Full characterization of vibrational coherence in a porphyrin chromophore by two-dimensional electronic spectroscopy

In this work we present experimental and calculated two-dimensional electronic spectra for a 5,15-bisalkynyl porphyrin chromophore. The lowest energy electronic Qy transition couples mainly to a single 380 cm–1 vibrational mode. The two-dimensional electronic spectra reveal diagonal and cross peaks which oscillate as a function of population time. We analyze both the amplitude and phase distribution of this main vibronic transition as a function of excitation and detection frequencies. Even though Feynman diagrams provide a good indication of where the amplitude of the oscillating components are located in the excitation-detection plane, other factors also affect this distribution. Specifically, the oscillation corresponding to each Feynman diagram is expected to have a phase that is a function of excitation and detection frequencies. Therefore, the overall phase of the experimentally observed oscillation will reflect this phase dependence. Another consequence is that the overall oscillation amplitude can show interference patterns resulting from overlapping contributions from neighboring Feynman diagrams. These observations are consistently reproduced through simulations based on third order perturbation theory coupled to a spectral density described by a Brownian oscillator model

Computer-designed repurposing of chemical wastes into drugs

As the chemical industry continues to produce considerable quantities of waste chemicals1,2, it is essential to devise ‘circular chemistry’3,4,5,6,7,8 schemes to productively back-convert at least a portion of these unwanted materials into useful products. Despite substantial progress in the degradation of some classes of harmful chemicals9, work on ‘closing the circle’—transforming waste substrates into valuable products—remains fragmented and focused on well known areas10,11,12,13,14,15. Comprehensive analyses of which valuable products are synthesizable from diverse chemical wastes are difficult because even small sets of waste substrates can, within few steps, generate millions of putative products, each synthesizable by multiple routes forming densely connected networks. Tracing all such syntheses and selecting those that also meet criteria of process and ‘green’ chemistries is, arguably, beyond the cognition of human chemists. Here we show how computers equipped with broad synthetic knowledge can help address this challenge. Using the forward-synthesis Allchemy platform16, we generate giant synthetic networks emanating from approximately 200 waste chemicals recycled on commercial scales, retrieve from these networks tens of thousands of routes leading to approximately 300 important drugs and agrochemicals, and algorithmically rank these syntheses according to the accepted metrics of sustainable chemistry17,18,19. Several of these routes we validate by experiment, including an industrially realistic demonstration on a ‘pharmacy on demand’ flow-chemistry platform20. Wide adoption of computerized waste-to-valuable algorithms can accelerate productive reuse of chemicals that would otherwise incur storage or disposal costs, or even pose environmental hazards

An Integrated Biorefinery Concept for Conversion of Sugar Beet Pulp into Value-added Chemicals and Pharmaceutical Intermediates

Over 8 million tonnes of sugar beet are grown annually in the UK. Sugar beet pulp (SBP) is the main by-product of sugar beet processing which is currently dried and sold as a low value animal feed. SBP is a rich source of carbohydrates, mainly in the form of cellulose and pectin, including D-glucose (Glu), L-arabinose (Ara) and D-galacturonic acid (GalAc). This work describes the technical feasibility of an integrated biorefinery concept for fractionation of SBP and conversion of these monosaccharides into value-added products. SBP fractionation is initially carried out by steam explosion under mild conditions to yield soluble pectin and insoluble cellulose fractions. The cellulose is readily hydrolysed by cellulases to release Glu that can then be fermented by a commercial Yeast strain to produce bioethanol with a high yield. The pectin fraction can be either fully hydrolysed, using physico-chemical methods, or selectively hydrolysed, using cloned arabinases and galacturonases, to yield Ara-rich and GalAc-rich streams. These monomers can be separated using either Centrifugal Partition Chromatography (CPC) or ultrafiltration into streams suitable for subsequent enzymatic upgrading. Building on our previous experience with transketolase (TK) and transaminase (TAm) enzymes, the conversion of Ara and GalAc into higher value products was explored. In particular the conversion of Ara into L-gluco-heptulose (GluHep), that has potential therapeutic applications in hypoglycaemia and cancer, using a mutant TK is described. Preliminary studies with TAm also suggest GluHep can be selectively aminated to the corresponding chiral aminopolyol. Current work is addressing upgrading of the remaining SBP monomer, GalAc, and modelling of the biorefinery concept to enable economic and Life Cycle Analysis (LCA)

The impact of recent developments in technologies which enable the increased use of biocatalysts

While biocatalytic transformations are very powerful in enantioselective synthesis, frequently occurring under mild conditions, and proceed with extraordinary selectivity, there are practical challenges associated with the use of biocatalysis, such as limited substrate scope, stability, and reusability. Recent technological developments, for example immobilization, continuous flow, and molecular biology, all contribute towards enhancing the use of enzymes in synthesis

Synthesis and linear and nonlinear optical properties of low-melting pi-extended porphyrins

A large and diverse library of trans-A2B2 and A 2BC-porphyrins possessing two arylethynyl substituents at the meso positions has been efficiently synthesized and tested for their two-photon absorption (2PA) behavior. All compounds fall into three general types A-π-A, D-π-D or D-π-A, where A is an electron-acceptor and D is an electron-donor moiety. These porphyrins contain two polyalkoxyaryl substituents, resulting in very low melting points (typically 110-125 °C) and superb solubility in non-polar solvents. Some of these porphyrins exhibit two different crystal phases in addition to an isotropic liquid state. Their linear and nonlinear optical properties were thoroughly elucidated and analyzed. π-Extended porphyrins emit light in the NIR and have moderate triplet state lifetimes. The increase of 2PA cross-section in the Soret region for porphyrins bearing strong electron-donating groups has been attributed to resonance enhancement (caused by intensification, redshift and broadening of the lowest Q-band) of gerade-gerade transition. The combination of high two-photon absorption cross-sections (>500 GM) and low melting points makes them perfect candidates for nonlinear optical materials in the 600-900 nm range. © 2013 The Royal Society of Chemistry