Formation of monofluorophosphate from fluoride in phosphoric acid – water and phosphoric acid – sulfuric acid – water mixturesKenneth E. Newman, Raymond E. Ortlieb, Nicole Pawlik, and Jason Reedyk Can. J. Chem./Rev. can. chim. 85(5): 346-351 (2007)
Abstract: When dissolved in concentrated phosphoric acid, sodium fluoride reacts rapidly to form monofluorophosphate. In less concentrated acid, the reaction does not proceed to completion, and the reaction kinetics become very much slower. The equilibrium and rate constants for the reaction have been determined. In ternary mixtures of phosphoric acid, sulfuric acid, and water, the formation of monofluorophosphate is considerably enhanced, and the kinetics are rapid. The results are interpreted in terms of the very low water activity coefficients in strong-acid solutions.
Key words: monofluorophosphate, monofluorophosphoric acid, 19F NMR, 31P NMR, phosphoric acid, sulufric acid, equilibria, kinetics.
Thursday, May 31, 2007
Sunday, May 20, 2007
J. Am. Chem. Soc., 129 (17), 5703 -5709, 2007.
Lamellar Structure in Poly(Ala-Gly) Determined by Solid-State NMR and Statistical Mechanical Calculations
Tetsuo Asakura,* Hirohiko Sato, Fumika Moro, Yasumoto Nakazawa, and Akihiro Aoki
Abstract:
Lamellar structure of poly(Ala-Gly) or (AG)n in the solid was examined using 13C solid-state NMR and statistical mechanical approaches. Two doubly labeled versions, [1-13C]Gly14[1-13C]Ala15- and [1-13C]Gly18[1-13C]Ala19 of (AG)15 were examined by two-dimensional (2D) 13C spin diffusion NMR in the solid state. In addition five doubly labeled [15N,13C]-versions of the same peptide, (AG) 15 and 15 versions labeled [3-13C] in each of the successive Ala residues were utilized for REDOR and 13C CP/MAS NMR measurements, respectively. The observed spin diffusion NMR spectra were consistent with a structure containing a combination of distorted -turns with a large distribution of the torsion angles and antiparallel -sheets. The relative proportion of the distorted -turn form was evaluated by examination of 13C CP/MAS NMR spectra of [3-13C]Ala-(AG)15. In addition, REDOR determinations showed five kinds of atomic distances between doubly labeled 13C and 15N nuclei which were also interpreted in terms of a combination of -sheets and -turns. Our statistical mechanical analysis is in excellent agreement with our Ala C 13C CP/MAS NMR data strongly suggesting that (AG)15 has a lamellar structure.
Tetsuo Asakura,* Hirohiko Sato, Fumika Moro, Yasumoto Nakazawa, and Akihiro Aoki
Abstract:
Lamellar structure of poly(Ala-Gly) or (AG)n in the solid was examined using 13C solid-state NMR and statistical mechanical approaches. Two doubly labeled versions, [1-13C]Gly14[1-13C]Ala15- and [1-13C]Gly18[1-13C]Ala19 of (AG)15 were examined by two-dimensional (2D) 13C spin diffusion NMR in the solid state. In addition five doubly labeled [15N,13C]-versions of the same peptide, (AG) 15 and 15 versions labeled [3-13C] in each of the successive Ala residues were utilized for REDOR and 13C CP/MAS NMR measurements, respectively. The observed spin diffusion NMR spectra were consistent with a structure containing a combination of distorted -turns with a large distribution of the torsion angles and antiparallel -sheets. The relative proportion of the distorted -turn form was evaluated by examination of 13C CP/MAS NMR spectra of [3-13C]Ala-(AG)15. In addition, REDOR determinations showed five kinds of atomic distances between doubly labeled 13C and 15N nuclei which were also interpreted in terms of a combination of -sheets and -turns. Our statistical mechanical analysis is in excellent agreement with our Ala C 13C CP/MAS NMR data strongly suggesting that (AG)15 has a lamellar structure.
J. Am. Chem. Soc., 129 (17), 5391 -5402, 2007.
Single Crystal 55Mn ENDOR of Concanavalin A: Detection of Two Mn2+ Sites with Different 55Mn Quadrupole Tensors
Kuppala V. Narasimhulu, Raanan Carmieli, and Daniella Goldfarb*
Abstract:
Concanavalin A is a member of the plant hemeagglutinin (or plant lectin) family that contains two metal binding sites; one, called S1, is occupied by Mn2+ and the other, S2, by Ca2+. 55Mn electron-nuclear double resonance (ENDOR) measurements were performed on a single crystal of concanavalin A at W-band (95 GHz, ~3.5 T) to determine the 55Mn nuclear quadrupole interaction in a protein binding site and its relation to structural parameters. Such measurements are easier at a high field because of the high sensitivity for size-limited samples and the reduction of second-order effects on the spectrum which simplifies spectral analysis. The analysis of the 55Mn ENDOR rotation patterns showed that two chemically inequivalent Mn2+ types are present at low temperatures, although the high-resolution X-ray structure reported only one site. Their quadrupole coupling constants, e2Qq/h, are significantly different; 10.7 ± 0.6 MHz for Mand only -2.7 ± 0.6 MHz for M. The ENDOR data also refined the hyperfine coupling determined earlier by single-crystal EPR measurements, yielding a small but significant difference between the two: -262.5 MHz for M and -263.5 MHz for M. The principal z-axis for M is not aligned with any of the Mn-ligand directions, but is 25 off the Mn-asp10 direction, and its orientation is different than that of the zero-field splitting (ZFS) interaction. Because of the small quadrupole interaction of M the orientation dependence was very mild, leading to larger uncertainties in the asymmetry parameter. Nonetheless, there too z is not along the Mn-ligand bonds and is rotated 90 with respect to MnA. These results show, that similar to the ZFS, the quadrupolar interaction is highly sensitive to small differences in the coordination sphere of the Mn2+, and the resolution of the two types is in agreement with the earlier observation of a two-site conformational dynamic detected through the ZFS interaction, which is frozen out at low temperatures and averaged at room temperature. To account for the structural origin of the different e2Qq/h values, the electric field gradient tensor was calculated using the point-charge model. The calculations showed that a relatively small displacement of the oxygen ligand of asp10 can lead to differences on the order observed experimentally.
Kuppala V. Narasimhulu, Raanan Carmieli, and Daniella Goldfarb*
Abstract:
Concanavalin A is a member of the plant hemeagglutinin (or plant lectin) family that contains two metal binding sites; one, called S1, is occupied by Mn2+ and the other, S2, by Ca2+. 55Mn electron-nuclear double resonance (ENDOR) measurements were performed on a single crystal of concanavalin A at W-band (95 GHz, ~3.5 T) to determine the 55Mn nuclear quadrupole interaction in a protein binding site and its relation to structural parameters. Such measurements are easier at a high field because of the high sensitivity for size-limited samples and the reduction of second-order effects on the spectrum which simplifies spectral analysis. The analysis of the 55Mn ENDOR rotation patterns showed that two chemically inequivalent Mn2+ types are present at low temperatures, although the high-resolution X-ray structure reported only one site. Their quadrupole coupling constants, e2Qq/h, are significantly different; 10.7 ± 0.6 MHz for Mand only -2.7 ± 0.6 MHz for M. The ENDOR data also refined the hyperfine coupling determined earlier by single-crystal EPR measurements, yielding a small but significant difference between the two: -262.5 MHz for M and -263.5 MHz for M. The principal z-axis for M is not aligned with any of the Mn-ligand directions, but is 25 off the Mn-asp10 direction, and its orientation is different than that of the zero-field splitting (ZFS) interaction. Because of the small quadrupole interaction of M the orientation dependence was very mild, leading to larger uncertainties in the asymmetry parameter. Nonetheless, there too z is not along the Mn-ligand bonds and is rotated 90 with respect to MnA. These results show, that similar to the ZFS, the quadrupolar interaction is highly sensitive to small differences in the coordination sphere of the Mn2+, and the resolution of the two types is in agreement with the earlier observation of a two-site conformational dynamic detected through the ZFS interaction, which is frozen out at low temperatures and averaged at room temperature. To account for the structural origin of the different e2Qq/h values, the electric field gradient tensor was calculated using the point-charge model. The calculations showed that a relatively small displacement of the oxygen ligand of asp10 can lead to differences on the order observed experimentally.
J. Am. Chem. Soc., 129 (17), 5318 -5319, 2007.
Chemical-Shift Anisotropy Measurements of Amide and Carbonyl Resonances in a Microcrystalline Protein with Slow Magic-Angle Spinning NMR Spectroscopy
Benjamin J. Wylie, Lindsay J. Sperling, Heather L. Frericks, Gautam J. Shah, W. Trent Franks, and Chad M. Rienstra*
Abstract:
Chemical shifts are fundamental to interpretation of NMR spectra and provide constraints for macromolecular structure determination, refinement, and validation, as well as details of active-site chemistry in enzymes. Insights into the origins of the chemical shift can be leveraged to improve chemical analysis, including conformation, bonding, and dynamics. To exploit this information fully, it is desirable to measure not only isotropic chemical shifts but also the full chemical-shift anisotropy (CSA) tensor. Until recently, most efforts to measure backbone amide and carbonyl CSAs have relied upon cross-correlated relaxation and residual anisotropic shifts in solution NMR. In solid-state NMR, analysis of sideband patterns has typically been reserved for site-specifically labeled samples or small peptides. Here we demonstrate that the Herzfeld-Berger method (Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021-6030) can be applied to highly 13C,15N-enriched solid proteins, using 2D heteronuclear correlation in combination with high magnetic fields (750 MHz 1H frequency) and pattern labeling of 13C sites. The experiments report on 42 pairs of amide and carbonyl tensors in the microcrystalline protein GB1.
Benjamin J. Wylie, Lindsay J. Sperling, Heather L. Frericks, Gautam J. Shah, W. Trent Franks, and Chad M. Rienstra*
Abstract:
Chemical shifts are fundamental to interpretation of NMR spectra and provide constraints for macromolecular structure determination, refinement, and validation, as well as details of active-site chemistry in enzymes. Insights into the origins of the chemical shift can be leveraged to improve chemical analysis, including conformation, bonding, and dynamics. To exploit this information fully, it is desirable to measure not only isotropic chemical shifts but also the full chemical-shift anisotropy (CSA) tensor. Until recently, most efforts to measure backbone amide and carbonyl CSAs have relied upon cross-correlated relaxation and residual anisotropic shifts in solution NMR. In solid-state NMR, analysis of sideband patterns has typically been reserved for site-specifically labeled samples or small peptides. Here we demonstrate that the Herzfeld-Berger method (Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021-6030) can be applied to highly 13C,15N-enriched solid proteins, using 2D heteronuclear correlation in combination with high magnetic fields (750 MHz 1H frequency) and pattern labeling of 13C sites. The experiments report on 42 pairs of amide and carbonyl tensors in the microcrystalline protein GB1.
J. Am. Chem. Soc., 129 (16), 5117 -5130, 2007
Solid-State NMR Study of Amyloid Nanocrystals and Fibrils Formed by the Peptide GNNQQNY from Yeast Prion Protein Sup35p
Patrick C. A. van der Wel, Józef R. Lewandowski, and Robert G. Griffin*
Abstract:
Sup35p is a prion protein found in yeast that contains a prion-forming domain characterized by a repetitive sequence rich in Gln, Asn, Tyr, and Gly amino acid residues. The peptide GNNQQNY7-13 is one of the shortest segments of this domain found to form amyloid fibrils, in a fashion similar to the protein itself. Upon dissolution in water, GNNQQNY displays a concentration-dependent polymorphism, forming monoclinic and orthorhombic crystals at low concentrations and amyloid fibrils at higher concentrations. We prepared nanocrystals of both space groups as well as fibril samples that reproducibly contain three (coexisting) structural forms and examined the specimens with magic angle spinning (MAS) solid-state nuclear magnetic resonance. 13C and 15N MAS spectra of both nanocrystals and fibrils reveal narrow resonances indicative of a high level of microscopic sample homogeneity that permitted resonance assignments of all five species. We observed variations in chemical shift among the three dominant forms of the fibrils which were indicated by the presence of three distinct, self-consistent sets of correlated NMR signals. Similarly, the monoclinic and orthorhombic crystals exhibit chemical shifts that differ from one another and from the fibrils. Collectively, the chemical shift data suggest that the peptide assumes five conformations in the crystals and fibrils that differ from one another in subtle but distinct ways. This includes variations in the mobility of the aromatic Tyr ring. The data also suggest that various structures assumed by the peptide may be correlated to the "steric zipper" observed in the monoclinic crystals.
Patrick C. A. van der Wel, Józef R. Lewandowski, and Robert G. Griffin*
Abstract:
Sup35p is a prion protein found in yeast that contains a prion-forming domain characterized by a repetitive sequence rich in Gln, Asn, Tyr, and Gly amino acid residues. The peptide GNNQQNY7-13 is one of the shortest segments of this domain found to form amyloid fibrils, in a fashion similar to the protein itself. Upon dissolution in water, GNNQQNY displays a concentration-dependent polymorphism, forming monoclinic and orthorhombic crystals at low concentrations and amyloid fibrils at higher concentrations. We prepared nanocrystals of both space groups as well as fibril samples that reproducibly contain three (coexisting) structural forms and examined the specimens with magic angle spinning (MAS) solid-state nuclear magnetic resonance. 13C and 15N MAS spectra of both nanocrystals and fibrils reveal narrow resonances indicative of a high level of microscopic sample homogeneity that permitted resonance assignments of all five species. We observed variations in chemical shift among the three dominant forms of the fibrils which were indicated by the presence of three distinct, self-consistent sets of correlated NMR signals. Similarly, the monoclinic and orthorhombic crystals exhibit chemical shifts that differ from one another and from the fibrils. Collectively, the chemical shift data suggest that the peptide assumes five conformations in the crystals and fibrils that differ from one another in subtle but distinct ways. This includes variations in the mobility of the aromatic Tyr ring. The data also suggest that various structures assumed by the peptide may be correlated to the "steric zipper" observed in the monoclinic crystals.
Monday, May 14, 2007
Inorganic Chemistry - updated to May 2007
Michelle A. M. Forgeron, Roderick E. Wasylishen, Michael Gerken, and Gary J. Schrobilgen. (2007).
"Solid-State 129Xe and 131Xe NMR Study of the Perxenate Anion XeO64-."
Inorg. Chem. 46(9): 3585-3592.
Abstract: Results of the first solid-state 131Xe NMR study of xenon-containing compounds are presented. The two NMR-active isotopes of xenon, 129Xe (I = 1/2) and 131Xe (I = 3/2), are exploited to characterize the xenon magnetic shielding and quadrupolar interactions for two sodium perxenate salts, Na4XeO6·xH2O (x = 0, 2), at an applied magnetic field strength of 11.75 T. Solid-state 129/131Xe NMR line shapes indicate that the local xenon environment in anhydrous Na4XeO6 adopts octahedral symmetry, but upon hydration, the XeO64- anion becomes noticeably distorted from octahedral symmetry. For stationary, anhydrous samples of Na4XeO6, the heteronuclear 129/131Xe-23Na dipolar interaction is the principal contributor to the breadth of the 129/131Xe NMR lines. For stationary and slow magic-angle-spinning samples of Na4XeO6·2H2O, the anisotropic xenon shielding interaction dominates the 129Xe NMR line shape, whereas the 131Xe NMR line shape is completely dominated by the nuclear quadrupolar interaction. The xenon shielding tensor is approximately axially symmetric, with a skew of -0.7 ± 0.3, an isotropic xenon chemical shift of -725.6 ± 1.0 ppm, and a span of 95 ± 5 ppm. The 131Xe quadrupolar coupling constant, 10.8 ± 0.5 MHz, is large for a nucleus at a site of approximate Oh symmetry, and the quadrupolar asymmetry parameter indicates a lack of axial symmetry. This study demonstrates the extreme sensitivity of the 131Xe nuclear quadrupolar interaction to changes in the local xenon environment.
Ling-I Hung, Sue-Lein Wang, Hsien-Ming Kao, and Kwang-Hwa Lii. (2007).
"Flux Synthesis, Crystal Structures, and Solid-State NMR Spectroscopy of Two Indium Silicates Containing Varied In-O Coordination Geometries."
Inorg. Chem. 46(8): 3301-3305.
Abstract: Two novel indium silicates, K5In3Si7O21 (1) and K4In2Si8O21 (2), have been synthesized by a flux-growth method and characterized by single-crystal X-ray diffraction. The structure of 1 consists of siebener single chains of corner-sharing SiO4 tetrahedra running along the b axis linked via corner-sharing by In2O9 face-sharing octahedral dimers and InO5 trigonal bipyramids to form a 3D framework. The structure of 2 consists of a 3D silicate framework containing 6- and 14-ring channels. InO5 square pyramids are located within the 14-ring channels sharing corners with the silicate framework. The solid-state 29Si MAS NMR spectrum of compound 1 was recorded; it shows the influence of the indium atoms in the second coordination sphere of the silicon on the chemical shift. Crystal data: 1, orthorhombic, Pna21 (No. 33), a = 12.4914(3) Å, b = 16.8849(3) Å, c = 10.2275(2) Å, V = 2157.1(1) Å3 and Z = 4; 2, monoclinic, P21/n (No. 14), a = 8.4041(3) Å, b = 11.4919(4) Å, c = 10.4841(3) Å, = 90.478(2), V = 1012.5(1) Å3 and Z = 2.
Tuesday, May 01, 2007
J. Am. Chem. Soc., 129 (15), 4612 -4619, 2007.
Synthesis of Nitric Oxide-Releasing Silica Nanoparticles
Jae Ho Shin, Sara K. Metzger, and Mark H. Schoenfisch*
Abstract:
The synthesis and characterization of a new nitric oxide (NO)-releasing scaffold prepared from amine-functionalized silica nanoparticles are reported. Inorganic-organic hybrid silica was prepared via cocondensation of tetraethoxy- or tetramethoxysilane (TEOS or TMOS) and aminoalkoxysilane with appropriate amounts of ethanol (or methanol), water, and ammonia. The amine functional groups in the silica were converted to N-diazeniumdiolate NO donors via exposure to high pressures of NO (5 atm) under basic conditions. Control over both the structure and concentration of the silane precursors (i.e., tetraalkoxy- and aminoalkoxysilanes) and specific synthetic conditions allowed for the preparation of NO donor silica particles of widely varying sizes (d = 20-500 nm), NO payloads (50-1780 nmol·mg-1), maximum amounts of NO released (10-5500 ppb·mg-1), half-lives (0.1-12 h), and NO release durations (up to 30 h). The silica nanoparticles were characterized by solid-state 29Si nuclear magnetic resonance (NMR), atomic force microscopy (AFM), elemental analysis, and gas adsorption-desorption isotherms. The advantages of silica-derived NO storage/delivery systems over previously reported macromolecular NO donors include the ability to (1) store large quantities of NO, (2) modulate NO release kinetics, and (3) readily tune particle size based on the composition of the particle. In addition, a one-pot strategy for preparing the NO donor silica allows for straightforward, high-throughput synthesis and purification.
Jae Ho Shin, Sara K. Metzger, and Mark H. Schoenfisch*
Abstract:
The synthesis and characterization of a new nitric oxide (NO)-releasing scaffold prepared from amine-functionalized silica nanoparticles are reported. Inorganic-organic hybrid silica was prepared via cocondensation of tetraethoxy- or tetramethoxysilane (TEOS or TMOS) and aminoalkoxysilane with appropriate amounts of ethanol (or methanol), water, and ammonia. The amine functional groups in the silica were converted to N-diazeniumdiolate NO donors via exposure to high pressures of NO (5 atm) under basic conditions. Control over both the structure and concentration of the silane precursors (i.e., tetraalkoxy- and aminoalkoxysilanes) and specific synthetic conditions allowed for the preparation of NO donor silica particles of widely varying sizes (d = 20-500 nm), NO payloads (50-1780 nmol·mg-1), maximum amounts of NO released (10-5500 ppb·mg-1), half-lives (0.1-12 h), and NO release durations (up to 30 h). The silica nanoparticles were characterized by solid-state 29Si nuclear magnetic resonance (NMR), atomic force microscopy (AFM), elemental analysis, and gas adsorption-desorption isotherms. The advantages of silica-derived NO storage/delivery systems over previously reported macromolecular NO donors include the ability to (1) store large quantities of NO, (2) modulate NO release kinetics, and (3) readily tune particle size based on the composition of the particle. In addition, a one-pot strategy for preparing the NO donor silica allows for straightforward, high-throughput synthesis and purification.
J. Am. Chem. Soc., 129 (15), 4567 -4574, 2007.
Cluster Growth and Fragmentation in the Highly Fluxional Platinum Derivatives of Sn94-: Synthesis, Characterization, and Solution Dynamics of Pt2@Sn174- and Pt@Sn9H3-
Banu Kesanli, Jordan E. Halsig, Peter Zavalij, James C. Fettinger, Yiu-Fai Lam, and Bryan W. Eichhorn*
Abstract:
Sn94- reacts with Pt(PPh3)4 in ethylenediamine/toluene solvent mixtures in the presence of 2,2,2-cryptand to give four different complexes: "Rudolph's complex" of proposed formula [Sn9Pt(PPh3)x]4- (2), the previously reported [Pt@Sn9Pt(PPh3)]2- ion (3), and the title complexes Pt2@Sn174- (4) and Pt@Sn9H3- (5). The use of Pt(norbornene)3 instead of Pt(PPh3)4 gives complex 4 exclusively. The structure of 4 contains two Pt atoms centered in a capsule-shaped Sn17 cage. The complex is highly dynamic in solution showing single, mutually coupled 119Sn and 195Pt NMR resonances indicative of an intramolecular liquidlike dynamic exchange process. Complex 5 has been characterized by selectively decoupled 1H, 119Sn, and 195Pt NMR experiments and shows similar liquidlike fluxionality. In addition, the H atom scrambles across the cage showing small couplings to both Sn and Pt atoms. Neither 3 nor 4 obeys Wades rules; they adopt structures more akin to the subunits in alloys such as PtSn4. The structural and chemical relevance to supported PtSn4 heterogeneous catalysts is discussed.
Banu Kesanli, Jordan E. Halsig, Peter Zavalij, James C. Fettinger, Yiu-Fai Lam, and Bryan W. Eichhorn*
Abstract:
Sn94- reacts with Pt(PPh3)4 in ethylenediamine/toluene solvent mixtures in the presence of 2,2,2-cryptand to give four different complexes: "Rudolph's complex" of proposed formula [Sn9Pt(PPh3)x]4- (2), the previously reported [Pt@Sn9Pt(PPh3)]2- ion (3), and the title complexes Pt2@Sn174- (4) and Pt@Sn9H3- (5). The use of Pt(norbornene)3 instead of Pt(PPh3)4 gives complex 4 exclusively. The structure of 4 contains two Pt atoms centered in a capsule-shaped Sn17 cage. The complex is highly dynamic in solution showing single, mutually coupled 119Sn and 195Pt NMR resonances indicative of an intramolecular liquidlike dynamic exchange process. Complex 5 has been characterized by selectively decoupled 1H, 119Sn, and 195Pt NMR experiments and shows similar liquidlike fluxionality. In addition, the H atom scrambles across the cage showing small couplings to both Sn and Pt atoms. Neither 3 nor 4 obeys Wades rules; they adopt structures more akin to the subunits in alloys such as PtSn4. The structural and chemical relevance to supported PtSn4 heterogeneous catalysts is discussed.
J. Am. Chem. Soc., 129 (14), 4440 -4455, 2007.
Coupling of Functional Hydrogen Bonds in Pyridoxal-5'-phosphate-Enzyme Model Systems Observed by Solid-State NMR Spectroscopy
Shasad Sharif, David Schagen, Michael D. Toney, and Hans-Heinrich Limbach*
Abstract:
We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.
Shasad Sharif, David Schagen, Michael D. Toney, and Hans-Heinrich Limbach*
Abstract:
We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.
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