Structure and Dynamics of the Lowest Triplet State in p-Benzoquinone I. An isotope effect study on the optical absorption, emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone-h4, -dh3, 2,6-d2h2, d4, and -CH3 in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B1g (nπ*) singlet-triplet transition in p-benzoquinone-h4 (PBQ-h4) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm-1 higher in energy in the pure PBQ-h4 crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration. These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ-h4 and the applicability of this model is critically examined. Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ-h4 in PBQ-d4 show hydrogen (deuterium) bonding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bonding effects also induce the electronic origin of the B1g (nπ*) triplet state in case of a translational inequivalent dimer. A detailed vibrational analysis of the phosphorescence spectrum of PBQ-h4 in a PBQ-d4 host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B1g (nπ*) triplet state of PBQ-h4 is located at 18609 ± 1 cm-1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm-1 in this mixed crystal system. An isotope effect study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent. The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ-h4 is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZFS parameters, induced by localization of the nπ* excitation on oxygen. Finally the dynamics of energy migration in the dilute PBQ-h4 in PBQ-d4 isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system

Structure and Dynamics of the Lowest Triplet State in p-Benzoquinone II. An optically detected EPR and proton ENDOR study

Optically detected high field EPR and proton ENDOR experiments on the zero-point (g-inversion) level of the lowest triplet state in p-benzoquinone-h4 as guest in p-benzoquinone-d4 are reported. The results are used to obtain the parameters of the molecular spin hamiltonian and the following results are obtained at 1.8 K: The fine-structure principal values are X = -798.2 ± 0.6 MHz, Y = -569.1 ± 0.6 MHz, and Z = 1367.3 ± 0.6 MHz; while the molecular g-values are found to be gxx = 2.0045 ± 0.0005, gyy = 2.0035 ± 0.0005, and gzz = 2.00994 ± 0.00008. The principal z axes of both the fine-structure and g-tensor are found to coincide with the carbonyl bond direction of the p-benzoquinone ground state structure. The orientation of the x and y axes of the fine-structure and proton hypefine-structure tensor however indicates that the molecule in the excited state is distorted into a centrosymmetric chair-form, which also implies that the symmetry of this state is only Ag. The 1/3 trace of the proton hyperfine interaction tensor is found to be 9.6 MHz and from this we calculate that 1.8% of the unpaired n-electron density is at each proton. We further report the effect of mono deuteration on the fine-structure and g-tensor principal values and these data are used to calculate the fine-structure parameter Z of the u-inversion level (20 cm-1 above the zero-paint level) in the triplet state of p-benzoquinone-h4. This method yields Z = 5.8 GHz which is to be compared with the value 6 ± 1.3 GHz obtained from the temperature dependence of the corresponding fine-structure parameter of the zero-point level. Finally, a new effect - the observation of radio-frequency induced changes in the phosphorescence cross relaxation signal - is reported and used to identify the election spin state, in which the (ENDOR) proton spin flips take place

Sculpting with stem cells: how models of embryo development take shape

During embryogenesis, organisms acquire their shape given boundary conditions that impose geometrical, mechanical and biochemical constraints. A detailed integrative understanding how these morphogenetic information modules pattern and shape the mammalian embryo is still lacking, mostly owing to the inaccessibility of the embryo in vivo for direct observation and manipulation. These impediments are circumvented by the developmental engineering of embryo-like structures (stembryos) from pluripotent stem cells that are easy to access, track, manipulate and scale. Here, we explain how unlocking distinct levels of embryo-like architecture through controlled modulations of the cellular environment enables the identification of minimal sets of mechanical and biochemical inputs necessary to pattern and shape the mammalian embryo. We detail how this can be complemented with precise measurements and manipulations of tissue biochemistry, mechanics and geometry across spatial and temporal scales to provide insights into the mechanochemical feedback loops governing embryo morphogenesis. Finally, we discuss how, even in the absence of active manipulations, stembryos display intrinsic phenotypic variability that can be leveraged to define the constraints that ensure reproducible morphogenesis in vivo

Hypoxia induces a transcriptional early primitive streak signature in pluripotent cells enhancing spontaneous elongation and lineage representation in gastruloids

The cellular microenvironment, together with intrinsic regulators, shapes stem cell identity and differentiation capacity. Mammalian early embryos are exposed to hypoxia in vivo and appear to benefit from hypoxic culture in vitro. Yet, how hypoxia influences stem cell transcriptional networks and lineage choices remain poorly understood. Here, we investigated the molecular effects of acute and prolonged hypoxia on embryonic and extra-embryonic stem cells as well as the functional impact on differentiation potential. We find a temporal and cell type-specific transcriptional response including an early primitive streak signature in hypoxic embryonic stem cells mediated by HIF1α. Using a 3D gastruloid differentiation model, we show that hypoxia-induced T expression enables symmetry breaking and axial elongation in the absence of exogenous WNT activation. When combined with exogenous WNT activation, hypoxia enhances lineage representation in gastruloids, as demonstrated by highly enriched signatures of gut endoderm, notochord, neuromesodermal progenitors and somites. Our findings directly link the microenvironment to stem cell function and provide a rationale supportive of applying physiological conditions in models of embryo development

Detailed analysis of the genetic and epigenetic signatures of iPSC-derived mesodiencephalic dopaminergic neurons.

Induced pluripotent stem cells (iPSCs) hold great promise for in vitro generation of disease-relevant cell types, such as mesodiencephalic dopaminergic (mdDA) neurons involved in Parkinson's disease. Although iPSC-derived midbrain DA neurons have been generated, detailed genetic and epigenetic characterizations of such neurons are lacking. The goal of this study was to examine the authenticity of iPSC-derived DA neurons obtained by established protocols. We FACS purified mdDA (Pitx3 (Gfp/+) ) neurons derived from mouse iPSCs and primary mdDA (Pitx3 (Gfp/+) ) neurons to analyze and compare their genetic and epigenetic features. Although iPSC-derived DA neurons largely adopted characteristics of their in vivo counterparts, relevant deviations in global gene expression and DNA methylation were found. Hypermethylated genes, mainly involved in neurodevelopment and basic neuronal functions, consequently showed reduced expression levels. Such abnormalities should be addressed because they might affect unambiguous long-term functionality and hamper the potential of iPSC-derived DA neurons for in vitro disease modeling or cell-based therapy

2017 Research & Innovation Day Program

A one day showcase of applied research, social innovation, scholarship projects and activities.https://first.fanshawec.ca/cri_cripublications/1004/thumbnail.jp

Factor Group Splitting in the Lowest Triplet State of p-Benzoquinone-d4 Crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d4 single crystals at 2 K show the factor group splitting in the origin of the lowest B1g (nπ*) triplet state at 18649 cm-1 to be 0.62±0.06 cm-1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group splitting in the 3Au (nπ*) state at 12.1 cm-1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be -10±3 GHz.