Spectroscopic and Theoretical Studies of Hg(II) Complexation with Some Dicysteinyl Tetrapeptides.

Tetrapeptides containing a Cys-Gly-Cys motif and a propensity to adopt a reverse-turn structure were synthesized to evaluate how -, -, -, and aromatic donor groups might contribute to mercury(II) complex formation. Tetrapeptides Xaa-Cys-Gly-Cys, where Xaa is glycine, glutamate, histidine, or tryptophan, were prepared and reacted with mercury(II) chloride. Their complexation with mercury(II) was studied by spectroscopic methods and computational modeling.

Dithiolated peptides incorporating bis(tryptophan)s for cooperative mercury(II) binding.

The indole side chain of tryptophan is a versatile π-donor that can participate in various types of cation-π interactions. An understanding of how it may contribute as an auxiliary binding group in mercury(II) complexes can provide valuable insights toward the design of effective chelators for optimal mercury immobilization. In this study, we investigate how the incorporation of two tryptophan residues in model dicysteinyl peptides might participate in peptide-mercury(II) complex stabilization.

Tryptic y(++) fragment ion distributions are guided by Coulombic repulsion.

Ideal tryptic peptides contain only a single basic residue, located at the C-terminus. Collisional fragmentation of their doubly- or triply-protonated ions generates doubly-charged y(++) fragment ions with modest intensities. The size distribution of the y(++) fragments, when averaged over many spectra, corresponds closely to the expectations from charge-directed backbone cleavage and a Coulomb-Boltzmann distribution of mobile protons.

N-Protonated isomers as gateways to peptide ion fragmentation.

According to the popular "mobile proton model" for peptide ion fragmentation in tandem mass spectrometry, peptide bond cleavage is typically preceded by intramolecular proton transfer from basic sites to an amide nitrogen in the backbone. If the intrinsic barrier to dissociation is the same for all backbone sites, the fragmentation propensity at each amide bond should reflect the stability of the corresponding N-protonated isomer. This hypothesis was tested by using ab initio and force-field computations on several polyalanines and Leu-enkephalin.

Atmospheric stability of arsine and methylarsines.

Arsenic (As) occurs in a variety of different chemical forms, among them volatile (gaseous) species, usually referred to as arsine and methylarsines. Here we demonstrate that arsine and methylarsines are stable in air in concentrations at the μg/L gas level. We determined half-lives of approximately 8 h under daytime conditions (UV light) for all methylated arsines, while the same species were found to be considerably more stable in night-time (dark) conditions.

Volatilization of organotin species from municipal waste deposits: novel species identification and modeling of atmospheric stability.

Organotin compounds are used as pesticides and fungicides as well as additives in plastics. This study identifies the de novo generation of novel volatile organotins in municipal waste deposits and their release via landfill gas. Besides tetramethyltin (Me(4)Sn), a strong neurotoxin, and 5 previously reported organotins, 13 novel ethylated, propylated, and butylated tetraalkyltin compounds were identified.

Rate constants of hydroperoxyl radical addition to cyclic nitrones: a DFT study.

Nitrones are potential synthetic antioxidants against the reduction of radical-mediated oxidative damage in cells and as analytical reagents for the identification of HO2* and other such transient species. In this work, the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) and PCM/mPW1K/6-31+G(d,p) density functional theory (DFT) methods were employed to predict the reactivity of HO2* with various functionalized nitrones as spin traps. The calculated second-order rate constants and free energies of reaction at both levels of theory were in the range of 100-103 M-1 s-1 and 1 to -12 kcal mol-1, respectively, and the rate constants for some nitrones are on the same order of magnitude as those observed experimentally.

Reactivity of superoxide radical anion with cyclic nitrones: role of intramolecular H-bond and electrostatic effects.

Limitations exist among the commonly used cyclic nitrone spin traps for biological free radical detection using electron paramagnetic resonance (EPR) spectroscopy. The design of new spin traps for biological free radical detection and identification using EPR spectroscopy has been a major challenge due to the lack of systematic and rational approaches to their design. In this work, density functional theory (DFT) calculations and stopped-flow kinetics were employed to predict the reactivity of functionalized spin traps with superoxide radical anion (O2*-).

Spectroscopic and computational studies of aqueous ethylene glycol solution surfaces.

The combination of Monte Carlo, ab initio, and DFT computational studies of ethylene glycol (EG) and EG-water hydrogen-bonding complexes indicate that experimental vibrational spectra of EG and EG-water solution surfaces have contributions from numerous conformations of both EG and EG-water. The computed spectra, derived from harmonic vibrational frequency calculations and a theoretical Boltzmann distribution, show similarity to the experimental surface vibrational spectra of EG taken by broad-bandwidth sum frequency generation (SFG) spectroscopy. This similarity suggests that, at the EG and aqueous EG surfaces, there are numerous coexisting conformations of stable EG and EG-water complexes.

Theoretical determinations of the ambient conformational distribution and unimolecular decomposition of n-propylperoxy radical.

The conformational distribution and unimolecular decomposition pathways for the n-propylperoxy radical have been generated at the CBS-QB3, B3LYP/6-31+G and mPW1K/6-31+G levels of theory. At each of the theoretical levels, the 298 K Boltzmann distributions and rotational profiles indicate that all five unique rotamers of the n-propylperoxy radical can be expected to be present in significant concentrations at thermal equilibrium. At the CBS-QB3 level, the 298 K distribution of rotamers is predicted to be 28.

Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline N-oxide (DMPO). 2. The thermodynamics of decay and EPR spectral properties.

The formation of the superoxide radical anion (O2*-) adduct of the nitrone 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as detected by electron paramagnetic resonance (EPR) spectroscopy is one of the most common techniques for O2*- detection in chemical and biological systems. However, the nature of DMPO-O2H has confounded spin-trapping investigators over the years, since there has been no independently synthesized DMPO-O2H to date. A density functional theory (DFT) approach was used to predict the isotropic hyperfine coupling constants arising from the N, beta-H, and gamma-H nuclei of DMPO-O2H using explicit interactions with water molecules as well as via a bulk dielectric effect employing the polarizable continuum model (PCM).

Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline n-oxide (DMPO). 1. The thermodynamics of formation and its acidity.

The nitrone 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been the most widely used spin trap for the detection of transient free radicals in chemical, biological, and biomedical research using electron paramagnetic resonance (EPR) spectroscopy. A density functional theory (DFT) approach was used to predict the thermodynamics of formation of the superoxide anion/hydroperoxyl radical (O2*-/*O2H) adduct of DMPO as well as its pK(a) in aqueous systems. At the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level, we predicted (in the gas phase and with a polarizable continuum model (PCM) for water) three conformational minima for both the DMPO-O2- and DMPO-O2H adducts.

Observation of the A-X electronic transition of the 1-C3H7O2 and 2-C3H7O2 radicals using cavity ringdown spectroscopy.

Cavity ringdown spectra of the A-X electronic transition of the 1-propyl and 2-propyl peroxy radicals are reported. Spectroscopic assignments are facilitated by implementing several production mechanisms, either isomer-specific or not. Assignments of specific spectral lines to particular conformers of a given isomer are suggested.