Spectroscopic and electronic structure studies of intermediate X in ribonucleotide reductase R2 and two variants: a description of the FeIV-oxo bond in the FeIII-O-FeIV dimer

Spectroscopic and electronic structure studies of the class I Escherichia coli ribonucleotide reductase (RNR) intermediate X and three computationally derived model complexes are presented, compared, and evaluated to determine the electronic and geometric structure of the FeIII-FeIV active site of intermediate X. Rapid freeze-quench (RFQ) EPR, absorption, and MCD were used to trap intermediate X in R2 wild-type (WT) and two variants, W48A and Y122F/Y356F. RFQ-EPR spin quantitation was used to determine the relative contributions of intermediate X and radicals present, while RFQ-MCD was used to specifically probe the FeIII/FeIV active site, which displayed three FeIV d-d transitions between 16,700 and 22,600 cm(-1), two FeIV d-d spin-flip transitions between 23,500 and 24,300 cm(-1), and five oxo to FeIV and FeIII charge transfer (CT) transitions between 25,000 and 32,000 cm(-1). The FeIV d-d transitions were perturbed in the two variants, confirming that all three d-d transitions derive from the d-pi manifold. Furthermore, the FeIV d-pi splittings in the WT are too large to correlate with a bis-mu-oxo structure. The assignment of the FeIV d-d transitions in WT intermediate X best correlates with a bridged mu-oxo/mu-hydroxo [FeIII(mu-O)(mu-OH)FeIV] structure. The mu-oxo/mu-hydroxo core structure provides an important sigma/pi superexchange pathway, which is not present in the bis-mu-oxo structure, to promote facile electron transfer from Y122 to the remote FeIV through the bent oxo bridge, thereby generating the tyrosyl radical for catalysis.     

Direct and Indirect Chemiluminescence: Reactions,Mechanisms and Challenges

Emission of light by matter can occur through a variety of mechanisms. When it results from an electronically excited state of a species produced by a chemical reaction, it is called chemiluminescence (CL). The phenomenon can take place both in natural and artificial chemical systems and it has been utilized in a variety of applications. In this review, we aim to revisit some of the latest CL applications based on direct and indirect production modes. The characteristics of the chemical reactions and the underpinning CL mechanisms are thoroughly discussed in view of studies from the very recent bibliography. Different methodologies aiming at higher CL efficiencies are summarized and presented in detail, including CL type and scaffolds used in each study. The CL role in the development of efficient therapeutic platforms is also discussed in relation to the Reactive Oxygen Species (ROS) and singlet oxygen (¹O₂) produced, as final products. Moreover, recent research results from our team are included regarding the behavior of commonly used photosensitizers upon chemical activation under CL conditions. The CL prospects in imaging, biomimetic organic and radical chemistry, and therapeutics are critically presented in respect to the persisting challenges and limitations of the existing strategies to date.     

Ricci flow on surfaces with cusps

We consider the normalized Ricci flow ? _t g = (ρ ? R)g with initial condition a complete metric g ₀ on an open surface M where M is conformal to a punctured compact Riemann surface and g ₀ has ends which are asymptotic to hyperbolic cusps. We prove that when χ(M) < 0 and ρ < 0, the flow g(t) converges exponentially to the unique complete metric of constant Gauss curvature ρ/2 in the conformal class.     

Ueber den Nachweis des Quecksilbers mittelst Elektrolyse

Ohne Zusammenfassung     

The classification of locally conformally flat Yamabe solitons

This paper addresses the classification of locally conformally flat gradient Yamabe solitons. In the first part it is shown that locally conformally flat gradient Yamabe solitons with positive sectional curvature are rotationally symmetric. In the second part the classification of all radially symmetric gradient Yamabe solitons is given and their correspondence to smooth self-similar solutions of the fast diffusion equation on Rⁿ${\mathbb{R}}^n$ is shown. In the last section it is shown that any eternal solution to the Yamabe flow with positive Ricci curvature and with the scalar curvature attaining an interior space–time maximum must be a steady Yamabe soliton.     

Möbius-like groups of homeomorphisms of the circle

An orientation preserving homeomorphism of is Möbius-like if it is conjugate in to a Möbius transformation. Our main result is: given a (noncyclic) group whose every element is Möbius-like, if has at least one global fixed point, then the whole group is conjugate in to a Möbius group if and only if the limit set of is all of . Moreover, we prove that if the limit set of is not all of , then after identifying some closed subintervals of to points, the induced action of is conjugate to an action of a Möbius group. Said differently, is obtained from a group which is conjugate to a Möbius group, by a sort of generalized Denjoy's insertion of intervals. In this case is isomorphic, as a group, to a Möbius group.

This result has another interpretation. Namely, we prove that a group of orientation preserving homeomorphisms of whose every element can be conjugated to an affine map (i.e., a map of the form ) is just the conjugate of a group of affine maps, up to a certain insertion of intervals. In any case, the group structure of is the one of an affine group.

     

A new type of a spectrometric microtitration set up

A spectrometric microtitrator was developed from a spectrometer with a microreaction chamber and a tri-colour light-emitting diode (LED) as the light source. A novel, vertical, optical geometry of the spectrometric microtitration chamber was introduced and tested. This novel geometry also required a new method for mixing the titrated solution. A laboratory-made 50 μl syringe pump was used for the addition of the titration reagent. The 10-channel module for light effects, which makes possible a low-cost hardware approach to changing the titration protocols, was used for coordinating the operation of the microtitration set up. The system, with 10 channels and a regulated speed of operation, is flexible enough to allow an operator to generate different titration protocols. The performance test showed that the speed of titration-reagent addition can be regulated in the range from 0.87 to 21.8 μl min⁻¹. The smallest achievable volume addition is equal to 35 nl. The mixing rate can be continuously regulated by an electrical pulse that initiates the mixing cycle. The quickest rate is every 1.6 s, and the slowest rate is every 4.8 s. The spectrometric microtitration set up was successfully tested for several different real-life spectrometric titrations, including an iodometric titration, a determination of CO₂ in deionised water, and EDTA titrations of copper(II) ions with no indicator. The volume of the examined solution can be as small as 220 μl. The titration-reagent consumption is usually between 10 and 35 μl. Coefficients of variation of the end point volume determination (n = 5) at different experimental conditions and different average volumes of consumed reagents (7.06, 12.17 and 22.88 μl) were 2.4, 1.3 and 1.2%, respectively. The novel geometry of the spectrometric microtitration chamber proved to be useful for real-life applications.     

Fatty acids of <Emphasis Type="Italic">Stachys milanii</Emphasis> seeds

For the first time the total acid composition of the seeds of Stachys milanii Petrovic, a species endemic to the central and eastern Balkan Peninsula and growing spontaneously at two localities, was examined by GC and GC/MS. The major fatty acid was linoleic or oleic acid. Besides fatty acids that represent the usual seed oil constituents, 6-octadecynoic and 9-oxononanoic acid were identified in considerable amounts. The high content of 6-octadecynoic acid makes S. milanii seed oil a good potential source for the active substance in antifungal preparations. From the chemotaxonomical point of view, small amounts of octadecatrienoic acid detected in the seeds suggested classification of S. milanii as an archaic taxon. Published in Khimiya Prirodnykh Soedinenii, No. 4, pp. 315–317, July–August, 2007.