The possibility of using thiocyanate to determine iron(II) and/or iron(III) in water-acetone mixture has been re-examined as part of a systematic and comparative study involving metallic complexes of pseudohalide ligands. Some parameters that affect the complete oxidation of the ferrous cations, their subsequent complexation and the system stability have been studied to optimize the experimental conditions. Our results show the viability and potentiality of this simply methodology as an alternative analytical procedure to determine iron cations with high sensitivity, precision and accuracy. Studies on the calibration, stability, precision, and effect of various different ions have been carried out by using absorbance values measured at 480 nm. The analytical curve for the total iron determination obeys Beer's law (r = 0.9993), showing a higher sensitivity (molar absorptivity of 2.10x10(4) L cm-1 mol-1) when compared with other traditional systems (ligands) or even with the "similar" azide ion [1.53x10(4) L cm-1 mol-1, for iron-III/azide complexes, in 70% (v/v) tetrahydrofuran/water, at 396 nm]. Under such optimized experimental conditions, it is possible to determine iron in the concentration range from 0.5 to 2 ppm (15-65% T for older equipments, quartz cells of 1.00 cm). Analytical applications have been tested for some different materials (iron ores), also including pharmaceutical products for anemia, and results were compared with atomic absorption determinations. Very good agreement was obtained with these two different techniques, showing the potential of the present experimental conditions for the total iron spectrophotometric determinations (errors < 5%). The possibility of iron speciation was made evident by using another specific and auxiliary method for iron(II) or (III).

No abstract

ABSTRACT. Some antimicrobial peptides have a broad spectrum of action against many different kinds of microorganisms. Gomesin and protegrin-1 are examples of such antimicrobial peptides, and they were studied by molecular dynamics in this research. Both have a β-hairpin conformation stabilized by two disulfide bridges and are active against Gram-positive and Gram-negative bacteria, as well as fungi. In this study, the role of the disulfide bridge in the maintenance of the tertiary peptide structure of protegrin-1 and gomesin is analyzed by the structural characteristics of these peptides and two of their respective variants, gomy4 and proty4, in which the four cysteines are replaced by four tyrosine residues. The absence of disulfide bridges in gomy4 and proty4 is compensated by overall reinforcement of the original hydrogen bonds and extra attractive interactions between the aromatic rings of the tyrosine residues. The net effects on the variants with respect to the corresponding natural peptides are: i) maintenance of the original β-hairpin conformation, with great structural similarities between the mutant and the corresponding natural peptide; ii) combination of positive Φ and Ψ Ra- Role of disulfide bridges in gomesin and protegrin-1 machandran angles within the hairpin head region with a qualitative change to a combination of positive (Φ) and negative (Ψ) angles, and iii) significant increase in structural flexibility. Experimental facts about the antimicrobial activity of the gomesin and protegrin-1 variants have also been established here, in the hope that the detailed data provided in the present study may be useful for understanding the mechanism of action of these peptides.

IntroductionThe principle of detection by piezoelectric crystal is based on the fact that the vibration frequency of an oscillating sensor decreases in the presence of an amount of material added to its surface. As shown by Sauerbrey [1], the vibrating frequency of a quartz crystal, to a first approximation, changes proportionally to the mass deposited onto or removed from one or both their faces. Because the TSMR (thickness shear mode resonator) fundamental frequency is rather low (10 MHz), it is assumed that viscosity effects contribute to the frequency changes due to gas sorption to a negligible extent only [2]. Gaseous pollutants can be selectively sorbed by the detector if an ideal coating is deposited on the metallic electrodes. The observed frequency change is a measure of the amount of sorbed gas. The type of interaction taking place between the analyte molecules and the active coating determines the sorption and desorption characteristics. Lowenergy, perfectly reversible interactions such as physisorption generally lack a high degree of selectivity. On the other hand, the formation of chemical bonds or the chemisorption process tend to be less reversible. One approach to overcoming these problems includes the application of sensor arrays to compensate the low selectivity [3,4]. Another solution to this problem includes searching for "intermediate interactions", i.e., interactions that are weaker than chemisorption (≈300 kJ mol -1 ) but stronger than physisorption (0-40 kJ mol -1 ), such as coordination [5]. A central metal atom surrounded by neutral or charged (often organic) ligands is responsible for these interactions, where one or more donor atoms on these ligands interact with the metal ion. So, selectivity can be influenced by the choice of both the metal ion and the ligand, from both an eletronic and steric point of view [5]. When gaseous molecules interact with metal complexes in this way, they themselves become coligands either by occupying free coordination sites or by displacing other ligands. Earlier work in the field of coordination chemistry on mass-sensitive Abstract: Carbon monoxide was detected and determined by a piezoelectric quartz crystal sensor coated with nickel(II)-phthalocyanine 50 % (v/v) solution in glycerine. Studies on the effect of temperature, flow rate, and some possible interferents were carried out. Calibration curves, sensor stability (lifetime) and the precision of measurements were also verified. The resulting selectivity is probably due to the coordinative binding between the electronically unsatured metal complexes and the analyte. The analytical curve is linear in the concentration range 0.10 to 1.0 % (v/v).

ABSTRACT:The total time reached by molecular dynamics simulation in the study of the interactions between hydrated bilayers and peptides is still very short. A scheme of fast heating and cooling cycles of simulated annealing (FHCCSA) is proposed to improve the efficiency of the search for the global minimum of the peptide/bilayer potential energy surface. In FHCCSA, the high temperatures facilitate the transitions between stable configurations; i.e., heating and cooling cycles make easier the escape of the system outside the local energy minimum. The FHCCSA efficiency is confirmed by comparing its results with conventional NpT simulations. The new scheme saves more than 90% of the total cpu time compared with ordinary NpT simulations.