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1.
Magn Reson Imaging ; 68: 83-94, 2020 05.
Artigo em Inglês | MEDLINE | ID: mdl-32007558

RESUMO

BACKGROUND: Spatially normalizing brain MRI data to a template is commonly performed to facilitate comparisons between individuals or groups. However, the presence of multiple sclerosis (MS) lesions and other MS-related brain pathologies may compromise the performance of automated spatial normalization procedures. We therefore aimed to systematically compare five commonly used spatial normalization methods for brain MRI - including linear (affine), and nonlinear MRIStudio (LDDMM), FSL (FNIRT), ANTs (SyN), and SPM (CAT12) algorithms - to evaluate their performance in the presence of MS-related pathologies. METHODS: 3 Tesla MRI images (T1-weighted and T2-FLAIR) were obtained for 20 participants with MS from an ongoing cohort study (used to assess a real dataset) and 1 healthy control participant (used to create a simulated lesion dataset). Both raw and lesion-filled versions of each participant's T1-weighted brain images were warped to the Montreal Neurological Institute (MNI) template using all five normalization approaches for the real dataset, and the same procedure was then repeated using the simulated lesion dataset (i.e., total of 400 spatial normalizations). As an additional quality-assurance check, the resulting deformations were also applied to the corresponding lesion masks to evaluate how each processing pipeline handled focal white matter lesions. For each normalization approach, inter-subject variability (across normalized T1-weighted images) was quantified using both mutual information (MI) and coefficient of variation (COV), and the corresponding normalized lesion volumes were evaluated using paired-sample t-tests. RESULTS: All four nonlinear warping methods outperformed conventional linear normalization, with SPM (CAT12) yielding the highest MI values, lowest COV values, and proportionately-scaled lesion volumes. Although lesion-filling improved spatial normalization accuracy for each of the methods tested, these effects were small compared to differences between normalization algorithms. CONCLUSIONS: SPM (CAT12) warping, ideally combined with lesion-filling, is recommended for use in future MS brain imaging studies requiring spatial normalization.

2.
Chemistry ; 2019 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-31650641

RESUMO

he mechanistic details of the aldol addition of N-amino cyclic carbamate (ACC) hydrazones is provided herein from both an experimental and computational perspective. When the transformation is carried out at room temperature the anti-aldol product is formed exclusively. Under these conditions the anti- and syn-aldolate intermediates are in equilibrium and the transformation is under thermodynamic control. The anti-aldolate that leads to the anti-aldol product was calculated to be 3.7 kcal mol-1 lower in energy at room temperature than that leading to the syn-aldol product, which sufficiently accounts for the exclusive formation of the anti-aldol product. When the reaction is conducted at -78 °C it is under kinetic control and favors formation of the syn-aldol addition product. In this case, it was found that a solvent separated aza-enolate anion and aldehyde form a σ-intermediate in which the lithium cation is coordinated to the aldehyde. The σ-intermediate collapses with a very small activation barrier to form the ß-alkoxy hydrazone intermediate. The chiral nonracemic lithium aza-enolate discriminates between the two diastereotopic faces of the pro-chiral aldehyde, and there is no rapid direct pathway that interconverts the two diastereomeric intermediates. Consequently, the reaction does not follow the Curtin-Hammett principle and the stereochemical outcome at low temperature instead depends on the relative energies of the two σ-intermediates.

3.
Adv Mater ; 31(49): e1905540, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31631407

RESUMO

Molecular surfactants are widely used to control low-dimensional morphologies, including 2D nanomaterials in colloidal chemical synthesis, but it is still highly challenging to accurately control single-layer growth for 2D materials. A scalable stacking-hinderable strategy to not only enable exclusive single-layer growth mode for transition metal dichalcogenides (TMDs) selectively sandwiched by surfactant molecules but also retain sandwiched single-layer TMDs' photoredox activities is developed. The single-layer growth mechanism is well explained by theoretical calculation. Three types of single-layer TMDs, including MoS2 , WS2 , and ReS2 , are successfully synthesized and demonstrated in solar H2 fuel production from hydrogen-stored liquid carrier-methanol. Such H2 fuel production from single-layer MoS2 nanosheets is COx -free and reliably workable under room temperature and normal pressure with the generation rate reaching ≈617 µmole g-1 h-1 and excellent photoredox endurability. This strategy opens up the feasible avenue to develop methanol-storable solar H2 fuel with facile chemical rebonding actualized by 2D single-layer photocatalysts.

4.
Sci Rep ; 9(1): 2500, 2019 02 21.
Artigo em Inglês | MEDLINE | ID: mdl-30792440

RESUMO

Various MRI techniques, including myelin water imaging, T1w/T2w ratio mapping and diffusion-based imaging can be used to characterize tissue microstructure. However, surprisingly few studies have examined the degree to which these MRI measures are related within and between various brain regions. Therefore, whole-brain MRI scans were acquired from 31 neurologically-healthy participants to empirically measure and compare myelin water fraction (MWF), T1w/T2w ratio, fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD) and mean diffusivity (MD) in 25 bilateral (10 grey matter; 15 white matter) regions-of-interest (ROIs). Except for RD vs. T1w/T2w, MD vs. T1w/T2w, moderately significant to highly significant correlations (p < 0.001) were found between each of the other measures across all 25 brain structures [T1w/T2w vs. MWF (Pearson r = 0.33, Spearman ρ = 0.31), FA vs. MWF (r = 0.73, ρ = 0.75), FA vs. T1w/T2w (r = 0.25, ρ = 0.22), MD vs. AD (r = 0.57, ρ = 0.58), MD vs. RD (r = 0.64, ρ = 0.61), AD vs. MWF (r = 0.43, ρ = 0.36), RD vs. MWF (r = -0.49, ρ = -0.62), MD vs. MWF (r = -0.22, ρ = -0.29), RD vs. FA (r = -0.62, ρ = -0.75) and MD vs. FA (r = -0.22, ρ = -0.18)]. However, while all six MRI measures were correlated with each other across all structures, there were large intra-ROI and inter-ROI differences (i.e., with no one measure consistently producing the highest or lowest values). This suggests that each quantitative MRI measure provides unique, and potentially complimentary, information about underlying brain tissues - with each metric offering unique sensitivity/specificity tradeoffs to different microstructural properties (e.g., myelin content, tissue density, etc.).

5.
J Org Chem ; 83(21): 12951-12964, 2018 11 02.
Artigo em Inglês | MEDLINE | ID: mdl-30200759

RESUMO

The α-alkylation of ketones is a fundamental synthetic transformation. The development of asymmetric variants of this reaction is important given that numerous natural products, drugs, and related compounds exist as α-functionalized ketones or derivatives thereof. We previously reported our preliminary studies on the development of a new enantioselective ketone α-alkylation procedure using N-amino cyclic carbamate (ACC) auxiliaries. In comparison to other auxiliary-based methods, ACC alkylation offers a number of advantages and is both highly enantioselective and high yielding. Herein, we provide a full account of our studies on the enantioselective ACC ketone α-alkylation method.

6.
Org Lett ; 20(13): 3723-3727, 2018 07 06.
Artigo em Inglês | MEDLINE | ID: mdl-29897766

RESUMO

A chiral auxiliary-based diastereoselective transformation that entirely avoids the stereochemically mismatched pairing, providing equally high levels of asymmetric induction in the formation of each diastereomer is described. In particular, we show that chiral nonracemic methyl ketones undergo α,α-bisalkylation using phenylalanine-derived N-amino cyclic carbamate (ACC) auxiliaries with essentially perfect diastereoselectivity, as well as excellent yield and regioselectivity. Significantly, with the use of a single enantiomer of the auxiliary, either diastereomeric product can be synthesized with an equally high level of asymmetric induction.

7.
Magn Reson Imaging ; 51: 35-43, 2018 09.
Artigo em Inglês | MEDLINE | ID: mdl-29680454

RESUMO

Tissue contrast can be enhanced by dividing T1-weighted (T1w) images by T2-weighted (T2w) images to map the so-called T1w/T2w ratio, which has become an increasingly popular technique for quantifying brain tissue changes associated with neurodevelopment, aging, and a variety of neurodegenerative diseases. However, although it is self-evident that T1w/T2w ratios increase with the amount of T2-weighting in the T2w image - which is determined by the echo time (TE), all else being equal - longer TEs also reduce the signal-to-noise ratio (SNR) of the T2w images, and it is not clear how these SNR characteristics affect the reliability of T1w/T2w measurements. Therefore, the current study systematically investigated how different amounts of T2-weighting affected T1w/T2w measurements in order to determine whether there is an optimal amount of T2-weighting. T1w images were acquired from 10 neurologically healthy adults using a 3D turbo field echo (TFE) sequence, and a series of T2-weighted images were extracted from a multi-echo 3D combined gradient- and spin-echo (GRASE) sequence. Analyses of 12 anatomically defined brain regions revealed that both the mean and standard deviation of the T1w/T2w measurements increased exponentially with TE of the T2w images, and that T2w images with TE ≈ 120-160 ms yielded the most consistent/reproducible contrast between white matter ROIs and the whole-brain T1w/T2w signal. Furthermore, comparisons between T1w/T2w measurements and multi-component T2-relaxation myelin water fractions (MWFs) in the same brain regions revealed that T2w images with TE ≥ 160 ms drastically reduced the degree of correlation between T1w/T2w measurements and MWF. Overall, these findings suggest that: 1) there is a substantial trade-off between increased T1w/T2w contrast (based on longer TEs for the T2w images) and the reliability of quantitative T1w/T2w signals; and 2) the optimal TE for T2w GRASE scans is between 120 ms and 160 ms for calculating T1w/T2w ratios.


Assuntos
Encéfalo/diagnóstico por imagem , Imagem Ecoplanar/métodos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Doenças Neurodegenerativas/diagnóstico por imagem , Substância Branca/diagnóstico por imagem , Adulto , Feminino , Humanos , Análise dos Mínimos Quadrados , Masculino , Bainha de Mielina/metabolismo , Valores de Referência , Razão Sinal-Ruído , Adulto Jovem
8.
Front Med (Lausanne) ; 5: 31, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-29515998

RESUMO

Purpose: Postmortem MRI can be used to reveal important pathologies and establish radiology-pathology correlations. However, quantitative MRI values are altered by tissue fixation. Therefore, the purpose of this study was to investigate time-dependent effects of formalin fixation on MRI relaxometry (T1 and T2), diffusion tensor imaging (fractional anisotropy, FA; and mean diffusivity, MD), and myelin water fraction (MWF) measurements throughout intact human brain specimens. Methods: Two whole, neurologically-healthy human brains were immersed in 10% formalin solution and scanned at 13 time points between 0 and 1,032 h. Whole-brain maps of longitudinal (T1) and transverse (T2) relaxation times, FA, MD, and MWF were generated at each time point to illustrate spatiotemporal changes, and region-of-interest analyses were then performed in eight brain structures to quantify temporal changes with progressive fixation. Results: Although neither of the diffusion measures (FA nor MD) showed significant changes as a function of formalin fixation time, both T1 and T2-relaxation times significantly decreased, and MWF estimates significantly increased with progressive fixation until (and likely beyond) our final measurements were taken at 1,032 h. Conclusion: These results suggest that T1-relaxation, T2-relaxation and MWF estimates must be performed quite early in the fixation process to avoid formalin-induced changes compared to in vivo values; and furthermore, that different ex vivo scans within an experiment must be acquired at consistent (albeit still early) fixation intervals to avoid fixative-related differences between samples. Conversely, ex vivo diffusion measures (FA and MD) appear to depend more on other factors (e.g., pulse sequence optimization, sample temperature, etc.).

9.
NMR Biomed ; 31(3)2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29315894

RESUMO

Given the growing popularity of T1 -weighted/T2 -weighted (T1 w/T2 w) ratio measurements, the objective of the current study was to evaluate the concordance between T1 w/T2 w ratios obtained using conventional fast spin echo (FSE) versus combined gradient and spin echo (GRASE) sequences for T2 w image acquisition, and to compare the resulting T1 w/T2 w ratios with histologically validated myelin water fraction (MWF) measurements in several subcortical brain structures. In order to compare these measurements across a relatively wide range of myelin concentrations, whole-brain T1 w magnetization prepared rapid acquisition gradient echo (MPRAGE), T2 w FSE and three-dimensional multi-echo GRASE data were acquired from 10 participants with multiple sclerosis at 3 T. Then, after high-dimensional, non-linear warping, region of interest (ROI) analyses were performed to compare T1 w/T2 w ratios and MWF estimates (across participants and brain regions) in 11 bilateral white matter (WM) and four bilateral subcortical grey matter (SGM) structures extracted from the JHU_MNI_SS 'Eve' atlas. Although the GRASE sequence systematically underestimated T1 w/T2 w values compared to the FSE sequence (revealed by Bland-Altman and mountain plots), linear regressions across participants and ROIs revealed consistently high correlations between the two methods (r2 = 0.62 for all ROIs, r2 = 0.62 for WM structures and r2 = 0.73 for SGM structures). However, correlations between either FSE-based or GRASE-based T1 w/T2 w ratios and MWFs were extremely low in WM structures (FSE-based, r2 = 0.000020; GRASE-based, r2 = 0.0014), low across all ROIs (FSE-based, r2 = 0.053; GRASE-based, r2 = 0.029) and moderate in SGM structures (FSE-based, r2 = 0.20; GRASE-based, r2 = 0.17). Overall, our findings indicated a high degree of correlation (but not equivalence) between FSE-based and GRASE-based T1 w/T2 w ratios, and low correlations between T1 w/T2 w ratios and MWFs. This suggests that the two T1 w/T2 w ratio approaches measure similar facets of subcortical tissue microstructure, whereas T1 w/T2 w ratios and MWFs appear to be sensitized to different microstructural properties. On this basis, we conclude that multi-echo GRASE sequences can be used in future studies to efficiently elucidate both general (T1 w/T2 w ratio) and myelin-specific (MWF) tissue characteristics.


Assuntos
Córtex Cerebral/anatomia & histologia , Imagem por Ressonância Magnética , Bainha de Mielina/metabolismo , Marcadores de Spin , Água/química , Adulto , Idoso , Idoso de 80 Anos ou mais , Feminino , Humanos , Processamento de Imagem Assistida por Computador , Modelos Lineares , Masculino , Pessoa de Meia-Idade , Adulto Jovem
10.
Environ Sci Pollut Res Int ; 24(28): 22402-22413, 2017 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-28803348

RESUMO

This paper aims to investigate the effect of the addition of 5% alcohol (butanol) with biodiesel-diesel blends on the performance, emissions, and combustion of a naturally aspirated four stroke multi-cylinder diesel engine at different engine speeds (1200 to 2400 rpm) under full load conditions. Three types of local Australian biodiesel, namely macadamia biodiesel (MB), rice bran biodiesel (RB), and waste cooking oil biodiesel (WCB), were used for this study, and the data was compared with results for conventional diesel fuel (B0). Performance results showed that the addition of butanol with diesel-biodiesel blends slightly lowers the engine efficiency. The emission study revealed that the addition of butanol additive with diesel-biodiesel blends lowers the exhaust gas temperature (EGT), carbon monoxide (CO), nitrogen oxide (NOx), and particulate matter (PM) emissions whereas it increases hydrocarbon (HC) emissions compared to B0. The combustion results indicated that in-cylinder pressure (CP) for additive added fuel is higher (0.45-1.49%), while heat release rate (HRR) was lower (2.60-9.10%) than for B0. Also, additive added fuel lowers the ignition delay (ID) by 23-30% than for B0. Finally, it can be recommended that the addition of 5% butanol with Australian biodiesel-diesel blends can significantly lower the NOx and PM emissions.


Assuntos
Biocombustíveis/análise , Butanóis/análise , Gasolina/análise , Emissões de Veículos/análise , Emissões de Veículos/prevenção & controle , Austrália , Biocombustíveis/normas , Monóxido de Carbono/análise , Gasolina/normas , Óxidos de Nitrogênio/análise , Material Particulado/análise
11.
Magn Reson Imaging ; 37: 21-26, 2017 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-27856398

RESUMO

OBJECTIVE: To determine accurate quantitative transverse relaxation times (T2) using retrospective clinical images and apply it to examine 7-year changes in multiple sclerosis (MS) brain. METHODS: A method for T2 mapping from retrospective proton density (PD) and T2-weighted fast spin echo images was recently introduced, but requires measurement of flip angles. We examined whether 1.5T flip angle variation in brain can be predicted, thus enabling T2 analysis of historical PD and T2-weighted images without a concurrent flip angle map. After method validation in healthy volunteers, retrospective longitudinal T2 analysis was performed in 14 MS subjects over seven years. Changes in patient T2 values were compared with brain atrophy, T2 lesion load and disability score in MS. RESULTS: Similar flip angle maps across volunteers enabled retrospective T2 from PD and T2-weighted images even when different refocusing angles were used. Over seven years, significant T2 changes of 2-4% were observed when using T2 modelling and the 7-year effect size for globus pallidus T2 was 0.56, which was more significant than brain atrophy. No significant T2 results were found when using exponential fit, which cannot account for refocusing angle variation. Moreover, change is T2 in globus pallidus and internal capsule correlated with MS disability score over time when using T2 modelling. CONCLUSIONS: Accurate quantitative T2 can be extracted from standard clinical 1.5T MRI exams that include PD and T2-weighted imaging even when no flip angle map is available. This method was applied retrospectively to examine seven year changes in MS.


Assuntos
Encéfalo/diagnóstico por imagem , Imagem por Ressonância Magnética , Esclerose Múltipla/diagnóstico por imagem , Adulto , Atrofia/patologia , Encéfalo/patologia , Encefalopatias/patologia , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Esclerose Múltipla/patologia , Prótons , Reprodutibilidade dos Testes , Estudos Retrospectivos , Índice de Gravidade de Doença
12.
Magn Reson Imaging ; 34(1): 51-9, 2016 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-26435459

RESUMO

PURPOSE: To investigate the brain iron dependence of transverse relaxation rate difference methods derived from spin echo and gradient echo measurements from two field strengths. METHODS: Transverse relaxation rates R2 and R2(⁎) were measured in human brain in 17 healthy subjects at 1.5T and 4.7T using multi-slice, multiecho spin echo and gradient echo sequences. R2 quantification used stimulated echo compensation and R2(⁎) quantification used linear background gradient correction at 4.7T only. Subtraction of R2 from R2(⁎) within each field strength yielded R2('), and R2 subtraction across fields yielded Field Dependent R2 Increase (FDRI). All transverse relaxation measures were then correlated with published post-mortem iron concentrations using linear regression analysis. Regional differences were tested using paired t-tests. Phantom measurements of FDRI were also performed. RESULTS: In deep grey matter, all transverse relaxation rates (R2, R2(⁎), R2(')) at both 1.5T and 4.7T, and FDRI had moderate to strong correlations (r>0.71, p<0.0001) with estimated non-heme iron. The 4.7T methods and FDRI had higher correlations (r>0.9) than 1.5T measures. R2, R2(⁎), R2' at 4.7T and FDRI had slopes 0.49, 1.96, 1.48 and 0.33 [s(-1)/mg Fe/100g wt. tissue] and intercepts 14.40, 16.87, 2.47 and 3.21 [s(-1)] respectively. Even though FDRI yielded a zero intercept in phantom, in vivo FDRI was found to be ineffective at fully removing non-iron contributions and yielded a large intercept. The slope for R2(') was 3.4 times greater at 4.7T than 1.5T. For white matter fiber tracts oriented predominantly perpendicular versus parallel to B0, R2(') increased by ~50% at 4.7T and ~30% at 1.5T, while R2 and FDRI in white matter was insensitive to its orientation with respect to B0. CONCLUSION: The transverse relaxation difference methods FDRI and R2(') at 4.7T had high correlations to predicted iron content similar to R2 and R2(⁎) at 4.7T. Although R2(') had smaller y-intercept with estimated iron concentration than FDRI, in white matter R2(') demonstrated strongest dependence on fiber orientation with respect to B0. These results suggest that for brain iron correlation, there is minimal value of transverse difference methods over a single R2(⁎) measurement at highest available field, which was 4.7T.


Assuntos
Algoritmos , Encéfalo/metabolismo , Interpretação de Imagem Assistida por Computador/métodos , Ferro/metabolismo , Imagem por Ressonância Magnética/métodos , Espectroscopia de Ressonância Magnética/métodos , Adulto , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Imagem Molecular/métodos , Valores de Referência , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Distribuição Tecidual
13.
Mult Scler ; 22(9): 1133-43, 2016 08.
Artigo em Inglês | MEDLINE | ID: mdl-26503237

RESUMO

BACKGROUND: Deep gray matter (DGM) is affected in relapsing-remitting multiple sclerosis (RRMS) and may be studied using short-term longitudinal MRI. OBJECTIVE: To investigate two-year changes in spin-echo transverse relaxation rate (R2) and atrophy in DGM, and its relationship with disease severity in RRMS patients. METHODS: Twenty six RRMS patients and 26 matched controls were imaged at 4.7 T. Multiecho spin-echo R2 maps and atrophy measurements were obtained in DGM at baseline and two-year follow-up. Differences between MRI measures and correlations to disease severity were examined. RESULTS: After two years, mean R2 values in the globus pallidus and pulvinar increased by ~4% (p<0.001) in patients and <1.7% in controls. Two-year changes in R2 showed significant correlation to disease severity in the globus pallidus, pulvinar, substantia nigra, and thalamus. Multiple regression of the two-year R2 difference using these four DGM structures as variables, yielded high correlation with disease severity (r=0.83, p<0.001). Two-year changes in volume and R2 showed significant correlation only for the globus pallidus in multiple sclerosis (MS) (p<0.05). CONCLUSIONS: Two-year difference R2 measurements in DGM correlate to disease severity in MS. R2 mapping and atrophy measurements over two years can be used to identify changes in DGM in MS.


Assuntos
Substância Cinzenta/patologia , Interpretação de Imagem Assistida por Computador/métodos , Imagem por Ressonância Magnética/métodos , Esclerose Múltipla Recidivante-Remitente/patologia , Adulto , Atrofia , Estudos de Casos e Controles , Progressão da Doença , Feminino , Humanos , Estudos Longitudinais , Masculino , Pessoa de Meia-Idade , Tamanho do Órgão , Valor Preditivo dos Testes , Índice de Gravidade de Doença , Fatores de Tempo
14.
Magn Reson Med ; 70(5): 1340-6, 2013 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-23325543

RESUMO

Transverse relaxation (T2 ) mapping has many applications, including imaging of iron accumulation in grey matter. Using the typical multiecho spin-echo sequence with long echo trains, stimulated echo compensation can enable T2 fitting under conditions of variable radio frequency homogeneity arising from slice profile and in-plane radio frequency variation. Substantial reduction in the number of refocusing pulses could enable use at high magnetic fields where specific absorption rate is a major limitation, and enable multislice use with reduced incidental magnetization transfer at all field strengths. We examine the effect of reduced echo train lengths and multislice imaging on T2 fitting using stimulated echo compensation applied to iron-rich subcortical grey matter in human brain at 4.7 T. Our findings indicate that reducing from 20 echoes to as few as four echoes can maintain consistent T2 values when using stimulated echo compensation in grey and white matter, but not for cerebrospinal fluid. All territories produce marginal results when using standard exponential fitting. Savings from reduced echoes can be used to substantially increase slice coverage. In multislice mode, the resulting incidental magnetization transfer decreased brain signal but had minimal effect on measured T2 values.


Assuntos
Algoritmos , Encéfalo/anatomia & histologia , Imagem Ecoplanar/métodos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Adulto , Feminino , Humanos , Masculino , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
15.
Langmuir ; 26(14): 12104-11, 2010 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-20536154

RESUMO

Variable-pressure deuterium nuclear magnetic resonance ((2)H NMR) has been used to study the pressure-temperature phase diagram of bicellar mixtures containing 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). Spectra were obtained for DMPC-d(54)/DHPC (3:1), DMPC-d(54)/DHPC (4.4:1), DMPC/DHPC-d(22) (3:1), and DMPC/DHPC-d(22) (4.4:1) in the range 10-68 degrees C at ambient pressure, 66 MPa, 102 MPa, and 135 MPa. Isotropic-to-nematic and nematic-to-lamellar transition temperatures were found to rise with pressure at approximately 0.15 and approximately 0.14 degrees C/MPa, respectively, for DMPC-d(54)/DHPC (3:1) and at at approximately 0.19 and approximately 0.18 degrees C/MPa, respectively, for DMPC-d(54)/DHPC (4.4:1). Pressure had little effect on the range of DMPC-d(54) chain orientational order through the nematic phase temperature range, but the behavior of chain orientational order at the nematic-to-lamellar transition was found to vary slightly with pressure. Comparison of differential scanning calorimetry (DSC) observations with ambient-pressure (2)H NMR observations of DMPC-d(54) in the bicellar mixtures suggests that absorption of heat persists for a few degrees above the onset of axially symmetric DMPC-d(54) reorientation.

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