Communication: Generalizing Rosenfeld's excess-entropy scaling to predict long-time diffusivity in dense fluids of Brownian particles: from hard to ultrasoft interactions

Computer simulations are used to test whether a recently introduced generalization of Rosenfeld's excess-entropy scaling method for estimating transport coefficients in systems obeying molecular dynamics can be extended to predict long-time diffusivities in fluids of particles undergoing Brownian dynamics in the absence of interparticle hydrodynamic forces. Model fluids with inverse-power-law, Gaussian-core, and Hertzian pair interactions are considered. Within the generalized Rosenfeld scaling method, long-time diffusivities of ultrasoft Gaussian-core and Hertzian particle fluids, which display anomalous trends with increasing density, are predicted (to within 20%) based on knowledge of interparticle interactions, excess entropy, and scaling behavior of simpler inverse-power-law fluids.     

Protonolysis of [((i)PrO)TiMo5O18]3-: access to a family of TiMo5 Lindqvist type polyoxometalates

The tetra-n-butylammonium (TBA) salts of [((i)PrO)TiMo(5)O(18)](3-) 1 and [((i)BuO)TiMo(5)O(18)](3-) 2 were prepared by hydrolysis of mixtures of (TBA)(2)[Mo(2)O(7)], (TBA)(4)α-[Mo(8)O(26)] and Ti(OR)(4) (R = (i)Pr or (i)Bu) in acetonitrile. Treatment of (TBA)(3)1 with alcohols ROH afforded primary and tertiary alkoxide derivatives [(RO)TiMo(5)O(18)](3-) (R = Me 3, (t)Bu 4), whilst aryloxides [(ArO)TiMo(5)O(18)](3-) were prepared by reacting 1 with phenols ArOH (Ar = C(6)H(4)Me-4 5, and C(6)H(4)CHO-2 6). Oxo-bridged [(μ-O)(TiMo(5)O(18))(2)](6-)7 rather than the hydroxo derivative [(HO)TiMo(5)O(18)](3-) was obtained upon hydrolysis of 1. X-Ray crystal structures of TBA salts of anions 3-7 show that titanium is six-coordinate in all cases, although titanium sites are disordered over two trans positions in 3. Mo-O bond length alternation is observed in the Mo(4)O(4) planes of 4 and 7 and in one of the two independent anions in the structure of 3. In solution, (17)O NMR spectra are consistent with the higher anionic charge compared to [Mo(6)O(19)](2-) and reveal an order of basicity for the anions [LM'Mo(5)O(18)](3-) associated with the ability of {LM'}(3+) to donate/withdraw electron density from {Mo(5)O(18)}(6-). Protonolysis reactions of 1 and 3 were slower than for tungstate analogues and the possibility of initial protonation at TiOM (M = Mo) rather than TiOR (M = W) in a proton-assisted S(N)1 mechanism for ligand exchange in [(RO)TiM(5)O(18)](3-) is discussed.     

Quantum Monte Carlo study of the CO interaction with a dimer model surface for Cr(110)

The chemisorption of CO on a Cr (110) surface is investigated using the quantum Monte Carlo method in the diffusion Monte Carlo (DMC) variant and a model Cr₂CO cluster. The present results are consistent with the earlier ab initio HF study with this model that showed the tilted/near-parallel orientation as energetically favoured over the perpendicular arrangement. The DMC energy difference between the two orientations is larger (1.9 eV) than that computed in the previous study. The distribution and reorganization of electrons during CO adsorption on the model surface are analysed using the topological electron localization function method that yields electron populations, charge transfer and clear insight on the chemical bonding that occurs with CO adsorption and dissociation on the model surface.     

Characterization of aromatic-thiol π-type hydrogen bonding and phenylalanine-cysteine side chain interactions through ab initio calculations and protein database analyses

In this study, the aromatic-thiol π hydrogen bonding and phenylalanine-cysteine side chain interactions are characterized through both molecular orbital calculations on a C₆H₆-HSCH₃ model complex and database analyses of 609 X-ray protein structures. The aromatic-thiol π hydrogen bonding interaction can achieve a stabilization energy of 2.60 kcal mol^?1, and is stronger than the already documented aromatic-hydroxyl and aromatic-amino hydrogen bonds. However, the occurrence of the aromatic-thiol hydrogen bond is rather rare in proteins. This is because most of the thiol groups participate in the formation of either disulphide bonds or stronger S—H…O (or N) ‘normal’ hydrogen bonds in a protein environment. Interactions between the side chains of phenylalanine and cysteine residues are characterized as the phenyl(Phe)(HSCH₂-)(Cys) interaction. The bonding energy for such interactions is approximately 3.71 kcal mol-¹ and is achieved in a geometric arrangement with an optimal phenyl(Phe)-(HS-)(Cys) π-type hydrogen bonding interaction. The interaction is very sensitive to the orientation of the two lone electron pairs on the sulphur atom relative to the π electron cloud of the phenyl ring. Accordingly, the interaction configurations that can accomplish a significant bonding energy exist only within a narrow configurational space. The database analysis of 609 experimental X-ray protein structures demonstrates that only 268 of the 1620 cysteine residues involve such phenylalanine-cysteine side chain interactions. Most of these interactions occur in the form of π (aromatic)-lone pair(sulphur) attractions, and correspond to a bonding energy less than 1.5 kcal mol^?1. A few were identified as the aromatic-thiol hydrogen bond with a bonding energy of 2.0–3.6 kcal mol^?1.     

A Non-linear Singular Perturbation Problem Arising in the Study of Chemical Flow Reactors

This paper presents a method of obtaining the complete asymptoticsolution of boundary value problems of the form for x [0,1] where b(x) is strictly positive andfor small and positive. Physically, the problem arises in determiningthe steady-state concentration of a substance in a chemicalflow reactor. A "two-variable" expansion procedure is used.     

DISTRIBUTIONS OF FREE RADICALS AMONG AMINO ACIDS FROM LYOPHILIZED ULTRAVIOLET-IRRADIATED PROTEINS

Abstract— The effects of u.v.-irradiation at 254 nm upon lyophilized ribonuclease, lysozyme, insulin, and chymotrypsinogen have been investigated by electron spin resonance (ESR). enzymatic assay, and labeling of free radical sites with tritiated hydrogen sulfide (HST). The ESR signal of the irradiated protein diminishes on exposure to HST, and tritium becomes covalently bound to carbon. The distribution of tritium among the amino acids of each protein. studied as an indicator of the carbon free radical distribution, differs markedly from those observed previously to result from exposure to gamma radiation, electrical discharge. or hydrogen atoms. However, the earlier observation that the tritium distribution is influenced by protein conformation holds true as well for u.v.-irradiation. Moreover, the distributions of tritium among the amino acids of u.v.-irradiated proteins indicate a broad scattering of free radicals. Tyrosine and phenylalanine, residues that absorb light energy in the region of the wavelength employed, are not particularly important as radical carriers. Thus, for ribonuclease, these residues incorporated 3.8 and 1.5 per cent of the total tritium, but they absorb 51 and 12 per cent of the light, respectively. These results, together with the observed low recoveries of methionine, an amino acid that does not absorb at 254 nm, add weight to the concept that a migration of energy ensues after the initial absorption of light energy and that photolytic damage may thus be due to destruction of amino acids other than those initially absorbing the u.v.-radiation.     

2-D soft ferromagnet based on [Wv(CN)8]3- and CuII with a Tc of 34 K

Self-assembly of [Cu(tetren)]2+ (tetren = tetraethylenepentamine) and [W(CN)8]3- in acidic aqueous solution yields the double-layered square grid cyanide-bridged polymer of ((tetrenH5)0.8CuII4[Wv(CN)8](4).7.2H2O)n with Cu(II) centres of square pyramidal geometry coordinatively saturated solely by CN bridges supplied by five [W(CN)8]3- ions; it exhibits soft ferromagnetic behaviour with an ordering temperature Tc of 34 K.