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 O-, N-, H-, and aromatic π 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. UV-vis studies confirmed that mercury(II) binds to the cysteinyl thiolates as indicated by characteristic ligand-to-metal-charge-transfer transitions for bisthiolated S-Hg-S complexes, which correspond to 1 : 1 mercury-peptide complex formation. ESI-MS data also showed dominant 1 : 1 mercury-peptide adducts that are consistent with double deprotonations from the cysteinyl thiols to form thiolates. These complexes exhibited a strong positive circular dichroism band at 210 nm and a negative band at 193 nm, indicating that these peptides adopted a β-turn structure after binding mercury(II). Theoretical studies confirmed that optimized 1 : 1 mercury-peptide complexes adopt β-turns stabilized by intramolecular hydrogen bonds. These optimized structures also illustrate how specific N-terminal side-chain donor groups can assume intramolecular interactions and contribute to complex stability. Fluorescence quenching results provided supporting data that the indole donor group could interact with the coordinated mercury. The results from this study indicate that N-terminal side-chain residues containing carboxylate, imidazole, or indole groups can participate in stabilizing dithiolated mercury(II) complexes. These structural insights on peripheral mercury-peptide interactions provide additional understanding of the chemistry of mercury(II) with side-chain donor groups in peptides.

Emission cross sections for energetic O+(4S,2D,2P)–N2 collisions

Measurements of emission cross sections for the [Formula: see text] collision system with the incident beam of 1–10 keV [Formula: see text] in the ground [Formula: see text]) and metastable [Formula: see text] and [Formula: see text] states are reported. The emission cross section induced by incident ions in the metastable state [Formula: see text] is much larger than that for the ground [Formula: see text] state. The emission cross section of [Formula: see text] ion for [Formula: see text], [Formula: see text], and [Formula: see text] bands system is measured and the ratio of intensities for these bands is established as [Formula: see text] It is shown that the cross sections for the [Formula: see text] ions emissions in the dissociative charge exchange processes increase with the increase of the incident ion energy. The energy dependence of the emission cross section of the band [Formula: see text] [Formula: see text] nm of the first-negative band system of the [Formula: see text] and degree of linear polarization of emission in [Formula: see text] collision are measured for the first time. An influence of an admixture of the ion metastable state on a degree of linear polarization is revealed. The mechanism of the processes realized during collisions of ground and metastable oxygen ions on molecular nitrogen have been established. It is demonstrated that for [Formula: see text] collision system the degree of linear polarization by metastable [Formula: see text] ions is less compared to those that are in the ground [Formula: see text] state and the sign of emission of degree of linear polarization of excited molecular ions does not change.

CO+ first-negative band emission: A tracer for CO in the Martian upper atmosphere

Context. Recently, the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite observed CO+ first-negative band limb emission in the Martian upper atmosphere. Aims. We aim to explore the photochemical processes in the Martian upper atmosphere, which drive this band emission. Methods. A photochemical model was developed to study the excitation processes of CO+ first-negative band emission (B2Σ+ → X2Σ+) in the upper atmosphere of Mars. The number density profiles of CO2 and CO from two different models, namely, Mars Climate Database (MCD) and Mars Global Ionosphere-Thermosphere (MGIT), were used to determine the limb intensity of this band emission. Results. By increasing the CO density by a factor of 4 and 8 in MCD and MGIT models, respectively, the modelled CO+ first-negative band limb intensity profile is found to be consistent with the IUVS/MAVEN observation. In this case, the intensity of this band emission is significantly determined by the ionisation of CO by solar photons and photoelectrons, and the role of dissociative ionisation of CO2 is negligible. Conclusions. Since CO is the major source of the CO+(B2Σ+), we suggest that the observed CO+ first-negative band emission intensity can be used to retrieve the CO density in the Martian upper atmosphere for the altitudes above 150 km.

Dissection of the Structural Features of a Fungicidal Antibody-Derived Peptide

The synthetic peptide T11F (TCRVDHRGLTF), derived from the constant region of human IgM antibodies, proved to exert a significant activity in vitro against yeast strains, including multidrug resistant isolates. Alanine substitution of positively charged residues led to a decrease in candidacidal activity. A more dramatic reduction in activity resulted from cysteine replacement. Here, we investigated the conformational properties of T11F and its alanine-substituted derivatives by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy. Peptide interaction with Candida albicans cells was studied by confocal and scanning electron microscopy. T11F and most of its derivatives exhibited CD spectra with a negative band around 200 nm and a weaker positive band around 218 nm suggesting, together with NMR coupling constants, the presence of a polyproline II (PPII) helix, a conformational motif involved in a number of biological functions. Analysis of CD spectra revealed a critical role for phenylalanine in preserving the PPII helix. In fact, only the F11A derivative presented a random coil conformation. Interestingly, the loss of secondary structure influenced the rate of killing, which turned out to be significantly reduced. Overall, the obtained results suggest that the PPII conformation contributes in characterising the cell penetrating and fungicidal properties of the investigated peptides.