Alkaline hydrogen peroxide bleaching of cellulose

A closed system bleaching apparatus was designed to determine the kinetics and effects of various factors on alkaline hydrogen peroxide bleaching of textile cellulose fabrics. It was confirmed that perhydroxyl anion is the primary bleaching moiety in alkaline hydrogen peroxide systems. The use of the apparatus in the measurement of fabric color, waste oxygen, and the subsequent calculation of hydroxyl ion, and molecular hydrogen peroxide confirmed that pH and titration of 'free' hydrogen peroxide in alkaline bleaching systems are not good indicators of bleaching mechanism. The role of the cellulose itself in the chemical bleaching system was determined. The rate of bleaching on cotton fabric was shown to be a first order reaction in concentration of perhydroxyl anion at 60 and 90°C. An activation energy of 17kcal/mole was estimated. Decomposition of H₂O₂ into waste oxygen was found to be second order kinetics.     

Refinement of NMR structures using implicit solvent and advanced sampling techniques

NMR biomolecular structure calculations exploit simulated annealing methods for conformational sampling and require a relatively high level of redundancy in the experimental restraints to determine quality three-dimensional structures. Recent advances in generalized Born (GB) implicit solvent models should make it possible to combine information from both experimental measurements and accurate empirical force fields to improve the quality of NMR-derived structures. In this paper, we study the influence of implicit solvent on the refinement of protein NMR structures and identify an optimal protocol of utilizing these improved force fields. To do so, we carry out structure refinement experiments for model proteins with published NMR structures using full NMR restraints and subsets of them. We also investigate the application of advanced sampling techniques to NMR structure refinement. Similar to the observations of Xia et al. (J.Biomol. NMR 2002, 22, 317-331), we find that the impact of implicit solvent is rather small when there is a sufficient number of experimental restraints (such as in the final stage of NMR structure determination), whether implicit solvent is used throughout the calculation or only in the final refinement step. The application of advanced sampling techniques also seems to have minimal impact in this case. However, when the experimental data are limited, we demonstrate that refinement with implicit solvent can substantially improve the quality of the structures. In particular, when combined with an advanced sampling technique, the replica exchange (REX) method, near-native structures can be rapidly moved toward the native basin. The REX method provides both enhanced sampling and automatic selection of the most native-like (lowest energy) structures. An optimal protocol based on our studies first generates an ensemble of initial structures that maximally satisfy the available experimental data with conventional NMR software using a simplified force field and then refines these structures with implicit solvent using the REX method. We systematically examine the reliability and efficacy of this protocol using four proteins of various sizes ranging from the 56-residue B1 domain of Streptococcal protein G to the 370-residue Maltose-binding protein. Significant improvement in the structures was observed in all cases when refinement was based on low-redundancy restraint data. The proposed protocol is anticipated to be particularly useful in early stages of NMR structure determination where a reliable estimate of the native fold from limited data can significantly expedite the overall process. This refinement procedure is also expected to be useful when redundant experimental data are not readily available, such as for large multidomain biomolecules and in solid-state NMR structure determination.     

In vitro bioactivity of wollastonite–titania materials obtained by sol–gel method or solid state reaction

In order to obtain materials for bone tissue replacement and regeneration that show an appropriate bioactivity, antibacterial behavior and higher mechanical properties than those of the existing bioactive systems, titania–wollastonite materials were obtained by both solid state reaction and sol–gel methods. In the solid state reaction process, titania and wollastonite powders were mixed and sintered. Two different types of materials were obtained by sol–gel: (i) using titanium butoxide, calcium nitrate tetrahydrate and tetraethyl orthosilicate as precursors or (ii) mixing titanium butoxide with wollastonite powder. The in vitro bioactivity was assessed by immersing samples in simulated body fluids for different periods of time. Alamar blue assays and osteoblast-like cells were used for testing citotoxicity. A higher bioactivity was observed on the samples synthesized by sol–gel. However, a higher cell proliferation was observed on the samples obtained by solid state reaction.     

Comparison of Approaches to Calibrate a Surface Complexation Model for U(VI) Sorption to Weathered Saprolite

A surface complexation model describing the sorption of uranyl ions and uranyl carbonate on weak and strong sites was used to analyze experiments conducted on pH-dependent U(VI) sorption to weathered shale/limestone saprolite. Sorption data were collected at two different solid to solution ratios. Various methods of estimating equilibrium reaction coefficients and site densities were investigated. As a first approximation, extractable iron oxides were assumed to behave as ferrihydrite with reaction coefficients as reported by Waite (Geochim Cosmochim Acta 58:5465–5478, 1994). A generalized composite (GC) approach was then employed with coefficients estimated by an inverse modeling method applied both in a stepwise fashion and simultaneously to whole data set. Uncertainty in model parameters and predictions was lowest using the simultaneous inverse method, but results from the stepwise method were very similar. The generalized reaction network accurately described pH-dependent U(VI) sorption on weathered saprolite between pH 4 and 9.