Brian L. Phillips

Effect of sugars on headgroup mobility in freeze-dried dipalmitoylphosphatidylcholine bilayers: solid-state 31P NMR and FTIR studies.

The effect of the carbohydrates trehalose, glucose, and hydroxyethyl starch (HES) on the motional properties of the phosphate headgroup of freeze-dried dipalmitoylphosphatidylcholine (DPPC) liposomes was studied by means of 31P NMR, Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The results show that trehalose, which is a strong glass former (Tg = 115 degreesC), elevates the onset of the lipid headgroup rotations and preserves some rotational mobility of the phosphate headgroups after cooling from the liquid-crystalline state. Glucose (Tg = 30 degreesC), a very effective depressant of the phase transition temperature of freeze-dried DPPC, markedly elevates the initiation of the temperature of headgroup rotations. On the other hand, the monosaccharide does not preserve the headgroup disordering when cooled from the liquid-crystalline state. These effects are consistent with formation of hydrogen bonds between the OH groups of the sugar and the polar headgroups of DPPC. They show, however, that hydrogen bonding is not sufficient for preservation of the dynamic properties of freeze-dried DPPC. HES, although a very good glass former (Tg > 110 degreesC), does not depress the phase transition temperature and affects only slightly the rotational properties of freeze-dried DPPC. This lack of effect of HES is associated with the absence of direct interactions with the lipid phosphates, as evidenced by the FTIR results. These data show that vitrification of the additive is not sufficient to affect the dynamic properties of dried DPPC.

Rates and mechanisms of oxygen exchanges between sites in the AlO4Al12(OH)24(H2O)127+(aq) complex and water: implications for mineral surface chemistry

Abstract Rate coefficients for steady exchange of oxygens between sites in the AlO4Al12(OH)24(H2O)127+(aq) (Al13) complex and bulk solution were determined over the temperature range of 9–19°C and 4.6

Mechanisms for fluoride-promoted dissolution of bayerite [β-Al(OH)3(s)] and boehmite [γ-AlOOH]: 19F-NMR spectroscopy and aqueous surface chemistry

Some reactions that control the dissolution of bayerite [β-Al(OH)3(s)] and boehmite [γ-AlOOH] were identified by comparing the adsorption chemistry, the dissolution rates, and solid-state 19F-NMR spectra of the reacting surfaces. The 19F-NMR spectra of bayerite distinguish two sites for fluoride reaction that vary in relative concentration with the total adsorbate density. One resonance at −131 ppm is assigned to fluoride bridges and the other resonance at −142 ppm is assigned to fluoride at terminal sites. These same resonances are observed on boehmite, in addition to a third resonance at −151 ppm that is tentatively assigned to aqueous AlFn(H2O)6−n(3−n)+(aq) complexes in pores. Peak broadening due to dipolar coupling between surface fluorides at high loading indicates that these sites are in close proximity. A consistent picture of dissolution is derived by considering the 19F-NMR results, the aqueous experiments, and information derived from the studies of aqueous complexes, particularly studies of the dissociation mechanisms of aqueous multimers. Both fluoride and adsorbed protons enhance the dissolution rates via a series of pathways that may be coupled to one another, and there is a profound dependence of the rate on the concentration of adsorbed protons and adsorbed fluorides. Particularly important are fluoride-substituted bridges and sites where aluminum atoms are bonded to several terminal fluorides or hydroxyls. These results illustrate that it is possible to test hypotheses about molecular-scale processes if adsorption studies are coupled to spectroscopy and ligand-promoted dissolution experiments where reaction via different pathways can be distinguished.

Vitreous forsterite (Mg2SiO4): Synthesis, structure, and thermochemistry

Here we report the first synthesis of a forsterite (Mg2SiO4) composition glass as an essentially phase-pure bulk material. Under containerless conditions, with heterogeneous nucleation sites minimized, glass forms by cooling ca. 1 mm liquid Mg2SiO4 droplets in oxygen at 700 K/s. 29Si NMR spectroscopic data indicate that the SiO4 tetrahedra and MgO6 octahedra exist in a corner sharing arrangement in the glass, but upon crystallization the polyhedral units reorganize to form edge-sharing linkages. Transposed temperature drop calorimetry shows that the glass is 61.4±1.3 kJ/mol higher in enthalpy than the crystal.

Kinetics of oxygen exchange between sites in the GaO4Al12(OH)24(H2O)127+(aq) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and sites in the GaO4Al12(OH)24(H2O)127+(aq) (GaAl12) aqueous complex were determined over the temperature range of 301 to 317 K and 4.1 < pH < 4.9 using 17O–nuclear magnetic resonance (NMR). The GaAl12 molecule, like the AlO4Al12(OH)24(H2O)127+(aq) (Al13) molecule studied previously, has 12 equivalent bonded water molecules (η1-OH2 sites), two structurally distinct sets of 12 hydroxyl bridges (μ2-OH; μ2-OH′ sites), and four four-coordinated oxo groups (μ4-O sites). The GaAl12 molecule is much less reactive than the Al13 molecule, and this decreased reactivity is not associated with any clear changes in the structural chemistry at the sites of exchange. The rate coefficients for exchange of the water molecules bonded to the complex with bulk water are as follows: kex298 = 227(±40) s−1, ΔH‡ = 63(±7) kJ mol−1, and ΔS‡ = 13(±21) J mol−1 K−1. The rate at 298 K is ≈5 times slower than the corresponding exchange reaction on the Al13 molecule, but it falls within the range measured for dissolved aluminum monomers. These data support our earlier speculation that rates of exchange of η1-OH2 sites at fully charged aluminum (hydr)oxide mineral surfaces are similar to rates for aqueous aluminum complexes in acids. The rates of isotopic exchange of the two hydroxyl bridges in the GaAl12 complex differ from one another, as we also observed for the Al13 complex, but to a much smaller extent. Likewise, the activation parameters for exchange at the two sites are much more similar to one another in the GaAl12 molecule than in the Al13. The rate coefficients for exchange of the more reactive hydroxyl bridge are as follows: kex298 = 1.8(±0.1) · 10−5 s−1, ΔH1‡ = 98(±3) kJ mol−1, ΔS1‡ = −8(±9) J mol−1 K−1, and for the less labile bridge, they are kex298 = 4.1(±0.2) · 10−7 s−1, with ΔH2‡ = 125(±4) kJ mol−1 and ΔS2‡ = 54(±12) J mol−1 K−1. There is no strong pH dependence to rates.

Solvent exchange in AlFx (H2O 6−x3−x (aq) complexes: Ligand-directed labilization of water as an analogue for ligand-induced dissolution of oxide minerals

We demonstrate, using dynamic 17O-NMR spectroscopy, that fluoride ions in the inner-coordination sphere of AlFx(H2O)6−x3−x(aq) complexes (0 ≤ x ≤ 2) progressively enhance, by orders of magnitude, the rate of exchange of waters from the innersphere to the bulk solvent. At 298K, the rate of the elementary ligand-exchange reaction increased approximately linearly with each fluoride substitution from about 2 s−1 for the fully hydrated [Al(H2O)63+(aq)] complex to about 2 × 104 s−1 for the difluoro [AlF2(H2O)4+(aq)] complex. A similar effect can be expected for isoelectronic complexes, such as the hydrolysis product: AlOH(H2O)52+(aq). By hypothesis, a similar phenomenon accounts for fluoride-enhanced release of metals from a dissolving mineral surface. If so, it should be possible to predict the effectiveness of different adsorbed ligands to enhance dissolution of minerals from spectroscopic rate measurements on dissolved complexes, as was recently shown for aminocarboxylate ligands and NiO(s) (Ludwig et al., 1995, 1996).

The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)128+(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, IVGeO4Al12(OH)24(OH2)128+(aq) (GeAl12), that is similar in structure to both the IVAlO4Al12(OH)24(OH2)127+(aq) (Al13) and IVGaO4Al12(OH)24(OH2)127+(aq) (GaAl12) molecules studied previously. All of these molecules have e-Keggin-like structures, but in the GeAl12 molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al13 and GaAl12. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using 17O-NMR. Apparent rate parameters for exchange of the bound water molecules (η-OH2) are kex298 = 200 (±100) s−1, ΔH‡ = 46 (±8) kJ · mol−1, and ΔS‡ = −46 (±24) J · mol−1 K−1 and are similar to those we measured previously for the GaAl12 and Al13 complexes. In contrast to the Al13 and GaAl12 molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl12 species. The two topologically distinct μ2-OH sites in the GeAl12 molecule exchange at greatly differing rates. The more labile set of μ2-OH sites in the GeAl12 molecule exchange at a rate that is faster than can be measured by the 17O-NMR isotopic-equilibration technique. The second set of μ2-OH sites have rate parameters of kex298 = 6.6 (±0.2) · 10−4 s−1, ΔH‡ = 82 (±2) kJ · mol−1, and ΔS‡ = −29 (±7) J · mol−1 · K−1, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ2-OH sites in the Al13 and GaAl12 molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ2-OH site with bulk solution for the GeAl12 molecule, which contrasts with Al13 and GaAl12 molecules.

Dynamics of Antifreeze Glycoproteins in the Presence of Ice

Antifreeze glycoproteins from the Greenland cod Boreogadus saida were dimethylated at the N-terminus (m*AFGP) and their dynamics and conformational properties were studied in the presence of ice using (13)C-NMR and FTIR spectroscopy. (13)C-NMR experiments of m*AFGP in D(2)O, in H(2)O, and of freeze-dried m*AFGP were performed as a function of temperature. Dynamic parameters ((1)H T(1 rho) and T(CH)) obtained by varying the contact time revealed notable differences in the motional properties of AFGP between the different states. AFGP/ice dynamics was dominated by fast-scale motions (nanosecond to picosecond time scale), suggesting that the relaxation is markedly affected by the protein hydration. The data suggest that AFGP adopts a similar type of three-dimensional fold both in the presence of ice and in the freeze-dried state. FTIR studies of the amide I band did not show a single prevailing secondary structure in the frozen state. The high number of conformers suggests a high flexibility, and possibly reflects the necessity to expose more ice-binding groups. The data suggest that the effect of hydration on the local mobility of AFGP and the lack of significant change in the backbone conformation in the frozen state may play a role in inhibiting the ice crystal growth.

THE RATES OF WATER EXCHANGE IN AI(III)-SALICYLATE AND AI(III)-SULFOSALICYLATE COMPLEXES

Rate parameters are reported for exchange of hydration waters from the inner coordination sphere of Al(III)-sulfosalicylate [Al(sSal)+] and Al(III)-salicylate [Al(Sal)+] complexes to bulk solution as determined with 17O-NMR. The rate parameters for the Al(sSal)+ complex are: kex298 = (3.0 ± 0.2) · 103 s−1, ΔH‡ = 37(±3) kJ mol−1, ΔS‡ = −54(±9) J mol−1 K−1; and for the Al(Sal)+ complex are: kex298 = 4.9(±0.3) · 103 s−1, ΔH‡ = 35(±3) kJ mol−1, ΔS‡ = −57(±11) J mol−1 K−1. These results, along with previous work, suggest that the lability of water molecules in bidentate carboxylate–phenolic complexes scales with the electron-donating properties of the ligand oxygens. Replacement of a coordinated carboxyl with a phenolic group in the ligand increases both the Lewis basicity and the value of kex298. A correlation between these parameters is proposed that can be used to predict rate coefficients for other bidentate aluminum complexes.

The rates of exchange of water molecules from Al(III)-methylmalonate complexes: the effect of chelate ring size

Abstract Rate coefficients are reported for exchange of hydration waters in the inner-coordination-sphere of Al(III)-methylmalonate complexes with bulk solution as determined via 17O-NMR. Surprisingly, water molecules in the thermodynamically less-stable complexes containing six-membered chelates are much more labile than those in five-membered oxalate-Al(III) complexes. The rate parameters for the Al(mMal)+ complex are: k ex 298 =6.6·10 2 s −1 ,ΔH ‡ =66(±1) kJ mol −1 ;ΔS ‡ =30.6(±2) J mol −1 K −1 and for the Al(mMal)2− complex are k ex 298 =6.9·10 3 s −1 ,ΔH ‡ =55(±3) kJ mol −1 ;ΔS ‡ =12.8(±11) J mol −1 K −1 The surprising trend in reactivity is attributable either to differences in the Lewis basicities of oxygens in bidentate oxalate and methylmalonate ligands, or to rapid dissociation/reassociation of one of the acetate groups to the metal center. These results identify a useful case where trends in the apparent labilities of dissolved and presumed surface complexes deviate sharply. This deviation could be usefully exploited to probe surfaces if ligand-promoted dissolution rates could be compared at conditions where inner-sphere and outer-sphere chelate complexes could be distinguished spectroscopically. We expect inner-sphere oxalate to have a smaller labilizing effect than malonate or methylmalonate. A contrary result would indicate structural dissimilarity between complexes on the surface and in solution, or perhaps steric hindrance.