Lithium cation enhances anion binding in a tripodal phosphine oxide-based ditopic receptor

A tripodal ditopic receptor presents H-bond donors and a phosphine oxide to potential guests. In the idealized binding conformation, an endohedral P=O functionality provides enhanced halide binding in the presence of lithium with the greatest ΔΔG° observed for bromide, while minimal changes in K(a) are observed in the presence of sodium.

Electron Paramagnetic Resonance Structure Investigation of Copper Complexation in a Hemicarcerand

The double-bridged hemicarcerand [A,B-(CH2OH)2-cavitand]-(CH2NHCH2)2-[A,B-(CH2OH)2-cavitand] 23 (and several other related compounds) was synthesized by the condensation of the two complementary precursors A,B-(CH2NH2)2(CH2OH)2-cavitand and A,B-(CH2Br)2(CH2OAc)2-cavitand followed by hydrolysis of the acetate groups. This hemicarcerand has nitrogen and oxygen donor atoms located on the interior of the spherical cavity and thus allows endohedral coordination of metal ions. The cavity has a volume of approximately 0.12 nm3, a value obtained by calculating a Connolly-type contact surface and the molecular electrostatic potential. The Cu2+ complex of hemicarcerand 23 was studied in detail by EPR and DFT calculations at the UB3LYP/6-31G level to verify the anticipated endohedral nature of the metal complex. It could be shown that the copper ion is coordinated to four oxygen donor atoms and no deviation from axial symmetry at the copper site could be detected. No direct coordination to nitrogen atoms of the hemicarcerand could be observed; however, complexation with DMF solvent molecules was detected by ESEEM and HYSCORE experiments. The closed structure of the hemicarcerand was also confirmed by an evaluation of proton-copper distances. Results from DFT calculations are in accord with the EPR results, and further support suggested coordination of the Cu(II) within the hemicarcerand cavity by four oxygen donor atoms.

Structural, redox, and mechanistic parameters for cysteine-sulfenic acid function in catalysis and regulation

A primary objective of this review is to facilitate the application of the chemical and structural approaches that are currently being employed in the identification of Cys-SOH, as both transient intermediates and stable redox forms, in biochemical systems where these derivatives are suspected of playing key roles in redox catalysis or regulation. These range from high-resolution crystallographic analyses benefiting from recent technological advances in rapid data collection at cryogenic temperatures to 13C NMR investigations of [3-(13)C]Cys-labeled proteins and chemical modification protocols that can be integrated with both UV-visible and fluorescence spectroscopic as well as mass spectrometric (especially ESI, MALDI-TOF, and even FT ion-cyclotron-resonance) analyses. In summarizing the diversity of biological functions currently identified with Cys-SH reversible Cys-SOH redox cycles (Fig. 17), it should also be [figure: see text] emphasized that in at least one protein (nitrile hydratase) stable Cys-SOH and Cys-SO2H derivatives play important structural roles while also modulating the electronic properties of the iron center; in neither case is the Cys-SOH residue itself involved in reduction and oxidation. The somewhat incomplete structural descriptions of the oxidized Cys forms involved in redox regulation of some transcription factors (e.g., BPV-1 E2 protein and activator protein-1) indicate that there is ample room for the application of the types of investigations employed, for example, with NADH peroxidase and the AhpC peroxiredoxin, with a view toward defining the potential roles of Cys-SOH in these very important contexts of intracellular redox signaling. These advances will also build on the recent progress in defining sulfenic acid stabilization and properties in small molecule model systems, as evidenced in the work of Okazaki, Goto, and others. When viewed in the perspective of Allison's 1976 review on the subject of sulfenic acids in proteins, the reader will hopefully come to appreciate the conclusion that the concept of protein-sulfenic acids has now become a very well-defined and established principle of biochemistry, with current efforts in this and other laboratories being directed to bring about still more detailed understanding of Cys-SOH function in both redox and nonredox modes of enzyme catalysis and regulation of protein function.

The first direct oxidative conversion of a selenol to a stable selenenic acid: experimental demonstration of three processes included in the catalytic cycle of glutathione peroxidase

[structure: see text]. A stable selenenic acid was synthesized by direct oxidation of a selenol bearing a novel bowl-type substituent with H2O2, and its structure was established by X-ray crystallographic analysis. Selenenyl sulfides obtained by the reaction of the selenenic acid with 1,4-dithiols were reduced to the corresponding selenol by treatment with a tertiary amine, thus achieving the experimental demonstration of three processes included in the catalytic cycle of glutathione peroxidase.