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Expanding Chemical Space in the Synthesis of Gold Bipyramids
No full textGold bipyramids (AuBPs), despite having superior properties compared to their spectroscopically similar counterparts, gold nanorods, have found comparatively limited applications. This discrepancy is primarily due to the lack of protocols to tailor their dimensions. Typically, concentration of Au seeds is virtually the sole factor that determines aspect ratio and thus, optical properties of AuBPs. As a result, varying the volumes of AuBPs while incurring minimal changes to their optical spectra remains a synthetically non-trivial task. Here, we expand the chemical space in the seeded growth of AuBPs, by exploiting the interplay between bromide, silver ions, and seed concentration for tuning the final dimensions and optical properties of AuBPs. Specifically, we achieved a 6-fold change in volumes of AuBPs while maintaining the fixed plasmon band position. Further overgrowth of as-prepared bipyramids broadens the realizable dimensions without compromising quality and initial morphology. Overall, our results expand the chemical toolbox in the wet-chemistry synthesis of anisotropic gold nanoparticles, which is relevant for health, colorimetric sensors, and energy applications
Cooling-Induced Order-Disorder Phase Transition in CsPbBr3 Nanocrystal Superlattices
No full textPerovskite nanocrystal superlattices are being actively studied after reports have emerged on collective excitonic properties at cryogenic temperatures, where energetic disorder is minimized due to the frozen lattice vibrations. However, an important issue related to structural disorder of superlattices at low temperatures has received little attention to date. In this work, we show that CsPbBr3 nanocrystal superlattices undergo a reversible order-disorder transition upon cooling to 90 K. The transition consists of the loss of structural coherence, i.e. increased nanocrystal misalignment, and contraction of the superlattices, as revealed by temperature-dependent X-ray diffraction, and is ascribed to the solidification of ligands (on the basis of Raman spectroscopy). Introducing shorter amines on the nanocrystal surface allows to mitigate these changes, improve order, and shorten interparticle distance. We demonstrate that the low temperature phase of the short ligand-capped nanocrystal superlattices is characterized by a strong exciton migration observable in the photoluminescence decay, which is due to the shrinkage of the inter-nanocrystal distance
Atomistic Simulations Reveal Crucial Role of Metal Ions for Ligand Binding in Gd-I Riboswitch
No full textRiboswitches are structured RNA segments that act as specific sensors for small molecules in bacterial metabolism. Due to the flexi- ble nature of these highly charged macromolecules, molecular dynamics simulations are instrumental to investigating the mechanistic details of their regulatory function. In the present study, a guanidinium sensing riboswitch (the Gd-I riboswitch) serves as example how atomistic simulations can shed light on the role of ions on structure and dynamics of the RNA and on ligand binding. Rely- ing on two crystal structures from orthologous forms from different bacterial species, it is demonstrated how the ion setup crucially determines whether the simulation yields meaningful insights into the conformational stability of the RNA, functionally relevant residues and RNA-ligand interactions. Ion setup in this context includes the free ions in solution and ions associated directly with the RNA, in particular a triad of 2 Mg2+ ions and a K+ ion in close proximity to the guanidinium binding site. A detailed investi- gation of the binding pocket reveals that K+ from the ion triad plays a decisive role in stabilizing the ligand binding by stabilizing important localized interactions which in turn contribute to the overall shape of the folded state of the RNA
Structures, energies and vibrational frequencies of the X and A states of haloacetylene cations, HCCX+ (X = F, Cl, Br, I)
No full textModeling charge migration resulting from the coherent superposition of cation ground and excited states requires information about the potential energy surfaces of the relevant cation states. Since these states are often of the same electronic symmetry as the ground state of the cation, conventional single reference methods such as coupled cluster cannot be used for the excited states. The EOMCCSD-IP (equation of motion coupled cluster with single and double excitations and ionization) is a convenient and reliable “black-box” method that can be used for the ground and excited states of cations, yielding results of CCSD (coupled cluster with singles and double excitation) quality. Charge migration in haloacetylene cations arises from the superposition of the X and A states of HCCX+ (X = F, Cl, Br and I). The geometries, ionization potentials and vibrational frequencies have been calculated by CCSD/cc-pVTZ for neutral HCCX and the X state of HCCX+ and by EOM CCSD-IP/cc-pVTZ for the X and A states of HCCX+. The results agree very well with each other and with experiment. The very good agreement between CCSD and EOMCCSD-IP for the X states demonstrates that EOMCCSD-IP is a suitable method for calculating the structure and properties of ground and excited states for the HCCX cations
Highly Tolerant Living/Controlled Anionic Polymerization of Dialkyl Acrylamides Enabled by Zinc Triflate/Phosphine Lewis Pair
No full textLiving polymerizations of polar vinyl monomers have been successful for decades. However, they still suffer the following challenges: fast propagation, air/moisture tolerance, and negligible side reactions even at elevated temperatures. Here, we developed an unprecedented polymerization that overcomes these limitations using a Lewis pair catalyst. The anionic polymerization of dialkyl acrylamides proceeded in a living/controlled matter using Zn(OTf)2/PPh3 within a wide temperature range of 25–100 °C for short times (1–10 min) even under open-air conditions. The recovery and reuse of Zn(OTf)2 without loss of polymerization activity were observed to be possible. The polymerization was retarded by excess Zn(OTf)2, additive methanol, and water, indicating equilibriums of the propagating species with them. The putative propagating zinc triflate-ate complex was tolerant to the protic additives and significantly selective for the propagation
Prefix and postfix versions of the SMILE chemical notation
No full textThe SMILE notation has become an open standard for the specification of the structure of chemical compounds. It is most suitable for computer processing of chemical data. In this work, two variants of this notation are introduced, namely the prefix and postfix versions. They are parenthesis-free notations that are designed to represent trees
Tellurium Empowered Catalysis for Enantioselective Seleno-Michael Addition Reaction
No full textThe development of a new catalyst, which has not been explored before, not only provides an alternative to
the existing organocatalysts but also leads to a distinct chemical reactivity. Here, an organotellurium catalyst consisting of a chiral quinine auxiliary displays remarkable activity for the enantioselective delivery of reactive arylselenols as a Michael donor in common solvents such as dichloromethane and acetonitrile. The developed chiral organotellurium-catalyzed arylselenol addition to alkenes shows a broad substrate scope as electron-rich and deficient arylselenols and diversely substituted enones are amenable to the reaction conditions. Control experiments, 77Se, and 125Te NMR, suggest that non-bonded Te...Se interaction between catalyst and arylselenol seems responsible for accelerated and highly enantioselective delivery of arylselenol to enone. The practically synthesized chiral organoselenides have also been readily post-derivatized by taking advantage of the leaving group ability and recyclability of arylselenium
Pharmacokinetics-Pharmacodynamics (PK-PD) Modeling: A translational path to brain-targeted drug delivery
No full textPharmacokinetic-pharmacodynamics (PK/PD) modeling has been recognized as an essential tool for the planning and execution of clinical pharmacology investigations throughout the formulation development process. The utilization of PK/PD modeling to optimize dosage recommendations and therapeutic medication monitoring is on the rise, and PK/PD model-based dose individualization is going to play a crucial role in personalized medicine. In recent years, higher costs and low productivity in drug development have got a lot of attention. Fewer than 10% of innovative medications that survive clinical trials are commercialized, while many more fail in preclinical research. Based on preclinical and clinical data, the FDA now defines model-based drug development as a key tool in the assessment of therapeutic efficacy and safety of medications. As successful drug delivery to central nervous system (CNS) diseases has been a tough challenge, effective use of PK/PD models can permit merging data from clinical trials with quantified exposure and provide highly convincing explanations for disparities in the results of distinct investigations. To improve CNS therapies and drug development, details of inter-species and inter-condition variations are needed to enable target site pharmacokinetics and associated CNS effects between disease states. The present review covers the potential of PK/PD modeling in the current scenario of drug development with a special emphasis on CNS-based targeted delivery. Additionally, challenges associated with the field have also been addressed
O2 Activation and Enzymatic C-H Bond Activation Mediated by a Dimanganese Cofactor
No full textDioxygen (O2) is a potent oxidant used by aerobic organisms for energy transduction and critical biosynthetic processes. Numerous metallocofactors, which most commonly feature iron or copper ions, harness O2 to mediate C-H bond hydroxylation reactions. In contrast, most manganese-dependent enzymes are redox-inert and infrequently activate O2 or C-H bonds. Here we report that the dimanganese-metalated form of the cambialistic monooxygenase SfbO (Mn2-SfbO) can efficiently mediate enzymatic C-H bond hydroxylation. Kinetic, spectroscopic and structural studies invoke a mixed-valent dimanganese cofactor (Mn2II/III) in catalytic O2 activation. Access to this redox form requires stoichiometric superoxide to mature a Mn2II cofactor to higher-valent forms that participate in catalysis. These findings establish the viability of proteinaceous dimanganese cofactors in mediating complex, multistep redox transformations
Distortion/Interaction Analysis via Machine Learning
No full textMachine learning (ML) models have provided a highly efficient pathway to quantum mechanical accurate reaction barrier predictions. Previous approaches have, however, stopped at prediction of these barriers instead of developing predictive capabilities in reactivity analysis tasks such as distortion/interaction-activation strain analysis. Such methods can provide insight into reactivity trends and ultimately guide rational reaction design. In this work we present the novel application of ML to the rapid and accurate prediction of distortion and interaction DFT energies across four datasets (three existing and one new dataset). We also show how our models can accurately predict on unseen, high impact literature examples where DFT-level distortion/interaction analysis has previously been used to explain reactivity trends for cycloadditions. This work thus provides support for ML to be utilised further in reactivity analysis across different reaction classes at a fraction of the cost of traditional methods such as DFT