the core component of these nanoparticles, held together by electrostatic interactions with the unstructured K(16) peptide. Cells internalize these particles by an endocytic process, and free particles are frequently seen in the cytosol, presumably because of the acid-dependent fusogenic properties of the anionic peptide. Among other applications, these particles have potential for the targeted delivery of single or multiple therapeutic moieties directly to the cytosol, and we report the successful delivery of a K(16)-linked pro-apoptosis peptide.

Generalized biaxial shearing of MQMAS NMR spectra

Two dimensional multiple quantum (MQ) MAS NMR experiments have become popular due to the wide applicability of this technique to structural questions in materials science, the abundance of half-integer spin nuclei in the periodic table, and the ease of implementation on typical solid state NMR instruments. In spite of the high-resolution theoretically possible from such experiments, the homogeneous and inhomogeneous broadening factors inherent in many samples of interest can make spectral analysis challenging. We present several possible spectral shearing schemes that may be useful for spectral analysis, and in particular we introduce shearing in the directly detected dimension. We suggest that for amorphous or disordered samples that give broad spectral features, shearing may be used as a general tool for optimal positioning of these features relative to one another and for the characterization of isotropic chemical and quadrupolar shifts.

Membrane interactions of antimicrobial peptides from Australian frogs

The membrane interactions of four antimicrobial peptides, aurein 1.2, citropin 1.1, maculatin 1.1 and caerin 1.1, isolated from Australian tree frogs, are reviewed. All four peptides are amphipathic alpha-helices with a net positive charge and range in length from 13 to 25 residues. Despite several similar sequence characteristics, these peptides compromise the integrity of model membrane bilayers via different mechanisms; the shorter peptides exhibit a surface interaction mechanism while the longer peptides may form pores in membranes.

Metal effects on the membrane interactions of amyloid-beta peptides

A beta (1-42) peptide, found as aggregated species in Alzheimer's disease brain, is linked to the onset of dementia. We detail results of 31P and 2H solid-state NMR studies of model membranes with A beta peptides and the effect of metal ions (Cu2+ and Zn2+), which are found concentrated in amyloid plaques. The effects on the lipid bilayer and the peptide structure are different for membrane incorporated or associated peptides. Copper ions alone destabilise the lipid bilayer and induce formation of smaller vesicles, but not when A beta(1-42) is associated with the bilayer membrane. A beta (25-35), a fragment from the C-terminal end of A beta(1-42), which lacks the metal coordinating sites found in the full length peptide, is neurotoxic to cortical cortex cell cultures. Addition of metal ions has little effect on membrane bilayers with A beta (25-35) peptides. 31P magic angle spinning NMR data show that A beta (1-42) and A beta (1-42)-Cu2+ complexes interact at the surface of anionic phospholipid membranes. Incorporated peptides, however, appear to disrupt the membrane more severely than associated peptides. Solid-state 13C NMR was used to compare structural changes of A beta (1-42) to those of A beta (25-35) in model membrane systems of anionic phospholipids and cholesterol. The A beta peptides appeared to have an increase in beta-strand structure at the C-terminus when added to phospholipid liposomes. The inclusion of Cu2+ also influenced the observed chemical shift of residues from the C-terminal half, providing structural clues for the lipid-associated A beta/metal complex. The results point to the complex pathway(s) for toxicity of the full-length peptide.

Boltzmann statistics rotational-echo double-resonance analysis

A new approach to rotational-echo double-resonance (REDOR) data analysis, analogous to Boltzmann maximum entropy statistics, is reported. This Boltzmann statistics REDOR (BS-REDOR) approach is useful for reconstructing an unbiased internuclear distance distribution for multiple internuclear distances from experimentally limited REDOR data sets on isolated spin pairs. The analysis is characterized by exploring reconstructions on model data and applied to both [1-(13)C,15N]-glycine and a long intramolecular distance in Abeta (16-22) peptide nanotubes. The approach also provides insight into the minimal number of REDOR data points required to allow faithful determination of dipolar couplings in systems with multiple internuclear distances.

Membrane interactions and the effect of metal ions of the amyloidogenic fragment Abeta(25-35) in comparison to Abeta(1-42)

Abeta(1-42) peptide, found as aggregated species in Alzheimer's disease brain, is linked to the onset of Alzheimer's disease. Many reports have linked metals to inducing Abeta aggregation and amyloid plaque formation. Abeta(25-35), a fragment from the C-terminal end of Abeta(1-42), lacks the metal coordinating sites found in the full-length peptide and is neurotoxic to cortical cortex cell cultures. We report solid-state NMR studies of Abeta(25-35) in model lipid membrane systems of anionic phospholipids and cholesterol, and compare structural changes to those of Abeta(1-42). When added after vesicle formation, Abeta(25-35) was found to interact with the lipid headgroups and slightly perturb the lipid acyl-chain region; when Abeta(25-35) was included during vesicle formation, it inserted deeper into the bilayer. While Abeta(25-35) retained the same beta-sheet structure irrespective of the mode of addition, the longer Abeta(1-42) appeared to have an increase in beta-sheet structure at the C-terminus when added to phospholipid liposomes after vesicle formation. Since the Abeta(25-35) fragment is also neurotoxic, the full-length peptide may have more than one pathway for toxicity.

Solution Structure and Interaction of Cupiennin 1a, a Spider Venom Peptide, with Phospholipid Bilayers†

The solution structure of cupiennin 1a, a 35 residue, basic antibacterial peptide isolated from the venom of the spider Cupiennius salei, has been determined by nuclear magnetic resonance (NMR) spectroscopy. The peptide was found to adopt a helix-hinge-helix structure in a membrane mimicking solvent. The hinge may play a role in allowing the amphipathic N-terminal helix and polar C-terminal helix to orient independently upon membrane binding, in order to achieve maximal antibacterial efficacy. Solid-state 31P and 2H NMR was used to further study the effects of cupiennin 1a on the dynamic properties of lipid membranes, using zwitterionic chain deuterated dimyristoylphosphatidylcholine (d54-DMPC) and anionic dimyristoylphosphatidylglycerol (DMPG) multilamellar vesicles. In d54-DMPC alone, cupiennin 1a caused a decrease in the 31P chemical shift anisotropy, indicating some interaction with the lipid head groups, and a decrease in order over the entire acyl chain. In contrast, for the mixed (d54-DMPC/DMPG) lipid system cupiennin 1a appeared to induce lateral separation of the two lipids as evidenced by the 31P spectra, in which the peptide preferentially interacted with DMPG. Little effect was observed on the deuterated acyl chain order parameters in the d54-DMPC/DMPG model membranes. Furthermore, 31P NMR relaxation measurements confirmed a differential effect on the lipid motions depending upon the membrane composition. Therefore, subtle differences are likely in the mechanism by which cupiennin 1a causes membrane lysis in either prokaryotic or eukaryotic cells, and may explain the specific spectrum of activity.

The influence of internuclear spatial distribution and instrument noise on the precision of distances determined by solid state NMR of isotopically enriched proteins

The relative merits of different isotopic enrichment strategies that might be used in solid state NMR protein structure determinations are explored. The basis for comparison of these merits is the determination of the relative uncertainties in rates measured by a generalized dipolar recoupling experiment. The different schemes considered use (13)C, (15)N and (2)H labeling of ubiquitin with homonuclear magnetization-transfer type experiments under magic-angle spinning (MAS). Specific attention is given to the sensitivity of the predicted relative precisions to variation in natural nuclear density distribution and noise levels. A framework is suggested to gauge the precision of measurement of a given dipolar coupling constant, and the potential for a set of such measurements to constrain structure calculations is explored. The distribution of nuclei in homonuclear (15)N and (1)H dipolar recoupling spin-exchange experiments appear to provide the most promising tertiary structure information for uniformly labeled ubiquitin.

Solution oligomerization of the rev protein of HIV-1: implications for function

rev is an RNA-binding protein of human immunodeficiency virus-1 and is required for the expression of incompletely spliced viral transcripts. Oligomerization of rev is thought to be associated with RNA binding and rev function. Here, we have characterized the oligomerization of rev using equilibrium analytical centrifugation. rev is predominantly monomeric at low concentrations, but reversibly polymerizes to produce large aggregates at higher concentrations. The data fit well to an unlimited isodesmic self-association model in which the association constants for the addition of a monomer to each aggregate are equal [K = 1.08 x 10(6) M-1 at 4 degrees C]. The association constant is essentially independent of monovalent salt concentration from 0.15 to 2 M at pH 6-9. Thermodynamic parameters derived from the temperature dependence of the association constant over the limited range of 0-30 degrees C reveal that the primary contribution to the free energy of oligomerization is a large negative enthalpy. Binding of rev to the rev-responsive element of RNA was characterized under the same conditions as the centrifugation experiments using a nitrocellulose filter assay. rev binds to the RRE at a protein concentration where rev is predominantly monomeric, suggesting that solution multimerization of rev is not required for rev function.

Endothelium-derived nitric oxide reduces baseline venous tone in awake instrumented rats

To determine whether nitric oxide, which is likely endothelium-derived relaxing factor (EDRF), modulates baseline venous tone, the effects of intravenous NG-monomethyl-L-arginine (L-NMMA) (3-25 mg/kg), an EDRF inhibitor, on mean circulatory filling pressure (MCFP) were determined in 10 awake instrumented rats. MCFP, the equilibrated systemic pressure occurring when the circulation is arrested by transient inflation of a balloon in the right atrium, is a measure of total venous capacitance. L-NMMA caused a dose-dependent increase in mean arterial pressure and a dose-dependent decrease in heart rate. MCFP rose from 6.6 +/- 0.2 to 7.6 +/- 0.2 mmHg at the highest L-NMMA dose. The effects of L-NMMA on MCFP were reversed with L-arginine. In an additional four rats, in which hexamethonium was administered to induce ganglionic blockade, L-NMMA (25 mg/kg) caused a similar increase in MCFP (4.1 +/- 0.6 to 5.0 +/- 0.7 mmHg, P = 0.22) during the ganglionic blocked state as during the control unblocked state. These findings suggest that nitric oxide, which is likely EDRF, reduces baseline venous tone.

Aging increases venous compliance in awake instrumented rats

To determine the effects of aging on total venous compliance, mean circulatory filling pressure (MCFP) was determined at several different blood volumes in 10 young (10-mo-old) and 10 older (30-mo-old) awake instrumented male Fischer 344/Brown Norway hybrid rats. Baseline weight and mean arterial pressure were similar in the two groups; heart rate was higher in the young (426 +/- 9 beats/min) than in the older rats (376 +/- 8 beats/min). Although MCFP was similar in the two groups at baseline blood volume, MCFP rose less with transient volume expansion and fell less with transient volume depletion in the older rats. The calculated venous compliance (reciprocal of the slope of the MCFP-to-volume relation) for the older rats was 25% greater than in the younger rats (3.28 +/- 0.21 vs. 2.63 +/- 0.12 ml.kg-1.mmHg-1; P = 0.014). In this conscious instrumented rat model, baseline total venous compliance is increased in older rats.

Adenosine increases total venous capacitance in awake instrumented rats

To determine the effect of adenosine on the venous system, mean circulatory filling pressure (MCFP) was determined during infusion of intravenous (i.v.) adenosine (66.5 to a maximum of 532 microgram.kg-1.min-1) in 9 awake instrumented rats before and during ganglionic blockade with i.v. hexamethonium, 0.6 mg.kg-1.min-1. MCFP, the equilibrated pressure (mm Hg) occurring when the circulation is arrested by transient inflation of a balloon in the right atrium, is inversely related to total venous capacitance. Both adenosine and hexamethonium caused a reduction in mean arterial pressure (MAP); heart rate (HR) decreased during adenosine infusion in the blocked, but not the unblocked, state. In the unblocked state, baseline MCFP was 6.5 +/- 0.3 mmHg; hexamethonium caused baseline MCFP to decrease to 5.3 +/- 0.3 mm Hg. In both the unblocked and the blocked state, adenosine caused a dose-related decrease in MCFP [6.5 +/- 0.3 to 5.5 +/- 0.6 mm Hg (532 microgram.kg-1.min-1 adenosine dose) unblocked state; and 5.3 +/- 0.3 to 4.3 +/- 0.3 mm Hg (400 microgram.kg-1.min-1 adenosine dose) blocked state]. This decrease in MCFP induced by adenosine was highly significant. Intravenous adenosine, in an awake instrumented rat model, increases venous capacitance, with and without ganglionic blockade.

Intracoronary adenosine and papaverine do not increase myocardial systolic thickening

STUDY OBJECTIVE

The aim was to determine if myocardial systolic thickening increases when coronary flow is augmented by infusing intracoronary vasodilators (adenosine and papaverine).

DESIGN

Systolic thickening fraction was measured with pulsed Doppler crystals and sonomicrometer crystals before and during the intracoronary infusion of adenosine and papaverine.

SUBJECTS

Sixteen anaesthetized mongrel dogs were studied.

MEASUREMENTS AND MAIN RESULTS

Intracoronary adenosine did not alter systemic haemodynamics, but did induce a three- to fourfold increase in myocardial blood flow. Intracoronary papaverine caused a slight decrease in systemic arterial pressure and rise in heart rate. Neither intracoronary adenosine nor intracoronary papaverine increased systolic thickening: control thickening fraction (TF%) = 20 (SEM 1)%, adenosine TF% = 18(1)%; control TF% = 22(2)%, papaverine TF% = 20(2)%.

CONCLUSIONS

These experiments do not support the hypothesis that an increase in myocardial blood flow induced by intracoronary vasodilators causes an increase in myocardial systolic function.