M Towrie's research while affiliated with Science and Technology Facilities Council and other places

Combining pulsed laser heating and time-resolved infrared (TR-IR) absorption spectroscopy provides a means of initiating and studying thermally activated chemical reactions and diffusion processes in heterogeneous catalysts on timescales from nanoseconds to seconds. To this end, we investigated single pulse and burst laser heating in zeolite catalysts under realistic conditions using TR-IR spectroscopy. 1 ns, 70 µJ, 2.8 µm laser pulses from a Nd:YAG-pumped optical parametric oscillator were observed to induce temperature-jumps (T-jumps) in zeolite pellets in nanoseconds, with the sample cooling over 1 – 3 ms. By adopting a tightly focused beam geometry, T-jumps as large as 145 °C from the starting temperature were achieved, demonstrated through comparison of the TR-IR spectra with temperature dependent IR absorption spectra and three dimensional heat transfer modelling using realistic experimental parameters. The simulations provide a detailed understanding of the temperature distribution within the sample and its evolution over the cooling period, which we observe to be bi-exponential. These results provide foundations for determining the magnitude of a T-jump in a catalyst/adsorbate system from its absorption spectrum and physical properties, and for applying T-jump TR-IR spectroscopy to the study of reactive chemistry in heterogeneous catalysts.

Time-resolved infrared spectroscopy reveals the flow of electron density through coenzyme B 12 in the light-activated, bacterial transcriptional regulator, CarH.

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor–bridge–acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(ii) trans-acetylide bridge (–C 00000000000000000 00000000000000000 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 01111111111111110 00000000000000000 00000000000000000 00000000000000000 C–Pt–CC–), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand ³IL state to the desired charge-separated ³CSS state. We observe two surprising effects of an increased strength of the acceptor in NDI vs. NAP: a ca. 70-fold slow-down of the ³CSS formation—(971 ps)⁻¹vs. (14 ps)⁻¹, and a longer lifetime of the ³CSS (5.9 vs. 1 ns); these are attributed to differences in the driving force ΔGet, and to distance dependence. The 100-fold increase in the rate of intersystem crossing—to sub-500 fs—by the stronger acceptor highlights the role of delocalisation across the heavy-atom containing bridge in this process. The close proximity of several excited states allows one to control the yield of ³CSS from ∼100% to 0% by solvent polarity. The new DBAs offer a versatile platform for investigating the role of bridge vibrations as a tool to control excited state dynamics.

Time-resolved infrared spectroscopy reveals the flow of electron density through coenzyme B 12 in the light-activated, bacterial transcriptional regulator, CarH. The protein stabilises a series of charge transfer states that result in a photoresponse that avoids reactive, and potentially damaging, radical photoproducts.

Knowledge of protein dynamics is fundamental to the understanding of biological processes, with NMR and 2D-IR spectroscopy being two of the principal methods for studying protein dynamics. Here, we combine these two methods to gain a new understanding of the complex mechanism of a cytokine:receptor interaction. The dynamic nature of many cytokines is now being recognised as a key property in the signalling mechanism. Interleukin-17s (IL-17) are proinflammatory cytokines which, if unregulated, are associated with serious autoimmune diseases such as psoriasis, and although there are several therapeutics on the market for these conditions, small molecule therapeutics remain elusive. Previous studies, exploiting crystallographic methods alone, have been unable to explain the dramatic differences in affinity observed between IL-17 dimers and their receptors, suggesting there are factors that cannot be fully explained by the analysis of static structures alone. Here, we show that the IL-17 family of cytokines have varying degrees of flexibility which directly correlates to their receptor affinities. Small molecule inhibitors of the cytokine:receptor interaction are usually thought to function by either causing steric clashes or structural changes. However, our results, supported by other biophysical methods, provide evidence for an alternate mechanism of inhibition, in which the small molecule rigidifies the protein, causing a reduction in receptor affinity. The results presented here indicate an induced fit model of cytokine:receptor binding, with the more flexible cytokines having a higher affinity. Our approach could be applied to other systems where the inhibition of a protein-protein interaction has proved intractable, for example due to the flat, featureless nature of the interface. Targeting allosteric sites which modulate protein dynamics, opens up new avenues for novel therapeutic development.

Solid, powdered samples are often prepared for infrared (IR) spectroscopy analysis in the form of compressed pellets. The intense scattering of incident light by such samples inhibits applications of more advanced IR spectroscopic techniques, such as two-dimensional (2D)-IR spectroscopy. We describe here an experimental approach that enables the measurement of high-quality 2D-IR spectra from scattering pellets of zeolites, titania, and fumed silica in the OD-stretching region of the spectrum under flowing gas and variable temperature up to ∼500 ◦ C. In addition to known scatter suppression techniques, such as phase cycling and polarization control, we demonstrate how a bright probe laser beam comparable in strength with the pump beam provides effective scatter suppression. The possible nonlinear signals arising from this approach are discussed and shown to be limited in consequence. In the intense focus of 2D-IR laser beams, a free-standing solid pellet may become elevated in temperature compared with its surroundings. The effects of steady state and transient laser heating effects on practical applications are discussed.

Introduction Dynamic processes play important roles in biological function, ranging from the separation of double stranded DNA during transcription and replication to protein conformational changes that constitute part of their biological mechanisms. The ability to observe these dynamic processes in real time would provide not only new insights into biological function but also experimental data for the validation of computational simulations that are used to predict solution phase biomolecular behaviour.

Host-guest complexation is an important supramolecular route to materials. Clear design rules have been developed for complexation in solution. This has proved more challenging for solid-state host-guest co-crystals because they often exhibit polymorphism, leading many researchers to focus instead on bonded frameworks, such as metal-organic frameworks. Here, we report an anthracene-based organic cage (1) that forms isoskeletal host-guest co-crystals with five similarly sized solid organic guests. The co-crystals were designed using inexpensive computational methods to identify appropriate guests that have packing coefficients (PCs) ranging from 44% to 50%, coupled with consideration of the guest shape. By complexing highly emissive BODIPY guests into the host structure, we enhanced its two-photon excited photoluminescent properties by a factor of six. Our crystal design approach was also transferrable to hard-to-design ternary organic crystals that were accessed by inserting specific guests into different sized voids in the host.

Glycine (Gly) is used as a model system to evaluate the ability of ultrafast two-dimensional infrared (2D-IR) spectroscopy to detect and quantify the low-molecular-weight proteinaceous components of blood serum. Combining data acquisition schemes to suppress absorption bands of H2O that overlap with the protein amide I band with analysis of peak patterns appearing in the off-diagonal region of the 2D-IR spectrum allows separation of the Gly spectral signature from that of the dominant protein fraction of serum in a transmission-mode 2D-IR measurement without any sample manipulation, e.g.,filtration or drying. 2D-IR spectra of blood serum samples supplemented with varying concentrations of Gly were obtained, and a range of data analysis methods compared, leading to a detection limit of∼3 mg/mL for Gly. The reported methodology provides a platform for a critical assessment of the sensitivity of 2D-IR for measuring the concentrations of amino acids, peptides, and low-molecular-weight proteins present in serum samples. We conclude that, in the case of several clinically relevant diagnostic molecules and their combinations, the potential exists for 2D-IR to complement IR absorption methods as the benefits of the second frequency dimension offered by 2D-IR spectroscopy outweigh the added technical complexity of the measurement.