The electronic coupling matrix element HAB is an essential ingredient of most electron-transfer theories. HAB depends on the overlap between donor and acceptor wave functions and is affected by the involved states' spin. We classify the spin-state effects into three categories orbital occupation, spin-dependent electron density, and density delocalization. The orbital occupancy reflects the diverse chemical nature and reactivity of the spin states of interest. The effect of spin-dependent density is related to a more compact electron density cloud at lower spin states due to decreased exchange interactions between electrons. Density delocalization is strongly connected with the covalency concept that increases the spatial extent of the diabatic state's electron density in specific directions. We illustrate these effects with high-level ab initio calculations on model direct donor-acceptor systems relevant to metal oxide materials and biological electron transfer. Obtained results can be used to benchmark existing methods for HAB calculations in complicated cases such as spin-crossover materials or antiferromagnetically coupled systems.Ruthenium carbenes, famously used in olefin metathesis, have impacted numerous research areas, ranging from synthesis to materials and biology. Although in the same group as ruthenium, iron carbenes showing similar reaction patterns have not been reported. Such targets are of high interest because the use of a sustainable metal would lead to a decreased cost, toxicity, and environmental impact of the corresponding compounds. Herein, we report the synthesis of an iron carbene complex, [PC(sp2)PFe(N2)(PMe3)] ([PC(sp2)P] = (bis[2-(di-isopropylphosphino)phenyl]methylene), which is capable of performing [2+2] cycloaddition reactions in the presence of alkynes. Specifically, η3-vinyl carbenes are formed stoichiometrically through a [2+2] cycloaddition between the alkyne and the metal carbene. Additional reactivity of the η3-vinyl carbenes with alkynes yields a second insertion product containing a new iron carbene moiety.The tandem process of phenol addition to a cyclic α,β-unsaturated ester followed by intramolecular transesterification and [1,5] sigmatropic rearrangement affords a series of helical coumarins based upon a previously unknown 3-amino-7-hydroxybenzo[3,4]cyclohepta[1,2-c]chromen-6-one core. These novel polarized coumarins, possessing a β-ketoester moiety, have been employed to synthesize more rigid and helical coumarin-pyrazolones, which display green fluorescence. The enhanced emission of coumarin-pyrazolones in polar solvents depends on the nature of the S1 state. The coumarin-pyrazolones are predicted to have two vertical states close in energy a weakly absorbing S1 (1LE) followed by a bright S2 state (1CT). In polar solvents, the 1CT can be stabilized below the 1LE and may become the fluorescent state. Solvatochromism of the fluorescence spectra confirms this theoretical prediction. The presence of an N-H···O═C intramolecular hydrogen bond in these coumarin-pyrazolone hybrids facilitates excited-state intramolecular proton transfer (ESIPT). This process leads to a barrierless conical intersection with the ground electronic state and opens a radiationless deactivation channel effectively competing with fluorescence. Solvent stabilization of the CT state increases the barrier for ESIPT and decreases the efficiency of the nonradiative channel. This results in the observed correlation between solvatochromism and an increase of fluorescence intensity in polar solvents.Recent calculations using coupled cluster on solids have raised the discussion of using a N-1/3 power law to fit the correlation energy when extrapolating to the thermodynamic limit, an approach which differs from the more commonly used N-1 power law, which is, for example, often used by quantum Monte Carlo methods. In this paper, we present one way to reconcile these viewpoints. Coupled cluster doubles calculations were performed on uniform electron gases reaching system sizes of 922 electrons for an extremely wide range of densities (0.1 less then rs less then 100.0) to study how the correlation energy approaches the thermodynamic limit. The data were corrected for the basis set incompleteness error and use a selected twist angle approach to mitigate the finite size error from shell filling effects. Analyzing these data, we initially find that a power law of N-1/3 appears to fit the data better than a N-1 power law in the large system size limit. selleck chemicals However, we provide an analysis of the transition structure factor showing that N-1 still applies to large system sizes and that the apparent N-1/3 power law occurs only at low N.The folding of triple-helical collagen, the most abundant protein in nature, relies on the nucleation and propagation along the strands. Hydrophobic moieties are crucial for the folding and stability of numerous proteins. Instead, nature uses for collagen a trimerization domain and cis-trans prolyl isomerases to facilitate and accelerate triple helix formation. Yet, pendant hydrophobic moieties endow triple-helical collagen with hyperstability and accelerate the cis-trans isomerization to an extent that thermally induced unfolding and folding of collagen triple helices take place at the same speed. Here, we systematically explored the effect of pendant fatty acids on the folding and stability of collagen triple helices. Thermal denaturation and kinetic studies with a series of collagen mimetic peptides (CMPs) bearing saturated and unsaturated fatty acids with different lengths revealed that longer and more flexible fatty acid appendages increase the stability and the folding rate of collagen triple helices. Molecular dynamics simulations combined with experimental data indicate that the hydrophobic appendages stabilize the triple helix by interaction with the grooves of the collagen triple helix and accelerate the folding and unfolding process by creating a molten globule-like intermediate.Medicinal chemists often bias toward working with scaffolds with which previously they have had direct experience and successes. In this way, it is often the case that scaffolds which have proven tractable within a research group are "reused" across multiple and sometimes unrelated drug targets. With this concept in mind, we designed a new computer algorithm AUTOSTERE which could systematically assess the opportunities to replace any part of any molecule within an entire database of known ligand structures with a target scaffold and automatically evaluate the potential designs in the context of the original ligand's protein environment. As such, it performs scaffold replacement on an unprecedented scale and suggests new target opportunities for preferred chemistries rather than the conventional reverse situation. The results of this approach for one scaffold, a substituted triazolinone, applied to a set of 10 426 ligand conformations extracted from the PDB are described. This led to the identification of ∼600 novel ligands incorporating the triazolinone scaffolds in complex with their predicted drug targets.