Modeling spin-forbidden monomer self-initiation reactions in spontaneous free-radical polymerization of acrylates and methacrylates

A spin-forbidden reaction is a reaction in which the total electronic spin-state changes. The standard transition-state theory that assumes a reaction occurs on a single potential energy surface with spin-conservation cannot be applied to a spin-forbidden reaction directly. In this work, we derive the crossing coefficient based on the Wentzel-Kramers-Brillouin (WKB) theory to quantify the effect of intersystem crossing on the kinetics of spin-forbidden reactions. Acrylates and methacrylates, by themselves, can generate free radicals that initiate polymerization at temperatures above 120 °C. Previous studies suggest that a triplet diradical is a key intermediate in the self-initiation. The formation of a triplet diradical from two closed-shell monomer molecules is a spin-forbidden reaction. This study provides a quantitative analysis of singlet-triplet spin crossover of diradical species in self-initiation of acrylates and methacrylates, taking into account the effect of intersystem crossing. The concept of crossing control is introduced and demonstrated computationally to be a new likely route to generate monoradicals via monomer self-initiation in high temperature polymerization.

Genetic and epigenetic regulation of the human prostacyclin synthase promoter in lung cancer cell lines

The importance of the arachidonic acid pathway has been established in colon and lung cancers, as well as in inflammatory diseases. In these diseases, prostacyclin I(2) (PGI2) and prostaglandin E(2) (PGE2) are thought to have antagonistic activities, with PGI2 exerting anti-inflammatory and antiproliferative activities, whereas PGE2 is proinflammatory and antiapoptotic. In human lung cancer, prostacyclin synthase (PGIS) and PGI2 are down-regulated, whereas PGE2 synthase (PGES) and PGE2 are up-regulated. Murine carcinogenesis models of human lung cancer reciprocate the relationship between PGIS and PGES expression. PGIS-overexpressing transgenic mice are protected from carcinogen- and tobacco smoke-induced lung tumor formation, suggesting that PGI2 may play a role in chemoprevention. We investigated several potential mechanisms for the down-regulation of PGIS in human lung cancer. Using transcription reporter assays, we show that single nucleotide polymorphisms in the PGIS promoter can affect transcriptional activity. In addition, PGIS expression in several human lung cancer cell lines is silenced by CpG methylation, and we have mapped these sites across the variable number of tandem repeats (VNTR) sequence in the promoter, as well as CpGs within exon 1 and the first intron. Finally, using fluorescence in situ hybridization, we show that human lung cancer cell lines and lung cancer tissues do not have a loss of the PGIS genomic region but multiple copies. These results show that an individual's PGIS promoter haplotype can play an important role in the predisposition for lung cancer and CpG methylation provides an epigenetic mechanism for the down-regulated PGIS expression.

Expression of human proteins at the Southeast Collaboratory for Structural Genomics

The human protein production group at the Southeast Collaboratory for Structural Genomics is charged with producing human proteins for both X-ray crystallography and NMR structural studies. Eukaryotic, and human proteins in particular, are notoriously difficult to express in bacterial systems. For various reasons, T7-based expression often results in protein expressed in an insoluble form. Overcoming this requires either introduction of a step to screen expression conditions or inclusion of a troublesome refolding step during purification. Our laboratory uses a trc-based expression vector that addresses many of the difficulties of the commonly used T7-based expression systems. Proteins expressed under the trc promoter, a weak promoter compared to the strong T7 promoter, are produced in a soluble form and include necessary cofactors. The details of this system will be discussed.