Distance makes a difference in crystalline photoluminescence

Crystallization-induced photoluminescence weakening was recently revealed in ultrasmall metal nanoparticles. However, the fundamentals of the phenomenon are not understood yet. By obtaining conformational isomer crystals of gold nanoclusters, we investigate crystallization-induced photoluminescence weakening and reveal that the shortening of interparticle distance decreases photoluminescence, which is further supported by high-pressure photoluminescence experiments. To interpret this, we propose a distance-dependent non-radiative transfer model of excitation electrons and support it with additional theoretical and experimental results. This model can also explain both aggregation-induced quenching and aggregation-induced emission phenomena. This work improves our understanding of aggregated-state photoluminescence, contributes to the concept of conformational isomerism in nanoclusters, and demonstrates the utility of high pressure studies in nanochemistry.

Competitive Inhibition of Cortisol by Prostaglandins at the Ligand Binding Domain of Glucocorticoid Receptors

Cortisol is a steroid hormone that binds to the glucocorticoid receptor with a diverse range of functions including metabolic, neurologic and immune responses. Prostglandins are lipid autacoids that participate in a wide range of functions including the inflammatory response and the vasoactivity of blood vessels and smooth muscle. Previously, a direct interaction between the two classes of biomolecules has not been studied, and here it is shown for the first time that there is competitive inhibition of cortisol and prostaglandins at the critical ligand binding domain of glucocorticoid receptors, which are found in nearly every cell and tissue in the human body. To assess molecular similarity that would enable competitive inhibition, it is first noted that cortisol (CORT, C21H30O5) and prostaglandin E2 (PGE2, C20H32O5) are of nearly the same molecular weight, and have nearly the same number of carbon, hydrogen and oxygen elements. The carbon-carbon double bonds on each of the two branches of PGE2, combined with the five-carbon cyclic centering structure, enable a conformational isomer of PGE2 to position the five oxygen-based functional groups at similar spatial positions as the corresponding five oxygen-based functional groups of CORT. Within the gene-regulating ligand binding domain (LBD) of the glucocorticoid receptor (GR), the average inter-molecular spacing of hydrogen bonding to the corresponding identical five functional groups is 1.778 Å for CORT versus 1.746 Å for PGE2. The conformational isomer of PGE2 within the LBD is at a lower energy state than CORT by 54.9%, indicating preference in stability. Both PGE2 and CORT exhibit a hydrophobic core within the LBD of GR, which is compatible with hydrophobic regions of the LBD. With the incorporation of two calcium ions into both PGE2 and CORT, the energy is further reduced by 58.9% and 113% respectively. PGE2 with calcium ion stabilization is lower by 15.5% in comparison to the equivalent arrangement of CORT. Because of the steroid ring structure of CORT, the spacing of the calcium ions is larger at 16.2Å, whereas PGE2, with just a single ring, collapses to 12.3Å. Analysis of experimental data associated with the adrenal response to an injection of lipopolysaccharide (LPS) is presented to indicate competitive inhibition of PGE2 is responsible for an observed lack of phosphorylation of the GR despite the presence of CORT. Competitive inhibition preventing normative cellular processing of cortisol, a stress hormone, by prostaglandins, which are associated with the inflammatory response, may be the causative source of certain signs and symptoms of disease, and thus is of significant medical importance.

The prenucleosome, a stable conformational isomer of the nucleosome

Chromatin comprises nucleosomes as well as nonnucleosomal histone–DNA particles. Prenucleosomes are rapidly formed histone–DNA particles that can be converted into canonical nucleosomes by a motor protein such as ACF. Here we show that the prenucleosome is a stable conformational isomer of the nucleosome. It consists of a histone octamer associated with ∼80 base pair (bp) of DNA, which is located at a position that corresponds to the central 80 bp of a nucleosome core particle. Monomeric prenucleosomes with free flanking DNA do not spontaneously fold into nucleosomes but can be converted into canonical nucleosomes by an ATP-driven motor protein such as ACF or Chd1. In addition, histone H3K56, which is located at the DNA entry and exit points of a canonical nucleosome, is specifically acetylated by p300 in prenucleosomes relative to nucleosomes. Prenucleosomes assembled in vitro exhibit properties that are strikingly similar to those of nonnucleosomal histone–DNA particles in the upstream region of active promoters in vivo. These findings suggest that the prenucleosome, the only known stable conformational isomer of the nucleosome, is related to nonnucleosomal histone–DNA species in the cell.

UV photoelectron spectroscopic and computational study of 7-substituted cycloheptatrienes

The He(I) photoelectron (PE) spectra of cycloheptatriene (1), 7-methylcycloheptatriene (2), 7-methoxycycloheptatriene (3), 7-methylthiocycloheptatriene (4), and 7-diemthylaminocycloheptatriene (5) were recorded and interpreted using MO energies and ionization potentials acquired from B3PW91 calculations. Partial simulated PE spectra were in good agreement with the experimental results. The axial and equatorial conformers have distinct PE spectra as illustrated by simulation. The PE spectra of 2, 3, and 5 are representative of the equatorial conformers, while the PE spectrum of 4 was in accord with a 1:1 mixture of equatorial and axial conformational isomers based on spectral simulation. The calculated free energy differences between the equatorial and axial conformers are in the order of 2 kcal mol–1 (1 cal = 4.184 J) for compounds 2, 3, and 5, where the equatorial conformational isomer is lowest in energy. In the case of 4, the equatorial conformer was only marginally lower in energy than its axial counterpart, where the free energy difference is 0.1 kcal mol–1.Key words: 7-substituted cycloheptatrienes, He(I) photoelectron spectroscopy, B3PW91, spectral simulation.

On the Structure of Tri- and Tetrahydroxolead(II) Complex Anions

Optimal geometries and corresponding electronic structures of various [Pb(OH)3]- and [Pb(OH)4]2- conformational isomers are investigated by the B3LYP and MP2 treatments. Unlike highly symmetric [Pb(OH)3]- structure (C3 symmetry), the most stable [Pb(OH)4]2- conformational isomer has only C2 symmetry. Hydrogen bonds exhibit a lower influence on the stereochemistry of lead(II) hydroxocomplexes in comparison with the steric effect of the Pb(II) lone electron pair. The picture of the Pb(II) lone electron pair cannot explain the lowered symmetry of isolated [Pb(OH)4]2- complexes with four equivalent hydrogen bonds.