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The type II isopentenyl diphosphate:dimethylallyl diphosphate isomerase (IDI-2) catalyzes the reversible isomerization of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). Although a growing body of experiments have suggested that the flavin coenzyme of IDI-2 serves a novel function as an acid-base catalyst, the detailed reaction mechanism of IDI-2 is still unknown. In this paper, a combined quantum-mechanical/molecular-mechanical (QM/MM) approach has been applied to investigate the detailed reaction mechanism of IDI-2. The one-base mechanism in which the N-5 nitrogen of the zwitterionic form of reduced FMN acts as the acid-base catalyst has been supported by our computational results, and a IPP-FMN adduct is also proposed for the first time. The mechanistic details including the fundamental reaction pathways, the complete energy profiles of the whole catalytic cycle, and the specific role of the coenzyme and key residues are all obtained. It is proved that IDI-2 employs novel flavin chemistry with the coenzyme acting as a general acid-base catalyst.

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The aging rules of rigid polyurethane foam (PUR) at indoor storage and different hygrothermal conditions have been studied. Four parameters, which are mass, dimension, compressive strength and compressive modulus were tested. At indoor storage, mass, dimension and compressive strength vary slowly with an increase in aging time, while compressive modulus decreases quickly. PUR is sensitive to relative humidity (RH) verified by accelerated hygrothermal aging, and hydrolysis of ester group is the main reason resulting in the decrease of compressive properties. The filling with fire retardant and glass beads had some effect on hygrothermal aging properties of PUR. The addition of fire retardant increased compressive strength with aging time in the total trend, but it made dimension stability worse. The addition of glass beads slightly improved hygrothermal aging properties.

Natural rubber (NR) blends are widely used in many industries because of their excellent integrated properties. However, a simple, easily operational, nondestructive, and accurate method for their quantitative analysis remains as a challenge. This has been always an important issue in the related industries, particularly for their daily quality control tests. One main reason is that NR ingredients vary according to their geographical origin and the harvest time, which renders it hard to set up a versatile analytical protocol for all NRs. Another reason is owing to the defects of the established methods themselves as having been revealed in those relying on TGA, Py-GC/MS, FTIR, and ATR-FTIR. In this study, a simple and feasible method based on near infrared spectroscopy combined with chemometric is proposed to solve this problem for the first time. NR/SBR (styrene-butadiene rubber) rubber blend, the most widely used NR blend, is selected as a typical research subject. Spectral calibration region, factor, and several different pretreatment methods are applied on the spectra data to optimize calibration models. The result shows the optimized calibration model provides a good accuracy (0.135 wt %), intraday precision (0.121 wt %) and interday precision (0.132 wt %) for 3 months. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41423.