Effects of mutating aromatic surface residues of the heme domain of human sulfite oxidase on its heme midpoint potential, intramolecular electron transfer, and steady-state kinetics.

Human sulfite oxidase (hSO), an essential molybdoheme enzyme, catalyzes the oxidation of toxic sulfite to sulfate. The proposed catalytic cycle includes two, one-electron intramolecular electron transfers (IET) between the molybdenum (Mo) and the heme domains. Rapid IET rates are ascribed to conformational changes that bring the two domains into close proximity to one another.

Approaches to the rational design of selective melanocortin receptor antagonists.

Introduction: When establishing the physiological roles of specific receptors in normal and disease states, it is critical to have selective antagonist ligands for each receptor in a receptor system with several subtypes. The melanocortin receptors have five subtypes referred to as the melanocortin 1 receptor, melanocortin 2 receptor, melanocortin 3 receptor, melanocortin 4 receptor and melanocortin 5 receptor, and they are of critical importance for many aspects of human health and disease.

Areas Covered: This article reviews the current efforts to design selective antagonistic ligands for the five human melanocortin receptors summarizing the currently published orthosteric and allosteric antagonists for each of these receptors.

Resonance and localization effects at a dipolar organic semiconductor interface.

The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0-1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage.

Cell-specific targeting by heterobivalent ligands.

Current cancer therapies exploit either differential metabolism or targeting to specific individual gene products that are overexpressed in aberrant cells. The work described herein proposes an alternative approach--to specifically target combinations of cell-surface receptors using heteromultivalent ligands ("receptor combination approach"). As a proof-of-concept that functionally unrelated receptors can be noncovalently cross-linked with high avidity and specificity, a series of heterobivalent ligands (htBVLs) were constructed from analogues of the melanocortin peptide ligand ([Nle(4), dPhe(7)]-α-MSH) and the cholecystokinin peptide ligand (CCK-8).

Magnetic self-assembly of gold nanoparticle chains using dipolar core-shell colloids.

The preparation of gold nanoparticle (AuNP) assemblies was conducted by the synthesis and dipolar assembly of ferromagnetic core-shell nanoparticles composed of AuNP cores and cobalt NP shells. Dissolution of metallic Co phases with mineral acids afforded self-assembled AuNP chains and bracelets.

Effects of large-scale amino acid substitution in the polypeptide tether connecting the heme and molybdenum domains on catalysis in human sulfite oxidase.

Sulfite oxidase (SO) is a molybdenum-cofactor-dependent enzyme that catalyzes the oxidation of sulfite to sulfate, the final step in the catabolism of the sulfur-containing amino acids, cysteine and methionine. The catalytic mechanism of vertebrate SO involves intramolecular electron transfer (IET) from molybdenum to the integral b-type heme of SO and then to exogenous cytochrome c. However, the crystal structure of chicken sulfite oxidase (CSO) has shown that there is a 32 Å distance between the Fe and Mo atoms of the respective heme and molybdenum domains, which are connected by a flexible polypeptide tether.

Image states at the interface with a dipolar organic semiconductor.

Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage.

Influence of electrostatic fields on molecular electronic structure: insights for interfacial charge transfer.

Molecular and interfacial electronic structure at organic semiconductor interfaces shows a rich and subtle dependence on short- and long-range electrostatic interactions. Interface dipoles can be controlled making use of the anisotropic charge distribution at the interface, often with direct consequences also for the molecular electronic structure. In this Perspective, we will discuss the emerging understanding of how local and collective electrostatic effects control energy level alignment and molecular electronic structure at organic semiconductor interfaces and highlight some of the ramifications for interfacial charge-transfer dynamics.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors.

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities.

Enantioselective organocatalytic alpha-sulfenylation of substituted diketopiperazines.

The asymmetric organocatalytic alpha-sulfenylation of substituted piperazine-2,5-diones is reported, with cinchona alkaloids as chiral Lewis bases and electrophilic sulfur transfer reagents. Catalyst loadings, the type of sulfur transfer reagent, temperature and solvent were investigated in order to optimize the reaction conditions. The effects of ring substitution and the type of catalyst on the yield and enantioselectivity of the reaction are reported.

Confocal single molecule fluorescence spectroscopy in ultrahigh vacuum.

We have constructed an ultrahigh vacuum confocal fluorescence microscope with the purpose of performing single molecule spectroscopy under highly defined conditions. The microscope is designed for high stability while affording the capability of sample preparation, sample transfer, and optical detection in ultrahigh vacuum. It achieves near-diffraction-limited performance and allows the observation of single molecule fluorescence dynamics over extended periods of time.

Single molecule power-law behavior on a crystalline surface.

Single perylene bisimide molecules deposited onto Al(2)O(3) (0001) and investigated under controlled ultrahigh vacuum conditions display fluorescence intermittency behavior characteristic of an interfacial charge transfer process. Remarkably, even though the molecules are deposited on a crystalline surface with reduced disorder, power-law-distributed bright and dark periods are observed. These data can be understood based on activated formation of localized small polaron states in Al(2)O(3) (0001).

Solid-Phase Synthesis of Heterobivalent Ligands Targeted to Melanocortin and Cholecystokinin Receptors.

Heteromultivalency provides a route to increase binding avidity and to high specificity when compared to monovalent ligands. The enhanced specificity can potentially serve as a unique platform to develop diagnostics and therapeutics. To develop new imaging agents based upon multivalency, we employed heterobivalent constructs of optimized ligands.

Noncovalent protein tetramers and pentamers with "n" charges yield monomers with n/4 and n/5 charges.

In recent years mass spectrometry based techniques have emerged as structural biology tools for the characterization of macromolecular, noncovalent assemblies. Many of these efforts involve preservation of intact protein complexes within the mass spectrometer, providing molecular weight measurements that allow the determination of subunit stoichiometry and real-time monitoring of protein interactions. Attempts have been made to further elucidate subunit architecture through the dissociation of subunits from the intact complex by colliding it into inert gas atoms such as argon or xenon.

Structures and reaction pathways of the molybdenum centres of sulfite-oxidizing enzymes by pulsed EPR spectroscopy.

SOEs (sulfite-oxidizing enzymes) are physiologically vital and occur in all forms of life. During the catalytic cycle, the five-co-ordinate square pyramidal oxo-molybdenum active site passes through the Mo(V) state, and intimate details of the structure can be obtained from variable frequency pulsed EPR spectroscopy through the hyperfine and nuclear quadrupole interactions of nearby magnetic nuclei. By employing variable spectrometer operational frequencies, it is possible to optimize the measurement conditions for difficult quadrupolar nuclei of interest (e.

Fragmentation of doubly-protonated peptide ion populations labeled by H/D exchange with CD(3)OD.

Doubly-protonated bradykinin (RPPGFSPFR) and an angiotensin III analogue (RVYIFPF) were subjected to hydrogen/deuterium (H/D) exchange with CD(3)OD in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. A bimodal distribution of deuterium incorporation was present for bradykinin after H/D exchange for 90 s at a CD(3)OD pressure of 4 x 10(-7) Torr, indicating the existence of at least two distinct populations. Bradykinin ion populations corresponding to 0-2 and 5-11 deuteriums (i.

Silica colloidal crystals as porous substrates for total internal reflection fluorescence microscopy of live cells.

Total internal reflection fluorescence (TIRF) microscopy is a powerful means of probing biological cells because it reduces autofluorescence, but the need for direct contact between the cell surface and the microscope slide hinders chemical access to the cell surface. In this work, a submicrometer crystalline layer of colloidal silica on the microscope coverslip is shown to allow TIRF microscopy while also allowing chemical access to the cell surface. A 750 nm layer of 165 nm silica colloidal crystals was sintered onto a fused silica coverslip, and Chinese hamster ovary cells were successfully grown on this surface.

Structural studies of the molybdenum center of the pathogenic R160Q mutant of human sulfite oxidase by pulsed EPR spectroscopy and 17O and 33S labeling.

Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A.

Molecules that target beta-amyloid.

The devastating effects of Alzheimer's and related amyloidogenic diseases have inspired the synthesis and evaluation of numerous ligands to understand the molecular mechanism of the aggregation of the beta-amyloid peptide. Our review focuses on the current knowledge in this field with respect to molecules that have been demonstrated to interact with either oligomeric or fibrillar forms of the beta-amyloid peptide. We describe natural proteins, peptides, peptidomimetics, and small molecules that have been found to interfere with beta-amyloid aggregation.

A theoretical study of zinc(II) interactions with amino acid models and peptide fragments.

Density functional theory calculations have been employed to study the interaction between the Zn2+ ion and some standard amino acid models. The highest affinities towards the Zn2+ ion are predicted for serine, cysteine, and histidine. Relatively high affinities are reported also for proline and glutamate/aspartate residues.

Investigation of the coordination structures of the molybdenum(v) sites of sulfite oxidizing enzymes by pulsed EPR spectroscopy.

Sulfite oxidizing enzymes (SOEs) are physiologically vital and occur in all forms of life. During the catalytic cycle the five-coordinate square-pyramidal oxo-molybdenum active site passes through the Mo(v) state, and intimate details of the structure can be obtained from pulsed EPR spectroscopy through the hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of nearby magnetic nuclei (e.g.