The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis

A genetic tool to express long fungal biosynthetic genes

BACKGROUND

Secondary metabolites (SMs) from mushroom-forming fungi (Basidiomycota) and early diverging fungi (EDF) such as Mucoromycota are scarcely investigated. In many cases, production of SMs is induced by unknown stress factors or is accompanied by seasonable developmental changes on fungal morphology. Moreover, many of these fungi are considered as non-culturable under laboratory conditions which impedes investigation into SM. In the post-genomic era, numerous novel SM genes have been identified especially from EDF. As most of them encode multi-module enzymes, these genes are usually long which limits cloning and heterologous expression in traditional hosts.

RESULTS

An expression system in Aspergillus niger is presented that is suitable for the production of SMs from both Basidiomycota and EDF. The akuB gene was deleted in the expression host A. niger ATNT∆pyrG, resulting in a deficient nonhomologous end-joining repair mechanism which in turn facilitates the targeted gene deletion via homologous recombination. The ∆akuB mutant tLK01 served as a platform to integrate overlapping DNA fragments of long SM genes into the fwnA locus required for the black pigmentation of conidia. This enables an easy discrimination of correct transformants by screening the transformation plates for fawn-colored colonies. Expression of the gene of interest (GOI) is induced dose-dependently by addition of doxycycline and is enhanced by the dual TetON/terrein synthase promoter system (ATNT) from Aspergillus terreus. We show that the 8 kb polyketide synthase gene lpaA from the basidiomycete Laetiporus sulphureus is correctly assembled from five overlapping DNA fragments and laetiporic acids are produced. In a second approach, we expressed the yet uncharacterized > 20 kb nonribosomal peptide synthetase gene calA from the EDF Mortierella alpina. Gene expression and subsequent LC-MS/MS analysis of mycelial extracts revealed the production of the antimycobacterial compound calpinactam. This is the first report on the heterologous production of a full-length SM multidomain enzyme from EDF.

CONCLUSIONS

The system allows the assembly, targeted integration and expression of genes of > 20 kb size in A. niger in one single step. The system is suitable for evolutionary distantly related SM genes from both Basidiomycota and EDF. This uncovers new SM resources including genetically intractable or non-culturable fungi.

Probing the Design Rationale of a High-Performing Faujasitic Zeotype Engineered to have Hierarchical Porosity and Moderated Acidity

Porosity and acidity are influential properties in the rational design of solid-acid catalysts. Probing the physicochemical characteristics of an acidic zeotype framework at the molecular level can provide valuable insights in understanding intrinsic reaction pathways, for affording structure-activity relationships. Herein, we employ a variety of probe-based techniques (including positron annihilation lifetime spectroscopy (PALS), FTIR and solid-state NMR spectroscopy) to demonstrate how a hierarchical design strategy for a faujasitic (FAU) zeotype (synthesized for the first time, via a soft-templating approach, with high phase-purity) can be used to simultaneously modify the porosity and modulate the acidity for an industrially significant catalytic process (Beckmann rearrangement). Detailed characterization of hierarchically porous (HP) SAPO-37 reveals enhanced mass-transport characteristics and moderated acidity, which leads to superior catalytic performance and increased resistance to deactivation by coking, compared to its microporous counterpart, further vindicating the interplay between porosity and moderated acidity.

Butane Isomerization as a Diagnostic Tool in the Rational Design of Solid Acid Catalysts

The growing demand for isobutane as a vital petrochemical feedstock and chemical intermediate has for many decades surpassed industrial outputs that can be supplied through liquified petroleum gases. Alternative methods to resource the isobutane market have been explored, primarily the isomerization of linear n-butane to the substituted isobutane. To date the isobutane market is valued at over 20 billion US dollars, enticing researchers to seek unique and novel catalytic materials to improve on current commercial practices. Two main classes of catalysts have dominated the butane isomerization literature in the last few decades; namely microporous zeolites and sulfated zirconia. Both have been widely researched for butane isomerization, to the point where key catalytic descriptors such as acidity, framework topology and metal doping are becoming well understood. While this provides new researchers with a roadmap for developing new materials, it is has also begun developing into an invaluable tool for diagnosing and understanding the effect of these individual descriptors on catalytic properties. In this review we explore the different factors that influence the active site behavior of particularly zeolites and sulfated zirconia catalysts towards understanding the use of butane isomerization as a diagnostic tool for solid-acid catalysts.

Surface Heterogeneous Nucleation-Mediated Release of Beta-Carotene from Porous Silicon

We demonstrate that the release of a poorly soluble molecule from nanoporous carriers is a complex process that undergoes heterogeneous surface nucleation events even under significantly diluted release conditions, and that those events heavily affect the dynamics of release. Using beta-carotene and porous silicon as loaded molecule and carrier model, respectively, we show that the cargo easily nucleates at the pore surface during the release, forming micro- to macroscopic solid particles at the pores surface. These particles dissolve at a much slower pace, compared to the rate of dissolution of pure beta-carotene in the same solvent, and they negatively affect the reproducibility of the release experiments, possibly because their solubility depends on their size distribution. We propose to exploit this aspect to use release kinetics as a better alternative to the induction time method, and to thereby detect heterogenous nucleation during release experiments. In fact, release dynamics provide much higher sensitivity and reproducibility as they average over the entire sample surface instead of depending on statistical analysis over a small area to find clusters.

Comprehensive Vibrational Spectroscopic Characterization of Nylon-6 Precursors for Precise Tracking of the Beckmann Rearrangement

As a key step in nylon-6 synthesis, the Beckmann rearrangement is an ongoing target of catalytic studies that seek to improve the sustainability of polymer manufacture. Whilst solid-acid catalysts (predominantly zeotypes) have proven effective for this transformation, the development of more active and selective systems demands an understanding of fundamental catalytic mechanisms. In this undertaking, in situ and operando characterization techniques can be informative, provided rigorous spectroscopic groundwork is in place. Thus, to facilitate mechanistic studies we present a detailed investigation of the vibrational spectra of cyclohexanone, cyclohexanone oxime, ϵ-caprolactam and their D10-isotopomers, in the solid state. Variable-temperature infrared (150-300 K) and Raman (10-300 K) spectra are reported alongside inelastic neutron scattering data. Moreover, where key vibrational modes have been assigned with the aid of periodic density functional theory calculations, it has been possible to include hydrogen-bonding interactions explicitly.