Rotational Echo Double Resonance Research Articles

Cisplatin interaction with phosphatidylserine bilayer studied by solid-state NMR spectroscopy

Cisplatin [cis-diamminedichloridoplatinum(II)] is used in chemotherapy where platinum-DNA adducts initiate tumor cell death. It is possible that side effects such as neurotoxicity and cellular cisplatin resistance can be due to interaction of cisplatin with lipids and the phospholipid bilayer. In this study, (13)C, (31)P, and (15)N solid-state NMR spectra of 1-palmitoyl-2-oleoyl phosphatidylserine (POPS) bilayers, POPS bilayers with 10 mol% cisplatin, and POPS bilayers with 30 mol% cisplatin were acquired. In addition, (15)N{(31)P} rotational echo double resonance spectra of POPS bilayers with 30 mol% cisplatin were acquired. The data demonstrate that the serine head group of phosphatidylserine binds to the aquated form of cisplatin and that cisplatin release of ammine takes place. It appears that the cisplatin release of ammine is followed by another cisplatin-POPS complex formation, possibly with cisplatin binding to one of the oxygen atoms of the POPS phosphate moiety.

Conformation of the phosphate D-alanine zwitterion in bacterial teichoic acid from nuclear magnetic resonance spectroscopy.

The conformation of d-alanine (d-Ala) groups of bacterial teichoic acid is a central, yet untested, paradigm of microbiology. The d-Ala binds via the C-terminus, thereby allowing the amine to exist as a free cationic NH(3)(+) group with the ability to form a contact ion pair with the nearby anionic phosphate group. This conformation hinders metal chelation by the phosphate because the zwitterion pair is charge neutral. To the contrary, the repulsion of cationic antimicrobial peptides (CAMPs) is attributed to the presence of the d-Ala cation; thus the ion pair does not form in this model. Solid-state nuclear magnetic resonance (NMR) spectroscopy has been used to measure the distance between amine and phosphate groups within cell wall fragments of Bacillus subtilis. The bacteria were grown on media containing (15)N d-Ala and beta-chloroalanine racemase inhibitor. The rotational-echo double-resonance (REDOR) pulse sequence was used to measure the internuclear dipolar coupling, and the results demonstrate (1) the metal-free amine-to-phosphate distance is 4.4 A and (2) the amine-to-phosphate distance increases to 5.4 A in the presence of Mg(2+) ions. As a result, the zwitterion exists in a nitrogen-oxygen ion pair configuration providing teichoic acid with a positive charge to repel CAMPs. Additionally, the amine of d-Ala does not prevent magnesium chelation in contradiction to the prevailing view of teichoic acids in metal binding. Thus, the NMR-based description of teichoic acid structure resolves the contradictory models, advances the basic understanding of cell wall biochemistry, and provides possible insight into the creation of new antibiotic therapies.

Structural Characterization of Rare-Earth Doped Yttrium Aluminoborate Laser Glasses Using Solid State NMR

The structure of laser glasses in the system (B2O3)0.6{(Al2O3)0.4−x(Y2O3)x} (0.1 ≤ x ≤ 0.25) has been investigated by means of 11B, 27Al, and 89Y solid state NMR as well as Y-3d core-level X-ray photoelectron spectroscopy. 11B magic-angle spinning (MAS) NMR spectra reveal that the majority of the boron atoms are three-coordinated, and a slight increase of four-coordinated boron content with increasing x can be noticed. 27Al MAS NMR spectra show that the alumina species are present in the coordination states four, five and six. All of them are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by 11B/27Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, nonsegregated glass structure. For the first time, 89Y solid state NMR has been used to probe the local environment of Y3+ ions in a glass-forming system. The intrinsic sensitivity problem associated with 89Y NMR has been overcome by combining the benefits of paramagnetic doping with those of signal accumulation via Carr-Purcell spin echo trains. Both the 89Y chemical shifts and the Y-3d core level binding energies are found to be rather sensitive to the yttrium bonding state and reveal that the bonding properties of the yttrium atoms in these glasses are similar to those found in the model compounds YBO3 and YAl3(BO3)4. Based on charge balance considerations as well as 11B NMR line shape analyses, the dominant borate species are concluded to be meta- and pyroborate anions.

Vancomycin and Oritavancin Have Different Modes of Action in Enterococcus faecium

The increasing frequency of Enterococcus faecium isolates with multidrug resistance is a serious clinical problem given the severely limited number of therapeutic options available to treat these infections. Oritavancin is a promising new alternative in clinical development that has potent antimicrobial activity against both staphylococcal and enterococcal vancomycin-resistant pathogens. Using solid-state NMR to detect changes in the cell-wall structure and peptidoglycan precursors of whole cells after antibiotic-induced stress, we report that vancomycin and oritavancin have different modes of action in E. faecium. Our results show the accumulation of peptidoglycan precursors after vancomycin treatment, consistent with transglycosylase inhibition, but no measurable difference in cross-linking. In contrast, after oritavancin exposure, we did not observe the accumulation of peptidoglycan precursors. Instead, the number of cross-links is significantly reduced, showing that oritavancin primarily inhibits transpeptidation. We propose that the activity of oritavancin is the result of a secondary binding interaction with the E. faecium peptidoglycan. The hypothesis is supported by results from 13C{ 19F} rotational-echo double-resonance (REDOR) experiments on whole cells enriched with l-[1- 13C]lysine and complexed with desleucyl [ 19F]oritavancin. These experiments establish that an oritavancin derivative with a damaged d-Ala– d-Ala binding pocket still binds to E. faecium peptidoglycan. The 13C{ 19F} REDOR dephasing maximum indicates that the secondary binding site of oritavancin is specific to nascent and template peptidoglycan. We conclude that the inhibition of transpeptidation by oritavancin in E. faecium is the result of the large number of secondary binding sites relative to the number of primary binding sites.

The Mineral Phase of Calcified Cartilage: Its Molecular Structure and Interface with the Organic Matrix

We have studied the atomic level structure of mineralized articular cartilage with heteronuclear solid-state NMR, our aims being to identify the inorganic species present at the surfaces of the mineral crystals which may interact with the surrounding organic matrix and to determine which components of the organic matrix are most closely involved with the mineral crystals. One-dimensional 1H and 31P and two-dimensional 1H-31P heteronuclear correlation NMR experiments show that the mineral component is very similar to that in bone with regard to its surface structure. 13C{31P} rotational echo double resonance experiments identify the organic molecules at the mineral surface as glycosaminoglycans, which concurs with our recent finding in bone. There is also evidence of γ-carboxyglutamic acid residues interacting with the mineral. However, other matrix components appear more distant from the mineral compared with bone. This may be due to a larger hydration layer on the mineral crystal surfaces in calcified cartilage.

Water Permeability of Spider Dragline Silk

The water permeability of spider dragline silk was studied by measuring changes in amide deuteration of D(2)O-soaked silk with solid-state NMR. (13)C-D rotational-echo double-resonance (REDOR) NMR experiments showed that chemical exchange of amide hydrogen occurs in a large fraction of amino acids, including over 50% of alanine residues, which are known to exist predominantly in beta-sheet crystallites. This suggests that a substantial fraction of the crystalline regions are permeable to water, at least on the time scale of hours, implying that they are more dynamic, and therefore susceptible to chemical exchange with water, than previously thought. Wideline deuterium NMR spectra of dried D(2)O-soaked silk showed a combination of quadrupolar broadened and motionally averaged isotropic components whose intensities change on the time scale of hours. These results are interpreted in terms of chemical exchange between deuterium on the protein backbone, residual water within the silk, and water vapor in the ambient atmosphere. A simple compartmental model fits the results well and yields rate constants for the exchange processes. The model requires the inclusion of a compartment that does not undergo exchange. This compartment, likely related to the crystalline region, is interesting because it is accessible to water in wet silk, but impervious to any remaining free water when the silk is dried.

Determination of the Structure of a Novel Anion Exchange Fuel Cell Membrane by Solid-State Nuclear Magnetic Resonance Spectroscopy

A novel anion exchange fuel cell membrane was successfully synthesized by chemically attaching proazaphosphatranium/phosphatranium cations under microwave treatment to the sulfonic groups of Nafion-F. Solid-state nuclear magnetic resonance (NMR) techniques were employed to determine the actual structure and composition of this anion exchange membrane. 31P NMR showed two main signals with a 2:1 intensity ratio and chemical shift changes of +89 and +46 ppm, respectively, from the main peak of phosphatranium chloride. 1H−31P heteronuclear correlation (HetCor) NMR and 1H−31P recoupling experiments indicated that the proton originally bonded to phosphorus in phosphatranium chloride is replaced in the major component of the Nafion−proazaphosphatranium/phosphatranium composite. 19F NMR experiments showed that the fluorine in the −SO2F group of the Nafion-F precursor is fully replaced. 31P{19F} rotational-echo double-resonance (REDOR) experiments measured a P−F internuclear distance of ∼0.4 nm, which showed that the proazaphosphatranium is covalently attached to Nafion through a S−P bond. 13C NMR and 1H−13C HetCor spectra indicated that the proazaphosphatranium structure is maintained even after the microwave treatment at 180 °C and also showed indications of entrapped dimethylformamide solvent.

Structure−Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 − x)P2O5:70SiO2

New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.

A solid-state NMR study of the formation of molecular sieve SAPO-34

This work examined the formation of a catalytically important microporous material, SAPO-34, in the presence of HF under hydrothermal synthesis conditions. The local environments of P, Al, F and Si atoms in several solid phases obtained at different stages of crystallization were characterized by several solid-state NMR techniques including 31P, 27Al, 19F and 29Si MAS, 27Al triple-quantum MAS, 31P{ 27Al} transfer of populations in double-resonance, 27Al{ 31P} rotational-echo double-resonance (REDOR), 27Al→ 31P heteronuclear correlation spectroscopy, 31P{ 19F} and 27Al{ 19F} REDOR as well as 1H→ 31P cross polarization. The NMR results provide the new insights into the formation of SAPO-34.

Sol-gel synthesis of sodium-modified AlPO4–SiO2 glasses and structural characterization by solid state NMR

Sodium aluminophosphosilicate gels and glasses along the composition line (Na2O)x–[(AlPO4)0.5(SiO2)0.5]1−x were synthesized via the sol-gel process using sodium acetate, aluminium lactate, phosphoric acid and tetraethyl orthosilicate (TEOS) as precursors. The structural evolution from solution to gel and to the final glass was monitored by 27Al, 31P, 29Si and 23Na magic-angle spinning (MAS) NMR. In the solution state aluminium forms a mixture of octahedral coordinated complexes coordinated to lactate, water and phosphate units with varying degrees of polymerization. With increasing temperature up to about 150 °C the lactate ligands are successively replaced by phosphate, resulting in an increased extent of Al–O–P linking. Annealing the xerogel at 400 °C results in a glassy network containing mostly four-coordinated Al species (Al(IV)), in addition to small concentrations of Al(V) and Al(VI). While the distribution of Al coordination numbers remains constant, the compositional trend of the 27Al chemical shift indicates a gradual replacement of Al–O–P linkages by Al–O–Si linkages with increasing sodium content. This conclusion is confirmed by 27Al{31P} Rotational Echo Double Resonance (REDOR) experiments, which reveal a reduction in the number of Al–O–P linkages. Concomitantly, the interaction of Na with the phosphate species is successively increased as indicated by 31P chemical shift trends and 31P{23Na} REDOR results. The results illustrate that the network segregation found in pure AlPO4–SiO2 sol-gel glasses can be overcome, at least partially, by introduction of the network modifier Na2O.

Application of REDOR NMR in natural product chemistry

Many natural products have molecular targets that are non-crystalline and insoluble biological matrices, such as proteins embedded in lipid membrane, cell membranes, and cell walls. To understand the action mechanisms, it is essential to determine the binding structure with atomic-level resolution. For structural studies of biological solids, high resolution distance measurements using solid-state nuclear magnetic resonance (NMR) are indispensable techniques, of which rotational-echo double-resonance (REDOR) is one of the most widely used methods. This feature article introduces the basic concepts of REDOR NMR and its application to the structural study of natural products in biological matrices.

Nanoscale Architecture in Atherosclerotic Calcification

Calcium mineral deposits are radiographically evident in the majority of significant atherosclerotic lesions.1 These deposits generally consist of a nonhomogeneous composite containing hydroxyapatite mineral nanocrystals embedded in an organic matrix including type I collagen nanofibers. The morphometry varies over a spectrum. Amorphous deposits lack organization at the light-microscopic scale, whereas bone-like deposits have varying degrees of architectural macro-organization similar to those of developing and mature skeletal bone, which also consist of a nonhomogeneous composite containing hydroxyapatite nanocrystals embedded in a collagenous organic matrix. Based on histopathologic features, the organized, bone-like deposits appear to arise from angiogenic invasion of the amorphous deposits.2 This is consistent with the known sequence of events that leads to bone formation in fracture repair, embryonic intramembranous ossification, and in embryonic endochondral ossification, where bone arises from a cartilage scaffold. See accompanying article on page 2030 The similarities between atherosclerotic calcification and osteogenesis are more than skin deep. Several investigative groups have demonstrated osteogenic features at the cellular and molecular levels, including dynamic osteogenic gene expression in vitro and in vivo as reviewed recently by Towler and colleagues.3 In this issue of ATVB , Duer and colleagues4 take this comparison to the next level—nanoscale architecture. Using solid-state nuclear magnetic resonance (NMR) techniques, they examined the mineral-organic interface in calcium deposits from atherosclerotic plaque and skeletal bone and found marked similarities. The NMR method (rotational echo double resonance) has the capacity to isolate the nanoscale features because it generates forces that act only within distances less …

Analytical solutions to several magic-angle spinning NMR experiments

Using the Anderson–Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.

Formation, Location, and Photocatalytic Reactivity of Methoxy Species on Keggin 12-H3PW12O40: A Joint Solid-State NMR Spectroscopy and DFT Calculation Study

Solid-state NMR experiments and quantum chemical Density Functional Theory (DFT) calculations were employed to investigate the formation, location, and photocatalytic reactivity of methoxy species on anhydrous 12-H3PW12O40. Two different types of methoxy species were identified on the methanol-adsorbed 12-H3PW12O40 catalyst. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that the two corresponding methyl groups reside on the O-c and O-d atoms of the Keggin anion, forming the surface OcCH3 and OdCH3 species. Photocatalytic experiments further indicated that the two methoxy species are both photochemically reactive species with the OdCH3 species being much more reactive, and the methoxy species are preferentially mineralized to the final product CO2 directly.

In SituObservation of the Internal Structure and Composition of BiomineralizedEmiliania huxleyiCalcite by Solid-State NMR Spectroscopy

Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular-level characterization of the resulting biominerals is a formidable task, solid-state magic angle spinning NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by Emiliania huxleyi, a unicellular alga distinguished by its exquisitely sculptured calcite cell coverings known as coccoliths. Strain 371 (CCMP) was grown and harvested from (15)N- and (13)C-enriched growth medium, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (cross-polarized) (13)C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4% with P and N moieties incorporated, were identified using (13)C rotational-echo double-resonance (REDOR) NMR. REDOR-derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (+/-0.2) A apart from a crystalline carbon carbonate. The phosphorus and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO4(3-) and NO3(-). The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity, as reflected by substantial T1((13)CO3(2-)) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments, as reflected by heterogeneous line broadening. This study is the first to directly unravel evidence on the incorporation of P/N moieties as structural defects within E. huxleyi biogenic calcite, and on the state of the adjacent crystalline carbonates.

Hydrophobic Side-Chain Length Determines Activity and Conformational Heterogeneity of a Vancomycin Derivative Bound to the Cell Wall of Staphylococcus aureus

Disaccharide-modified glycopeptides with hydrophobic side chains are active against vancomycin-resistant enterococci and vancomycin-resistant Staphylococcus aureus. The activity depends on the length of the side chain. The benzyl side chain of N-(4-fluorobenzyl)vancomycin (FBV) has the minimal length sufficient for enhancement in activity against vancomycin-resistant pathogens. The conformation of FBV bound to the peptidoglycan in whole cells of S. aureus has been determined using rotational-echo double resonance NMR by measuring internuclear distances from the (19)F of FBV to (13)C and (15)N labels incorporated into the cell-wall peptidoglycan. The hydrophobic side chain and aglycon of FBV form a cleft around the pentaglycyl bridge. FBV binds heterogeneously to the peptidoglycan as a monomer with the (19)F positioned near the middle of the pentaglycyl bridge, approximately 7 A from the bridge link. This differs from the situation for N-(4-(4-fluorophenyl)benzyl)vancomycin complexed to the peptidoglycan where the (19)F is located at the end of pentaglycyl bridge, 7 A from the cross-link.

A Solid-State NMR Spectroscopic Study of the Adsorption of Toluene in Zeolite LiK-L

The adsorption of toluene in partially Li+-exchanged zeolite K-L (LiK-L) was examined by solid-state NMR spectroscopy. The environment of the Li+ ions at different sites in the zeolite before and after adsorption was characterized by 7Li magic-angle spinning (MAS) NMR. 1H → 7Li cross polarization (CP) and 7Li{1H} rotational-echo double-resonance (REDOR) experiments were performed to probe the cation−sorbate interactions. The data were also used for spectral assignment. The 7Li NMR results indicate that most of the Li+ ions occupy the sites A (39%) and B (43%) and that there are fewer Li+ ions at sites C (17%) and D (1%). The four 7Li NMR signals corresponding to the Li+ ions at four sites are not resolved in the spectrum of the dehydrated zeolite, but are well-separated after the adsorption of toluene. The NMR results show that a toluene molecule is directly coordinated to the Li+ ion at site D near the wall of the main channel via formation of a π-complex, resulting in a significant change in 7Li chemical shift toward a more shielded direction. Although not directly accessible by the toluene molecules, the Li+ ions at sites A and C also exhibit an observable shift upon adsorption. Only the Li+ ions at site B do not experience a significant change in chemical shift. The effect of the adsorption on 7Li chemical shift is discussed. Dynamic behavior of toluene molecules inside the channels was also investigated by wide-line 2H NMR.

Mineral Surface in Calcified Plaque Is Like That of Bone

Cell biological studies demonstrate remarkable similarities between mineralization processes in bone and vasculature, but knowledge of the components acting to initiate mineralization in atherosclerosis is limited. The molecular level microenvironment at the organic-inorganic interface holds a record of the mechanisms controlling mineral nucleation. This study was undertaken to compare the poorly understood interface in mineralized plaque with that of bone, which is considerably better characterized. Solid state nuclear magnetic resonance (SSNMR) spectroscopy provides powerful tools for studying the organic-inorganic interface in calcium phosphate biominerals. The rotational echo double resonance (REDOR) technique, applied to calcified human plaque, shows that this interface predominantly comprises sugars, most likely glycosaminoglycans (GAGs). In this respect, and in the pattern of secondary effects seen to protein (mainly collagen), calcified plaque strongly resembles bone. The similarity between biomineral formed under highly controlled (bone) and pathological (plaque) conditions suggests that the control mechanisms are more similar than previously thought, and may be adaptive. It is strong further evidence for regulation of plaque mineralization by osteo/chondrocytic vascular smooth muscle cells.

REDOR NMR Characterization of DNA Packaging in Bacteriophage T4

Bacteriophage T4 is a large-tailed Escherichia coli virus whose capsid is 120×86 nm. ATP-driven DNA packaging of the T4 capsid results in the loading of a 171-kb genome in less than 5 min during viral infection. We have isolated 50-mg quantities of uniform 15N- and [ε-15N]lysine-labeled bacteriophage T4. We have also introduced 15NH4+ into filled, unlabeled capsids from synthetic medium by exchange. We have examined lyo- and cryoprotected lyophilized T4 using 15N{31P} and 31P{15N} rotational-echo double resonance. The results of these experiments have shown that (i) packaged DNA is in an unperturbed duplex B-form conformation; (ii) the DNA phosphate negative charge is balanced by lysyl amines (3.2%), polyamines (5.8%), and monovalent cations (40%); and (iii) 11% of lysyl amines, 40% of –NH2 groups of polyamines, and 80% of monovalent cations within the lyophilized T4 capsid are involved in the DNA charge balance. The NMR evidence suggests that DNA enters the T4 capsid in a charge-unbalanced state. We propose that electrostatic interactions may provide free energy to supplement the nanomotor-driven T4 DNA packaging.

The Mixed-Network Former Effect in Phosphate Glasses: NMR and XPS Studies of the Connectivity Distribution in the Glass System (NaPO3)1−x(B2O3)x

The structural organization of sodium borophosphate glasses with composition (NaPO3)1−x(B2O3)x (0.0 ≤ x ≤ 0.3) has been investigated by X-ray photoelectron spectroscopy (XPS), as well as single- and double-resonance 11B and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. O-1s XPS data provides a quantitative distinction between B−O−B, B−O−P, and P−O−P linkages as well as nonbridging oxygen atoms. 11B and 31P MAS NMR data indicate that within the compositional region 0 ≤ x ≤ 0.20 the entire boron inventory is present in the form of anionic BO4− units, resulting in the repolymerization of an equivalent fraction of the phosphate units (conversion of anionic metaphosphate (P(2)) into neutral branching groups (P(3)) species. Both XPS as well as 31P{11B} and 11B{31P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in the dominant formation of B−O−P linkages. In addition, the ...