Nigel J. Clayden

Solid state 27Al NMR and FTIR study of lanthanum aluminosilicate glasses

The structure of lanthanum aluminosilicate (LAS) glasses, containing: (15–25) mol% La2O3, (15–35) mol% Al2O3 and (40–70) mol% SiO2, were studied by 27Al nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The effects of La2O3 and Al2O3 content on the properties and structure of the LAS glasses were evaluated. Al3+ ion enters into the glass structure mainly in fourfold co-ordination, forming (AlO4/2)− tetrahedra, and only in small amounts in fivefold and sixfold co-ordination. La3+ acts as network modifier, producing more non-bridging oxygen atoms as its content increases.

Solid-state nuclear magnetic resonance spectroscopic study of γ-zirconium phosphate

An idealised model for the structure of γ-Zr(HPO4)2·2H2O is proposed on the basis of solid-state 31P magic-angle spinning (m.a.s.) n.m.r. spectroscopy. Two types of phosphate environment are seen of which only one undergoes phosphate exchange with phosphate and phosphonate esters. 13C Cross-polarisation m.a.s. n.m.r. confirms the phenyl phosphate exchange and indicates that the phenyl rings are rigid between the layers.

Characterisation and prediction of phase separation in hot-melt extruded solid dispersions: a thermal, microscopic and NMR relaxometry study.

ABSTRACTPurposeTo develop novel analytical approaches for identifying both miscibility and phase separation in hot-melt extruded formulations.MethodsFelodipine-Eudragit® E PO solid dispersions were prepared using hot-melt extrusion. The fresh and aged formulations were characterised using scanning electron microscopy, differential scanning calorimetry, heat capacity (Cp) measurements using modulated temperature DSC and nuclear magnetic resonance relaxometry.ResultsThe solubility of the drug in polymer was predicted as being ≤10% w/w using a novel model proposed in this study. Freshly prepared HME formulations were found to show no evidence for phase separation despite drug loadings greatly in excess of this figure. Conventional DSC showed limitations in directly detecting phase separation. However, a novel use of Cp measurements indicated that extensive phase separation into crystalline domains was present in all aged samples, a conclusion supported by SEM studies. The NMR relaxometry study confirmed the existence of phase separation in all aged formulations and also allowed the estimation of separated domains sizes in different formulations.ConclusionsThis study has presented a series of novel approaches for the identification, quantification and prediction of phase separation in HME formulations. Supersaturation of drug in the polymer caused the phase separation of the aged felodipine-Eudragit® E PO formulations.

Comparative quantitative study on the crystallinity of poly(tetrafluoroethylene) including Raman, infra-red and 19F nuclear magnetic resonance spectroscopy

Abstract Wide-angle X-ray scattering (WAXS), 19 F nuclear magnetic resonance (n.m.r.) spectroscopy, differential scanning calorimetry (d.s.c.) and density measurements have been employed to determine the crystallinity of melt-quenched poly(tetrafluoroethylene) samples of different molecular weight. The methods have been checked for consistency before a routinely used infra-red (i.r.) spectroscopic and a recently devised Raman spectroscopic procedure have been correlated with crystallinity as derived by the four conventional methods. The usefulness of the two vibrational spectroscopic methods for determining crystallinity has then been assessed. It was found that WAXS, d.s.c. and density give consistent results in that they clearly show the same trends in the sample crystallinity. In contrast, neither free induction decay nor solid echo pulse sequence n.m.r. correlate with any of these methods, and possible reasons for this behaviour are discussed. Raman as well as i.r. data correlate well with the crystallinity by WAXS, d.s.c. and density. In this respect, both methods are found to be equally useful in estimating crystallinity.

Structural dynamics of the membrane translocation domain of colicin E9 and its interaction with TolB

In order for the 61 kDa colicin E9 protein toxin to enter the cytoplasm of susceptible cells and kill them by hydrolysing their DNA, the colicin must interact with the outer membrane BtuB receptor and Tol translocation pathway of target cells. The translocation function is located in the N-terminal domain of the colicin molecule. (1)H, (1)H-(1)H-(15)N and (1)H-(13)C-(15)N NMR studies of intact colicin E9, its DNase domain, minimal receptor-binding domain and two N-terminal constructs containing the translocation domain showed that the region of the translocation domain that governs the interaction of colicin E9 with TolB is largely unstructured and highly flexible. Of the expected 80 backbone NH resonances of the first 83 residues of intact colicin E9, 61 were identified, with 43 of them being assigned specifically. The absence of secondary structure for these was shown through chemical shift analyses and the lack of long-range NOEs in (1)H-(1)H-(15)N NOESY spectra (tau(m)=200 ms). The enhanced flexibility of the region of the translocation domain containing the TolB box compared to the overall tumbling rate of the protein was identified from the relatively large values of backbone and tryptophan indole (15)N spin-spin relaxation times, and from the negative (1)H-(15)N NOEs of the backbone NH resonances. Variable flexibility of the N-terminal region was revealed by the (15)N T(1)/T(2) ratios, which showed that the C-terminal end of the TolB box and the region immediately following it was motionally constrained compared to other parts of the N terminus. This, together with the observation of inter-residue NOEs involving Ile54, indicated that there was some structural ordering, resulting most probably from the interactions of side-chains. Conformational heterogeneity of parts of the translocation domain was evident from a multiplicity of signals for some of the residues. Im9 binding to colicin E9 had no effect on the chemical shifts or other NMR characteristics of the region of colicin E9 containing the TolB recognition sequence, though the interaction of TolB with intact colicin E9 bound to Im9 did affect resonances from this region. The flexibility of the translocation domain of colicin E9 may be connected with its need to recognise protein partners that assist it in crossing the outer membrane and in the translocation event itself.

29Si and 27Al NMR study of the thermal transformation of barium exchanged zeolite-A to celsian

The thermal transformation of Ba exchanged zeolite A to celsian has been studied by 27Al and 29Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the 29Si NMR spectra after thermal treatment at 200 °C. Confirmation is provided by the 29Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single phase. The 27Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite A. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian. No evidence could be seen for significant amounts of nanometre scale crystalline domains in the amorphous matrix, prior to the crystallisation of monoclinic celsian.

Solid state 1H NMR study, humidity sensitivity and protonic conduction of gel derived phosphosilicate glasses

1H Magic Angle Spinning (MAS) NMR and 1H T1 and T1ρ relaxation times measurements were used to investigate structural aspects of gel derived glasses of composition 10P2O5·90SiO2 (10P) and 30P2O5·70SiO2 (30P), with the aim of understanding the different humidity sensitive behaviour observed for films of the same composition. The experimental evidence suggests that 10P and 30P gel samples heat treated at 100 °C have a similar POH environment while further heating causes a different structural evolution of POH units. In particular, the 30P sample heat treated at 400 °C shows a single 1H resonance while a number of resonances are seen in the 1H spectrum of the 10P sample heat treated at 400 °C. Further evidence of the different structure of 10P and 30P samples was given by the NMR relaxation times measurements, the gels were found to be heterogeneous on a nanometric scale. Domains, about 60 nm in size, related to 1H resonance in POH were evenly distributed in the matrix of both gels, only in the 10P gel also larger domains (about 700 nm) were found that can be related to 1H resonance in SiOH groups. Finally, MAS in combination with variable temperature static NMR spectra provided detailed information on the network structural features present in the 30P sample heat treated at 300 °C as well as in the 10P sample heat treated at 400 °C and allowed investigation of the dynamic processes taking place in these samples.

Solid state 29Si NMR and FT Raman spectroscopy of the devitrification of lithium metasilicate glass

Lithium metasilicate glass was found to devitrify in two steps. In the primary step the glass undergoes a transformation which is exothermic and in which there is almost no crystal growth. The nature of this process and the modification that it causes in the glass has been investigated by a combination of solid state nuclear magnetic resonance spectroscopy and Raman spectroscopy. During the primary step a redistribution of QN units occurs as shown by changes in the intensities of the resonances associated with these structural units. The absence of narrow resonances in samples heated to the primary step suggests that most of the sample is not crystalline. This amorphous state is consistent with the powder X-ray diffraction where only diffraction peaks with small intensities are seen.

NMR characterisation of the relationship between frustration and the excited state of Im7.

Previous work shows that Im9 folds in a two-state transition while its homologue Im7 folds in a three-state transition via an on-pathway kinetic intermediate state (KIS), with this difference being related to frustration in the structure of Im7. We have used NMR spectroscopy to study conformational dynamics connected to the frustration. A combination of equilibrium peptide N(1)H/N(2)H exchange, model-free analyses of backbone NH relaxation data and relaxation dispersion (RD)-NMR shows that the native state of Im7 is in equilibrium with an intermediate state that is lowly populated [equilibrium intermediate state (EIS)]. Comparison of kinetic and thermodynamic parameters describing the EIS native-state equilibrium obtained by RD-NMR with previously reported parameters describing the KIS native-state equilibrium obtained from stopped-flow fluorescence studies of refolding His-tagged Im7 shows that the KIS and the EIS are the same species. (15)N chemical shifts of the EIS obtained from the RD-NMR analysis show that residues forming helix III in the native state are unstructured in the EIS while other residues experiencing frustration in the native state are in structured regions of the EIS. We show that binding of Im7 and its L53A/I54A variant (which resembles the EIS as shown in previous work) to the cognate partner for Im7, the DNase domain of colicin E7, causes the dynamic processes associated with the frustration to be dampened.

Correlations between 31P n.m.r. chemical shifts and structural parameters in crystalline inorganic phosphates

Correlations have been observed between the 31P n.m.r. chemical shifts of crystalline inorganic phosphates and the bond strengths at the phosphate oxygen atoms; these provide a basis for structural studies and for peak assignment in phosphates that give more than one resonance.