The nature of singlet exciton fission in carotenoid aggregates.

Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.This work was supported by the EPSRC (UK) (EP/G060738/ 1), the European Community (LASERLAB-EUROPE, grant agreement no. 284464, EC’s Seventh Framework Programme; and Marie-Curie ITN-SUPERIOR, PITN-GA-2009-238177), and the Winton Programme for the Physics of Sustainability. G.C. acknowledges support by the European Research Council Advanced Grant STRATUS (ERC-2011-AdG No. 291198). J.C. acknowledges support by the Royal Society Dorothy Hodgkin Fellowship and The University of Sheffield’s Vice- Chancellor’s Fellowship scheme.This is the final published version. It was first made available by ACS at http://pubs.acs.org/doi/abs/10.1021/jacs.5b01130

Ultrafast transient absorption spectroscopy: principles and application to photosynthetic systems

The photophysical and photochemical reactions, after light absorption by a photosynthetic pigment–protein complex, are among the fastest events in biology, taking place on timescales ranging from tens of femtoseconds to a few nanoseconds. The advent of ultrafast laser systems that produce pulses with femtosecond duration opened up a new area of research and enabled investigation of these photophysical and photochemical reactions in real time. Here, we provide a basic description of the ultrafast transient absorption technique, the laser and wavelength-conversion equipment, the transient absorption setup, and the collection of transient absorption data. Recent applications of ultrafast transient absorption spectroscopy on systems with increasing degree of complexity, from biomimetic light-harvesting systems to natural light-harvesting antennas, are presented. In particular, we will discuss, in this educational review, how a molecular understanding of the light-harvesting and photoprotective functions of carotenoids in photosynthesis is accomplished through the application of ultrafast transient absorption spectroscopy

Adsorption of human carbonic anhydrase II onto silicon oxides surfaces : The effects of truncation in the N-terminal region

The adsorption of human carbonic anhydrase II pseudo-wild type (HCAIIpwt) and an N-terminally truncated version thereof onto silica surfaces were studied. The amount adsorbed and the adsorption kinetics were measured using in situ ellipsometry. A substantial difference was seen between the two proteins. The adsorbed amount of the truncated version (2.53 mg/m2) indicates an end-on orientation, while the HCAIIpwt seems to adsorb side-on (1.84 mg/m2). It is suggested that the orientation effects arise from the truncation. The truncation is known to unfold the two most N-terminal helical segments, which could inhibit adsorption with the N-terminal region facing the surface, due to steric repulsion

The use of mutant proteins in protein adsorption studies

This paper presents some of the authors work on adsorption of proteins to solid interfaces, using sets of mutant proteins. The benefit of using mutants is that specific well-known changes can be made on the protein. Thus, only one property of the protein might be altered and a series of protein varying gradual in a protein characteristic can be produced. The systems presented in this paper are T4 lysozyme, b-lactoglobulin, and human carbonic anhydrase II

Spatial imaging and evaluation of humectants impact on stratum corneum hydration with confocal Raman microscpectroscopy

Objective: Confocal Raman microspectroscopy (CRM) enables non-invasive depth-scanning of biological tissues. The technique has been used to obtain information about the molecular composition of the skin, tracking of externally applied compounds and to determine molecular concentration profiles. The objective of this study is to use CRM in order to evaluate the changes in stratum corneum hydration when applying polyethylene glycol and the humectants urea and glycerol, and thereby also varying the external chemical potential of water. In the present study we also utilize the advantages of CRM to create novel spatial high- resolution Raman images of stratum corneum. Methodology: Excised porcine skin membranes (500 nm in thickness) were equilibrated from the surface with phosphate buffered saline (PBS) together with different types of humectants using Franz cells. The Raman measurements were performed with a WITec alpha300 system (Ulm, Germany) equipped with a 532 nm laser. The change in stratum corneum hydration after treatment with different humectants was determined from the relative intensity of the water and protein spectra. Raman images were created along a cross section by integrating the Raman intensities for specific vibrational modes. Results and conclusions: The results show that the hydration profiles of stratum corneum correlates well with the gradient in water chemical potential created by the applied humectants, i.e. low molecular weight humectants enable increased hydration of stratum corneum compared to a high molecular weight, non-penetrating polyethylene glycol. In addition the novel results from the Raman imaging experiments illustrates that it is possible to distinguish between water rich domains and the extracellular lipid rich domains along a cross section of the intact skin membrane

Effects of water gradients and use of urea on skin ultrastructure evaluated by confocal Raman microspectroscopy.

The rather thin outermost layer of the mammalian skin, stratum corneum (SC), is a complex biomembrane which separates the water rich inside of the body from the dry outside. The skin surface can be exposed to rather extreme variations in ambient conditions (e.g. water activity, temperature and pH), with potential effects on the barrier function. Increased understanding of how the barrier is affected by such changes is highly relevant for regulation of transdermal uptake of exogenous chemicals. In the present study we investigate the effect of hydration and the use of a well-known humectant, urea, on skin barrier ultrastructure by means of confocal Raman microspectroscopy. We also perform dynamic vapor sorption (DVS) microbalance measurements to examine the water uptake capacity of SC pretreated with urea. Based on novel Raman images, constructed from 2D spectral maps, we can distinguish large water inclusions within the skin membrane exceeding the size of fully hydrated corneocytes. We show that these inclusions contain water with spectral properties similar to that of bulk water. The results furthermore show that the ambient water activity has an important impact on the formation of these water inclusions as well as on the hydration profile across the membrane. Urea significantly increases the water uptake when present in skin, as compared to skin without urea, and it promotes formation of larger water inclusions in the tissue. The results confirm that urea can be used as a humectant to increase skin hydration