Dynamic Phosphorylation of the C Terminus of Hsp70 Regulates the Mitochondrial Import of SOD2 and Redox Balance.

The import of superoxide dismutase-2 (SOD2) into mitochondria is vital for the survival of eukaryotic cells. SOD2 is encoded within the nuclear genome and translocated into mitochondria for activation after translation in the cytosol. The molecular chaperone Hsp70 modulates SOD2 activity by promoting import of SOD2 into mitochondria. In turn, the activity of Hsp70 is controlled by co-chaperones, particularly CHIP, which directs Hsp70-bound proteins for degradation in the proteasomes. We investigated the mechanisms controlling the activity of SOD2 to signal activation and maintain mitochondrial redox balance. We demonstrate that Akt1 binds to and phosphorylates the C terminus of Hsp70 on Serine631, which inhibits CHIP-mediated SOD2 degradation thereby stabilizing and promoting SOD2 import. Conversely, increased mitochondrial-H2O2 formation disrupts Akt1-mediated phosphorylation of Hsp70, and non-phosphorylatable Hsp70 mutants decrease SOD2 import, resulting in mitochondrial oxidative stress. Our findings identify Hsp70 phosphorylation as a physiological mechanism essential for regulation of mitochondrial redox balance.

An Electron-Rich Calix[4]arene-Based Receptor with Unprecedented Binding Affinity for Nitric Oxide.

Calixarenes have found widespread application as building blocks for the design and synthesis of functional materials in host-guest chemistry. The ongoing desire to develop a detailed understanding of the nature of NO bonding to multichromophoric π-stacked assemblies led us to develop an electron-rich methoxy derivative of calix[4]arene (3), which we show exists as a single conformer in solution at ambient temperature. Here, we examine the redox properties of this derivative, generate its cation radical (3+. ) using robust chemical oxidants, and determine the relative efficacy of its NO binding in comparison with model calixarenes. We find that 3/3+. is a remarkable receptor for NO+ /NO, with unprecedented binding efficacy. The availability of precise experimental structures of this calixarene derivative and its NO complex, obtained by X-ray crystallography, is critically important both for developing novel functional NO biosensors, and understanding the role of stacked aromatic donors in efficient NO binding, which may have relevance to biological NO transport.

Towards the rational design of novel charge-transfer materials: biaryls with a dihedral angle-independent hole delocalization mechanism.

Biaryl cation radicals are important electroactive materials, which show two mechanisms of hole delocalization: static delocalization at small interplanar dihedral angles and dynamic hopping at larger angles, reflecting the interplay between electronic coupling and structural reorganization. Herein, we describe the rational design of biaryls possessing an invariant hole delocalization mechanism.

Modeling of S-Nitrosothiol-Thiol Reactions of Biological Significance: HNO Production by S-Thiolation Requires a Proton Shuttle and Stabilization of Polar Intermediates.

Nitroxyl (HNO), a reduced form of the important gasotransmitter nitric oxide, exhibits its own unique biological activity. A possible biological pathway of HNO formation is the S-thiolation reaction between thiols and S-nitrosothiols (RSNOs). Our density functional theory (DFT) calculations suggested that S-thiolation proceeds through a proton transfer from the thiol to the RSNO nitrogen atom, which increases electrophilicity of the RSNO sulfur, followed by nucleophilic attack by thiol, yielding a charge-separated zwitterionic intermediate structure RSS+ (R)N(H)O- (Zi), which decomposes to yield HNO and disulfide RSSR. In the gas phase, the proton transfer and the S-S bond formation are asynchronous, resulting in a high activation barrier (>40 kcal mol-1 ), making the reaction infeasible. However, the barrier can decrease below the S-N bond dissociation energy in RSNOs (≈30 kcal mol-1 ) upon transition into an aqueous environment that stabilizes Zi and provides a proton shuttle to synchronize the proton transfer and the S-S bond formation. These mechanistic features suggest that S-thiolation can easily lend itself to enzymatic catalysis and thus can be a possible route of endogenous HNO production.

Metallic-like bonding in plasma-born silicon nanocrystals for nanoscale bandgap engineering.

Based on ab initio molecular dynamics simulations, we show that small nanoclusters of about 1 nm size spontaneously generated in a low-temperature silane plasma do not possess tetrahedral structures, but are ultrastable. Apparently small differences in the cluster structure result in substantial modifications in their electric, magnetic, and optical properties, without the need for any dopants. Their non-tetrahedral geometries notably lead to electron deficient bonds that introduce efficient electron delocalization that strongly resembles the one of a homogeneous electron gas leading to metallic-like bonding within a semiconductor nanocrystal. As a result, pure hydrogenated silicon clusters that form by self-assembly in a plasma reactor possess optical gaps covering most of the solar spectrum from 1.0 eV to 5.2 eV depending simply on their structure and, in turn, on their degree of electron delocalization. This feature makes them ideal candidates for future bandgap engineering not only for photovoltaics, but also for many nano-electronic devices employing nothing else but silicon and hydrogen atoms.

A deeper insight into strain for the sila-bi[6]prismane ( Si18H12) cluster with its endohedrally trapped silicon atom, Si19H12.

A new family of over-coordinated hydrogenated silicon nanoclusters with outstanding optical and mechanical properties has recently been proposed. For one member of this family, namely the highly symmetric Si19 H12 nanocrystal, strain calculations have been presented with the goal to question its thermal stability and the underlying mechanism of ultrastability and electron-deficiency aromaticity. Here, the invalidity of these strain energy (SE) calculations is demonstrated mainly based on a fundamentally wrong usage of homodesmotic reactions, the miscounting of atomic bonds, and arithmetic errors. Since the article in question is entirely anchored on those erroneous SE values, all of its conclusions and predictions become without meaning. We provide evidence here that the nanocrystal in question suffers from such low levels of strain that its thermodynamical stability should be largely sufficient for device fabrication in a realistic plasma reactor. Most remarkably, the two "alternative," irregular isomers explicitly proposed in the aforementioned article are also electron-deficient, nontetrahedral, ultrastable, and aromatic nicely underlining the universality of the ultrastability concept for nanometric hydrogenated silicon clusters.

On the possible biological relevance of HSNO isomers: a computational investigation.

Thionitrous acid (HSNO), the smallest S-nitrosothiol, has been identified as a potential biologically active molecule that connects the biochemistries of two important gasotransmitters, nitric oxide (NO) and hydrogen sulfide (H2S). Here, we computationally explore possible isomerization reactions of HSNO that may occur under physiological conditions using high-level coupled-cluster as well as density functional theory and composite CBS-QB3 methodology calculations. Gas-phase calculations show that the formation of the tautomeric form HONS and the Y-isomer SN(H)O is thermodynamically feasible, as they are energetically close, within ∼6 kcal mol(-1), to HSNO, while the recently proposed three-membered ring isomer is not thermodynamically or kinetically accessible. The gas-phase intramolecular proton-transfer reactions required for HSNO isomerization into HONS and SN(H)O are predicted to have prohibitively high reaction barriers, 30-50 kcal mol(-1). However, the polar aqueous environment and water-assisted proton shuttle should decrease these barriers to ∼9 kcal mol(-1), which makes these two isomers kinetically accessible under physiological conditions. Our calculations also support the possibility of an aqueous reaction between the Y-isomer SN(H)O and H2S leading to biologically active nitroxyl HNO. These results suggest that the formation of HSNO in biological milieu can lead to various derivative species with their own, possibly biologically relevant, activity.