Paraquat substituent effect on complexation with a dibenzo-24-crown-8-based cryptand

Dibenzo-24-crown-8-based cryptand 4 forms 1:1 inclusion complexes with three paraquat derivatives, P1, P2, and P3, as demonstrated by proton NMR spectroscopy and X-ray analysis. However, it was found that methyl-substituted paraquat derivatives, P2 and P3, can bind cryptand 4 more strongly than non-methyl-substituted paraquat derivative P1. The association constants (Ka) were determined in acetone by using a UV-vis titration method to be 5.0 x 10(3) M(-1) for 4.P1, 1.0 x 10(5) M(-1) for 4.P2, and 1.2 x 10(5) M(-1) for 4.P3, respectively. In the solid state, 4.P2 and 4.P3 have similar T-type inclusion complexation conformations, which are very different from the pseudorotaxane-type complexation conformation of 4.P1. Theoretical calculations were done to explain these experimental results.     

Diversification of catalytic function in a synthetic family of chimeric cytochrome p450s

We report initial characterization of a synthetic family of more than 3000 cytochrome P450s made by SCHEMA recombination of 3 bacterial CYP102s. A total of 16 heme domains and their holoenzyme fusions with each of the 3 parental reductase domains were tested for activity on 11 different substrates. The results show that the chimeric enzymes have acquired significant functional diversity, including the ability to accept substrates not accepted by the parent enzymes. K-means clustering analysis of the activity data allowed the enzymes to be classified into five distinct groups based on substrate specificity. The substrates can also be grouped such that one can be a "surrogate" for others in the group. Fusion of a functional chimeric heme domain with a parental reductase domain always reconstituted a functional holoenzyme, indicating that key interdomain interactions are conserved upon reductase swapping.     

Intramolecular Hydrogen Bonding Restricts Gd–Aqua-Ligand Dynamics

Aqua ligands can undergo rapid internal rotation about the M−O bond. For magnetic resonance contrast agents, this rotation results in diminished relaxivity. Herein, we show that an intramolecular hydrogen bond to the aqua ligand can reduce this internal rotation and increase relaxivity. Molecular modeling was used to design a series of four Gd complexes capable of forming an intramolecular H-bond to the coordinated water ligand, and these complexes had anomalously high relaxivities compared to similar complexes lacking a H-bond acceptor. Molecular dynamics simulations supported the formation of a stable intramolecular H-bond, while alternative hypotheses that could explain the higher relaxivity were systematically ruled out. Intramolecular H-bonding represents a useful strategy to limit internal water rotational motion and increase relaxivity of Gd complexes.     

The perimeter of words

We define $\left[k\right]=\{1,2,\dots ,k\}$ to be a (totally ordered) alphabet on $k$ letters. A word $w$ of length $n$ on the alphabet $\left[k\right]$ is an element of ${\left[k\right]}^n$. A word can be represented by a bargraph (i.e., by a column-convex polyomino whose lower edges lie on the $x$-axis) in which the height of the $i$th column equals the size of the $i$th part of the word. Thus these bargraphs have heights which are less than or equal to $k$. We consider the perimeter, which is the number of edges on the boundary of the bargraph. By way of Cramer’s method and the kernel method, we obtain the generating function that counts the perimeter of words. Using these generating functions we find the average perimeter of words of length $n$ over the alphabet $\left[k\right]$. We also show how the mean and variance can be obtained using a direct counting method.     

A robust approach for finding all well-separated solutions of sparse systems of nonlinear equations

Tearing is a long-established decomposition technique, widely adapted across many engineering fields. It reduces the task of solving a large and sparse nonlinear system of equations to that of solving a sequence of low-dimensional ones. The most serious weakness of this approach is well-known: It may suffer from severe numerical instability. The present paper resolves this flaw for the first time. The new approach requires reasonable bound constraints on the variables. The worst-case time complexity of the algorithm is exponential in the size of the largest subproblem of the decomposed system. Although there is no theoretical guarantee that all solutions will be found in the general case, increasing the so-called sample size parameter of the method improves robustness. This is demonstrated on two particularly challenging problems. Our first example is the steady-state simulation a challenging distillation column, belonging to an infamous class of problems where tearing often fails due to numerical instability. This column has three solutions, one of which is missed using tearing, but even with problem-specific methods that are not based on tearing. The other example is the Stewart–Gough platform with 40 real solutions, an extensively studied benchmark in the field of numerical algebraic geometry. For both examples, all solutions are found with a fairly small amount of sampling.     

Synthesis,Crystal Structure,and Characterization of Two Novel One‐ and Two‐Dimensionally Polymeric Copper(II) Phosphonoacetates

Blue single crystals of Cu[μ₂‐OOC(CH₂)PO₃H] · 2H₂O ( 1 ) and Cu_1.5[μ₃‐OOC(CH₂)PO₃] · 5H₂O ( 2 ) were prepared in aqueous solution. In compound 1 [space group C2/c (no. 15) with a = 1623.3(2), b = 624.0(1), c = 1495.5(2) pm, β = 122.45(1)°], Cu is coordinated by three oxygen atoms stemming from the hydrogenphosphonoacetate dianion and three water molecules to form a distorted octahedron. The Cu–O bonds range from 190.4(3) to 278.5(3) pm. The connection between the Cu²⁺ cations and the hydrogenphosphonoacetate dianions leads to a two‐dimensional structure with layers parallel to (101). The layers are linked by hydrogen bonds. In compound 2 [space group P1 (no. 2) with a = 608.2(1), b = 800.1(1), c = 1083.6(1) pm, α = 94.98(1)°, β = 105.71(1)°, γ = 109.84(1)°], two crystallographically independent Cu²⁺ cations are coordinated in a square pyramidal and an octahedral fashion, respectively. The Cu–O bonds range from 192.9(2) to 237.2(2) pm. The coordination of the phosphonoacetate trianion to Cu(1) results in infinite polyanionic chains parallel to [100] with a composition of {Cu(H₂O)[OOC(CH₂)PO₃]}_n^n–. Hydrated Cu(2) cations are accommodated between the chains as counterions. 1 and 2 show structural features of cation exchangers. Magnetic measurements reveal a paramagnetic Curie‐Weiss behavior. Compound 2 shows antiferromagnetic coupling between Cu²⁺ ions due to a super‐superexchange coupling. The UV/Vis spectra of 1 suggest three d–d transition bands at 763 nm (²B₁ → ²E), 878 nm (²B₁ → ²B₂), and 1061 nm (²B₁ → ²A₁). Thermoanalytical investigations in air show that compound 1 is stable up to 165 °C, whereas decomposition of 2 begins at 63 °C.     

Multiporphyrin coordination arrays based on complexation of magnesium(II) porphyrins with porphyrinylphosphine oxides

Di- and triporphyrin arrays consisting of 5,15-diphenylporphyrinatomagnesium(II) (MgDPP) coordinated to free-base and Ni(II) porphyrinyl mono- and bis-phosphine oxides, as well as the self-coordinating diphenyl[10,20-diphenylporphyrinatomagnesium(II)-5-yl]phosphine oxide [MgDPP(Ph(2)PO)], were synthesised in excellent yields and characterised by various spectroscopic techniques. Phosphine oxides stabilise Mg(II) coordination to porphyrins and the resulting complexes have convenient solubilities, while the Ni(II) complexes exhibit interesting intramolecular fluorescence quenching behaviour. The binding constant of MgDPP to triphenylphosphine oxide (5.3 +/- 0.1 x 10(5) M(-1)) and the very high self-association constant of [MgDPP(Ph(2)PO)] (5.5 +/- 0.5 x 10(8) M(-1)) demonstrate the strong affinity of phosphine oxides towards Mg(II) porphyrins. These complexes are the first strongly bound synthetic Mg(II) multiporphyrin complexes and could potentially mimic the "special pair" in the photosynthetic reaction centre.     

Molecular conformations in a phospholipid bilayer extracted from dipolar couplings: a computer simulation study

This paper describes an analysis of NMR dipolar couplings in a bilayer formed by dimyristoylphosphatidylcholine (DMPC). The couplings are calculated from a trajectory generated in a molecular dynamics (MD) simulation based on a realistic atom-atom interaction potential. The analysis is carried out employing a recently developed approach that focuses on the construction of the conformational distribution function. This approach is a combination of two models, the additive potential (AP) model and the maximum entropy (ME) method, and is therefore called APME. In contrast to the AP model, the APME procedure does not require an intuition-based choice of the functional form of the torsional potential and is, unlike the ME method, applicable to weakly ordered systems. The conformational distribution function for the glycerol moiety of the DMPC molecule derived from the APME analysis of the dipolar couplings is in reasonable agreement with the "true" distributions calculated from the trajectory. Analyses of dipolar couplings derived from MD trajectories can, in general, serve as guidelines for experimental investigations of bilayers and other complex biological systems.     

Unusual regioselective mercuration of metalloporphyrins and its potential applications

The reaction of Hg(CF3CO2)2 with metalloporphyrins produces mercurated porphyrins regioselectively, the reaction, surprisingly occurring at the most hindered betaB-position; this behavior is in marked contrast to the usual electrophilic substitution reactions of porphyrins, whose reactions produce meso-substituted porphyrins; the obtained mercurated porphyrins are active to transition metal-catalyzed coupling reactions, such as the Mizoroki-Heck reaction.     

Vancomycin architecture dependence on the capture efficiency of antibody-modified microbeads by magnetic nanoparticles

We show that the ability to control the architecture/orientation of vancomycin on the surface of magnetic nanoparticles has a drastic effect on the ability of the nanoparticles to magnetically confine vancomycin-antibody modified polystyrene microbeads.