Torsten Schaller

A 31P MAS NMR study of glasses in the system xNa2O–(1−x)Al2O3–2SiO2–yP2O5

Abstract The structural role of phosphorus in glass samples of variable alkali/aluminium ratio has been investigated in the range of 1 to 6 mol% P2O5, using a combination of one and two dimensional 31 P MAS NMR spectroscopy. The samples have the general formula xNa2O–(1−x)Al2O3–2SiO2–yP2O5 ranging from Al-free (x=1.0) to peraluminous (x=0.44). Spectra of Al-free compositions are consistent with the presence of Na3PO4 and Na4P2O7 complexes residing outside of the silicate network, as previously proposed in the literature. In contrast to solution mechanisms proposed in the literature no evidence for extended metaphosphate chains (NaPO3)n was found in aluminium-bearing peralkaline glasses. Alternatively, our spectra suggest that, in addition to Na3PO4 and Na4P2O7, phosphate tetrahedra are attached to the aluminosilicate framework through replacement of Na by tetrahedral aluminium charge balanced by sodium. The average number of Na replaced by Al increases with decreasing peralkalinity. At the metaluminous join all three sodium atoms of the Na3PO4 tetrahedron are replaced by Al without the need to disrupt a stable sodium aluminate complex. These solution mechanisms are shown to be consistent with available spectroscopic and physical property data reported in the literature. In peraluminous compositions we propose that P interacts with excess aluminium to form AlPO4 complexes. However, our spectra also imply that in mildly peraluminous glasses a fraction of phosphorus continues to interact with tetrahedral Al associated with Na.

Protons in the magnesium phosphates phosphoellenbergerite and holtedahlite; an IR and NMR study

Abstract Two structurally related magnesium phosphates, phosphoellenbergerite (P63mc) and holtedahlite (P63), have been investigated using a combination of high-frequency IR and 1H MAS, 31P MAS, and 31P CP MAS NMR spectroscopy on synthetic material. For phosphoellenbergerite, the two proton sites, H1 and H2, detected in single-crystal XRD studies, are confirmed. The protons on the first site, H1, delocalized around the threefold axis, form OH groups with the unshared apical O atom of PO4 tetrahedra. Those on the second site, H2, belong to Mg2O8OH dimers of face-sharing Mg octahedra, which form double chains. The spectroscopic data reveal additional protons (H3) in phosphoellenbergerite. By combining both crystallographic and spectroscopic data, it can be inferred that the H3 protons are associated with Mg vacancies in the single chains off ace-sharing Mg octahedra. Furthermore, a quantitative inspection of signal intensities indicates that the proton sites in synthetic phosphoellenbergerite are not all saturated. Therefore, the substitution P5+ = Si4+ + H+ might account for part of the Si-P replacement in the silicate-to-phosphate ellenbergerite series. The results obtained for the proton distribution in holtedahlite, Mg2PO4OH, were used as a reference to facilitate the interpretation of the IR and NMR spectra of phosphoellenbergerite.

TRAPDOR NMR investigations of phosphorus-bearing aluminosilicate glasses

Abstract The local environment of phosphorus in aluminosilicate glasses of variable sodium/aluminum ratio has been investigated using double-resonance magic angle spinning (MAS) nuclear magnetic resonance spectroscopy. The studied glasses have the general composition x Na 2 O–(1− x )Al 2 O 3 –2SiO 2 and contain approximately 4 mol% P 2 O 5 . The short-range dipolar couplings of phosphorus atoms to aluminum and sodium are used to deduce the connectivities of phosphate units to AlO 4 tetrahedra and/or sodium by observing the effects on a 31 P MAS echo spectrum caused by simultaneous irradiation of the 27 Al or 23 Na resonance frequency. In this way, different phosphorus environments were identified. The observed resonances can be assigned to Na 3 PO 4 and Na 4 P 2 O 7 units, as well as phosphate tetrahedra coordinated to n aluminum atoms in the aluminosilicate framework (with n varying from 1 to 3). The results provide unambiguous evidence for the interaction of P with aluminum even in peralkaline compositions, confirming a recently proposed model describing the solution of phosphorus in aluminosilicate melts.

A Mechanism of Efficient G6PD Inhibition by a Molecular Clip

Triple duty: A synthetic molecular clip traps nicotinamide adenine dinucleotide phosphate (NADP(+); see picture) as well as occupying both the cofactor- and the substrate-binding site in glucose-6-phosphate (G6P) dehydrogenase. This combination of two inhibition mechanisms makes the clip highly effective and selective for this enzyme over other dehydrogenases.

Stannacyclohexanes and spiro-tin compounds with s stannole or a stannolene group

1,1-Diethynyl-1-stannacyclohexane (5) was prepd., and its reaction with triethylborane or 9-isobutyl-9-borabicyclo[3.3.1]nonane gave the corresponding spiro-Sn compds. 6 and 7 (shown as I) with a stannole fragment. The mol. structure of 7 was detd. by x-ray anal. (triclinic, space group P.hivin.1; a 835.2(7), b 1117.5(2), c 1165.6(2) pm; a 72.05(3), b 74.32(3), g 50.61(3) Deg). Treatment of 6 with trimethyltin ethoxide gave at 1st quant. the spiro-Sn compd. 8 with a 2-stannolene unit which rearranges upon heating into the spiro-Sn compd. 9 with a 3-stannolene unit. The reaction of 7 with trimethyltin butoxide gave quant. another tetracyclic compd. 10, analogous to 8. All compds. were characterized by 1H, 11B, 13C and 119Sn NMR data. Numerous coupling signs nJ(119Sn1H) and nJ(119Sn13C) and 2J(119Sn117Sn) were detd. by 2-dimensional 13C/1H and 119Sn/1H heteronuclear shift correlations. 7 Was studied by solid-state 13C and 119Sn CPMAS NMR. This revealed a 2nd cryst. phase in the bulk of the solid material, as confirmed by powder x-ray diffraction data. 119Sn CPMAS NMR spectra of 7 show resolved scalar 119Sn-11B coupling (3J(119Sn11B) = 68 +- 5 and 65 +- 5 Hz).

Effect of molecular clips and tweezers on enzymatic reactions by binding coenzymes and basic amino acids

The tetramethylene-bridged molecular tweezers bearing lithium methanephosphonate or dilithium phosphate substituents in the central benzene or naphthalene spacer-unit and the dimethylene-bridged clips containing naphthalene or anthracene sidewalls substituted by lithium methanephosphonate, dilithium phosphate, or sodium sulfate groups in the central benzene spacer-unit are water-soluble. The molecular clips having planar naphthalene sidewalls bind flat aromatic guest molecules preferentially, for example, the nicotinamide ring and/or the adenine-unit in the nucleotides NAD(P)+, NMN, or AMP, whereas the benzene-spaced molecular tweezers with their bent sidewalls form stable host–guest complexes with the aliphatic side chains of basic amino acids such as lysine and argenine. The phosphonate-substituted tweezer and the clips having an extended central naphthalene spacer-unit or extended anthracene and benzo[k]fluoranthene sidewalls, respectively, form highly stable self-assembled dimers in aqueous solution, evidently due to non-classical hydrophobic interactions. The phosphate-substituted molecular clip containing naphthalene sidewalls inhibits the enzymatic, ADH-catalyzed ethanol oxidation by binding the cofactor NAD+ in a competitive reaction. Surprisingly, tweezer-bearing phosphate substituents in the central benzene spacer-unit are more efficient inhibitors for the ethanol oxidation than the correspondingly substituted naphthalene clip, even though the tweezer does not bind the cofactor NAD+ within the limits of detection. The phosphate-substituted naphthalene clip is, however, a highly efficient inhibitor of the enzymatic oxidation of glucose-6-phosphate (G6P) with NADP+ catalyzed by glucose-6-phosphate dehydrogenase (G6PD), whereas the phosphonate-substituted clip only functions as an inhibitor by forming a complex with the cofactor. Detailed kinetic, thermodynamic, and computational modeling studies provide insight into the mechanism of these novel enzyme inhibition reactions.

Structure and Properties of Cocrystals of Phenazine and Fumaric-, 2,3-Dihydroxyfumaric-, and Oxalic Acid

Phenazine and the dicarboxylic acids fumaric-, 2,3-dihydroxyfumaric-, and oxalic acid form 1: 1 cocrystals. X-ray anal. shows that the mols. are arranged as linear tapes, mainly held together by strong O-H.tplbond.N and weak C(sp2)-H.tplbond.O H bonds. Individual mols. form staples which are surrounded by staples of the other mols. The angle between neighboring tapes varies from .apprx.90 Deg in the cocrystal of phenazine and fumaric acid to .apprx.70 Deg in the cocrystal of phenazine and 2,3-dihydroxyfumaric acid, and .apprx.25 Deg in the cocrystal of phenazine and oxalic acid. The mols. assume an offset face-to-face arrangement in individual phenazine staples. Negligible p-stacking is obsd. in the cocrystals of phenazine with fumaric- and 2,3-dihydroxyfumaric acid. The absence of the CC double bond as spacer in oxalic acid leads to appreciable p-overlap of phenazine mols. in the cocrystal. As a consequence, the latter cocrystal displays special properties. An irreversible light-induced electron transfer generates initially singlet and triplet biradicals with the unpaired electrons positioned on neighboring phenazine mols. Partially, the electrons are transformed to magnetically independent electrons which show strong exchange narrowing in the EPR spectrum at temps.>0 Deg. The proposed model is supported by UV/visible-ESR-, and SQUID measurements.

29Si CP/MAS NMR of phosphorus-bearing organosilicon compounds

While liquid-state 29Si NMR of phosphorus-bearing organosilicon compounds with more than one phosphorus per molecule can take advantage of the presence of J-coupling nJ(31P29Si) for purposes of structural assignment from J-coupling patterns, conventional 29Si CP/MAS spectra of such molecular solids do not reveal structural details in a straightforward manner. For such compounds it is necessary to obtain 29Si CP/MAS spectra under conditions of simultaneous 1H- and 31P-high power decoupling in order to derive reliable 29Si chemical shift information. 29Si CP/MAS NMR spectra, obtained with and without 31P high power decoupling during the acquisition time, of several organosilicon compounds containing SixPy (x = 1-10, y = 1-10) moieties are reported.

Determination of 113Cd shielding tensor components in cadmium-phosphine complexes from 31-decoupled 113Cd cross-polarisation/magic angle spinning spectra

Owing to the simultaneous presence of various anisotropic interactions within the isolated M-31P and M(31P)2 fragments in solid transition metal phosphine complexes MX2.PR3 and MX2.2PR3 with, for instance, M = Cd, Hg or Pt, it is not normally possible to unequivocally determine the shielding tensor components of the metal nucleus M from simple 113Cd, 199Hg or 195Pt cross-polarisation/magic angle spinning (CP/MAS) spectra. In this paper it is shown that additional 31P on-resonance high-power decoupling during the acquisition time does allow the determination of the shielding tensor components of M from CP/MAS spectra. Two cadmium(II) complexes, Cd(OAc)2.Pchex3, (chex = cyclohexyl) and Cd(ClO4)2.2Pchex3 were chosen as examples; the 113Cd CP/MAS experiments were carried out at a low external magnetic field strength B0 = 2.35 T.

High-Affinity Copolymers Inhibit Digestive Enzymes by Surface Recognition

This account presents a general method for the construction of polymeric surface binders for digestion enzymes. Two prominent parts, namely, the modification of the copolymer composition and the screening assay for the most powerful inhibitors are both amenable to parallelization. The concept hinges on the appropriate selection of amino-acid-selective comonomers, their free radical copolymerization, and subsequent screening of the resulting copolymer library for efficient enzyme inhibition. A microscale synthetic procedure for the copolymerization process was developed, which produces water-soluble affinity polymers that can be stored for years at room temperature. Initial parallel screening was conducted in standard enzyme assays to identify polymeric inhibitors, which were subsequently subjected to determination of IC50 values for their target enzyme. For all digestion enzymes, except elastase, a number of polymer inhibitors were found, some of which were selective toward one or two protein targets. Since the key monomers of the best inhibitors bind to amino acid residues in the direct vicinity of the active site, we conclude that efficient coverage of the immediate environment by the copolymers is critical. Strong interference with enzymatic activity is brought about by blocking the substrate access and product exit to and from the active site.