Solvent Influence on Cellulose 1,4-β-Glycosidic Bond Cleavage: A Molecular Dynamics and Metadynamics Study

We explore the influence of two solvents, namely water and the ionic liquid 1-ethyl-3-methylimidazolium acetate (EmimAc), on the conformations of two cellulose models (cellobiose and a chain of 40 glucose units) and the solvent impact on glycosidic bond cleavage by acid hydrolysis by using molecular dynamics and metadynamics simulations. We investigate the rotation around the glycosidic bond and ring puckering, as well as the anomeric effect and hydrogen bonds, in order to gauge the effect on the hydrolysis mechanism. We find that EmimAc eases hydrolysis through stronger solvent–cellulose interactions, which break structural and electronic barriers to hydrolysis. Our results indicate that hydrolysis in cellulose chains should start from the ends and not in the centre of the chain, which is less accessible to solvent

The Electronic Nature of the 1,4-β-Glycosidic Bond and Its Chemical Environment: DFT Insights into Cellulose Chemistry

The molecular understanding of the chemistry of 1,4-β-glucans is essential for designing new approaches to the conversion of cellulose into platform chemicals and biofuels. In this endeavor, much attention has been paid to the role of hydrogen bonding occurring in the cellulose structure. So far, however, there has been little discussion about the implications of the electronic nature of the 1,4-β-glycosidic bond and its chemical environment for the activation of 1,4-β-glucans toward acid-catalyzed hydrolysis. This report sheds light on these central issues and addresses their influence on the acid hydrolysis of cellobiose and, by analogy, cellulose. The electronic structure of cellobiose was explored by DFT at the BB1 K/6-31++G(d,p) level. Natural bond orbital (NBO) analysis was performed to grasp the key bonding concepts. Conformations, protonation sites, and hydrolysis mechanisms were examined. The results for cellobiose indicate that cellulose is protected against hydrolysis not only by its supramolecular structure, as currently accepted, but also by its electronic structure, in which the anomeric effect plays a key role

Identification of reconstruction in Pt films deposited on Pd(110) at room temperature

We have studied the properties of Pt films on Pd(110), grown by deposition at 300 K and annealed up to 900 K, using low-energy electron diffraction and Auger-electron spectroscopy. We observe (1×2) and (1×3) superstructures, depending upon Pt coverage and annealing temperature. At one monolayer, the (1×1) periodicity is unperturbed. Between one and three monolayers, a broad and streaky (1×2) develops upon annealing, then fades to (1×1) as the film dissolves. At three monolayers and above, the broad and streaky (1×2) splits to a (1×3), then fades again to (1×1) at high temperature. Adsorption of CO causes (1×3)→(1×1) reversion at relatively low temperature, 430 K. Based upon the known behavior of Pt(110) reconstructions, this is strong evidence that the (1×3) structure of the Pt film is a surface reconstruction

Long-term predictability of mean daily temperature data

International audienceWe quantify the long-term predictability of global mean daily temperature data by means of the Rényi entropy of second order K2. We are interested in the yearly amplitude fluctuations of the temperature. Hence, the data are low-pass filtered. The obtained oscillatory signal has a more or less constant frequency, depending on the geographical coordinates, but its amplitude fluctuates irregularly. Our estimate of K2 quantifies the complexity of these amplitude fluctuations. We compare the results obtained for the CRU data set (interpolated measured temperature in the years 1901-2003 with 0.5° resolution, Mitchell et al., 2005)with the ones obtained for the temperature data from a coupled ocean-atmosphere global circulation model (AOGCM, calculated at DKRZ). Furthermore, we compare the results obtained by means of K2 with the linear variance of the temperature data

Liver Transplantation in Older Patients

To the Editor: The impact of liver transplantation on public health policy has not been delineated, partly because of uncertainty about the upper age limit for candidacy. We have compared the outlook in 363 adult recipients of livers who were less than 50 years old (mean +SD, 35.8+8.5) and 92 recipients who were 50 to 77 years old (mean +SD, 55.7+4.8). Cyclosporine and steroids1,2 were used for immunosuppression, and monoclonal antilymphocyte globulin3 and azathioprine were added as needed. The techniques of liver transplantation4 were not influenced by the age of the patient. The indications for transplantation are shown in Figure 1…., No extract is available for articles shorter than 400 words. © 1987, Massachusetts Medical Society. All rights reserved

Accuracy of computerized tomography in determining hepatic tumor size in patients receiving liver transplantation or resection

Computerized tomography (CT) of liver is used in oncologic practice for staging tumors, evaluating response to treatment, and screening patients for hepatic resection. Because of the impact of CT liver scan on major treatment decisions, it is important to assess its accuracy. Patients undergoing liver transplantation or resection provide a unique opportunity to test the accuracy of hepatic-imaging techniques by comparison of finding of preoperative CT scan with those at gross pathologic examination of resected specimens. Forty-one patients who had partial hepatic resection (34 patients) or liver transplantation (eight patients) for malignant (30 patients) or benign (11 patients) tumors were evaluable. Eight (47%) of 17 patients with primary malignant liver tumors, four (31%) of 13 patients with metastatic liver tumors, and two (20%) of 10 patients with benign liver tumors had tumor nodules in resected specimens that were not apparent on preoperative CT studies. These nodules varied in size from 0.1 to 1.6 cm. While 11 of 14 of these nodules were 1.0 cm. These results suggest that conventional CT alone may be insufficient to accurately determine the presence or absence of liver metastases, extent of liver involvement, or response of hepatic metastases to treatment