Directed evolution of enzymatic silicon-carbon bond cleavage in siloxanes.

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Synthesis and Biodegradation Studies of Low-Dispersity Poly(acrylic acid).

Poly(acrylic acid) (PAA) is produced on an industrial scale and widely-used in applications such as personal care products and cleaning formulations that end up "down-the-drain." Relatively high molecular weight PAA is considered poorly biodegradable, but little is known about the biodegradability of low molecular weight PAA at the wastewater treatment plant according to current regulatory and industrial Organization for Economic Co-operation and Development (OECD) standards. The synthesis, separation, and characterization of a series of ultralow dispersity PAA oligomers (i.e., D < 1.10) in the molecular weight range Mn ≈ 350-1200 Da and the results of biodegradability testing are reported. Miniaturized, high-throughput screening studies in a parallel respirometer reveals a strong trend toward lower biodegradation at higher molecular weight; these results are confirmed and expanded using standardized method OECD 301F. Biodegradability reaches ≈40% at Mn = 380 Da, ≈26% at Mn = 770 Da, and ≈17% at Mn = 1190 Da for discrete polyacid oligomers. These data not only shed light on potential biodegradation mechanisms for linear PAA, but also may inspire the future design of biodegradable PAA-containing macromolecules.

Characterization of low and intermediate molecular weight hydrogen silsesquioxanes by mass spectrometry.

Hydrogen silsesquioxanes (HSQ or H-resin), represented by (HSiO3/2)2n or TH(2n), are an important class of polymers that have gained popularity as spin-on dielectrics by the electronic industry. Previously in the literature, small oligomeric species such as (HSiO3/2)2n, where n = 4-16, have been identified by GC-MS. However, nondestructive mass spectral results for larger H-resin molecules have not been reported, likely due to the nonpolar nature of these molecules. We have utilized a number of "soft" ionization techniques such as field desorption (FD), desorption chemical ionization (DCI), and matrix-assisted laser desorption/ionization (MALDI), and demonstrated that they are amenable to hydrogen silsesquioxanes. The [TH(2n) - H]+* ions were observed by FD-MS while [TH(2n) + NH4]+ and [TH(2n) + Na]+ ions were found utilizing DCI and MALDI, respectively. Based upon the MS results, the polymer compositions as well as molecular weight information can be easily obtained. The detailed structures of H-resin components, however, remain a difficult issue, which cannot be answered by MS data alone. With these preliminary MS results, we have clearly demonstrated that mass spectrometry with the above-mentioned ionization techniques is an invaluable tool that can be utilized when attempting to solve the challenge set forth by the complexity of hydrogen silsesquioxane materials.