Probing P-H+-P hydrogen bonds: structures, binding energies, and spin-spin coupling constants

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of 22 open and 3 cyclic complexes formed from the sp2 [H(2)C=PH and HP=PH (cis and trans)] and sp3 [PH2(CH3) and PH3] hybridized phosphorus bases and their corresponding protonated ions. EOM-CCSD calculations have been carried out to obtain (31)P-(31)P and (31)P-(1)H coupling constants across P-H+-P hydrogen bonds. Two equilibrium structures with essentially linear hydrogen bonds have been found along the proton-transfer coordinate, except for complexes with P(CH3)H3+ as the proton donor to the sp2 bases. Although the isomer having the conjugate acid of the stronger base as the proton donor lies lower on the potential energy surface, it has a smaller binding energy relative to the corresponding isolated monomers than the isomer with the conjugate acid of the weaker base as the donor. The hydrogen bond of the latter has increased proton-shared character. All of the complexes are stabilized by traditional hydrogen bonds, as indicated by positive values of the reduced coupling constants (2h)K(P-P) and (1)K(P-H), and negative values of (1h)K(H-P). (2h)J(P-P) correlates with the P-P distance, a correlation determined primarily by the nature of the proton donor. For open complexes, (1)J(P-H) always increases relative to the isolated monomer, while (1h)J(H-P) is relatively small and negative. (2h)J(P-P) values are quite large in open complexes, but are much smaller in cyclic complexes in which the P-H+-P hydrogen bonds are nonlinear. Thus, experimental measurements of (2h)J(P-P) should be able to differentiate between open and cyclic complexes.     

Response of DNA fragments to potentiometric sensors studied using HPLC

Potentiometric sensors are studied as viable candidates for the construction of high throughput DNA arrays. For preliminary investigations, such sensors were used in an HPLC setup in the present work. This avoided errors due to ionic contaminants or additives in the commercial samples. The oligonucleotides dT(10), dT(20) and dT(30) were used as test substances. The potentiometric sensors were of the coated wire type, containing PVC, DOP, MTDDACl and a synthetic podand urea receptor. The HPLC system consisted of a reversed phase column eluted with a phosphate buffer, triethylammoniumacetate (TEAA), and an acetonitrile gradient. Molar responses and sensitivities increased with increasing chain length of oligonucleotides, yielding detection limits as low as 10(-6)M (dT(30), injected concentration). The slopes of the calibration graphs were at least 23 mV/decade (dT(10)), which was much higher than expected. The results are discussed in view of the potential use of this sensor type in high throughput microarrays.     

Photophysical behavior of Ru(II) and Os(II) terpyridyl phenylene vinylene complexes: perturbation of MLCT state by intra-ligand charge-transfer state

The photophysical behavior of novel bimetallic Ru(II) and Os(II) complexes having a bridging ligand consisting of two terpyridyl moieties covalently linked in the 4′ position through a distyrylbenzene bridge (tp vp vpt) is reported. The Ru(II) complex has a unique red emission with an excited state lifetime nearly 2000-times longer than the parent complex, [Ru(mpt)₂](PF6)₂ (mpt=4′-(methylphenyl)-2,2′,6′,2″-terpyridine). Combined spectral data suggest the presence of an emissive intra-ligand charge-transfer (ILCT) state lower in energy than the metal-to-ligand charge transfer (MLCT) state. The Os(II) complex exhibits red emission that is similar to that of the parent complex [Os(mpt)₂](PF₆)₂. However, the excited state absorption spectrum reveals a unique transient absorption in the far red that suggests perturbation of the MLCT state by the ILCT state.     

Black Bean (Phaseolus vulgaris L.) Polyphenolic Extract Exerts Antioxidant and Antiaging Potential

Phenolic compounds present in common beans (Phaseolus vulgaris L.) have been reported to possess antimicrobial, anti-inflammatory and ultraviolet radiation (UVR) protective properties. UVR from sunlight, which consists of UV-B and UV-A radiations, induces reactive oxygen species (ROS) and free radical formation, consequently activating proteinases and enzymes such as elastase and tyrosinase, leading to premature skin aging. The objective of this work was to extract, characterize and evaluate the antioxidant and antiaging potential of polyphenols from a black bean endemic variety. The polyphenolic extract was obtained from black beans by supercritical fluid extraction (SFE) using CO₂ with a mixture of water–ethanol as a cosolvent and conventional leaching with a mixture of water–ethanol as solvent. The polyphenolic extracts were purified and characterized, and antioxidant potential, tyrosinase and elastase inhibitory potentials were measured. The extract obtained using the SFE method using CO₂ and H₂O–Ethanol (50:50 v/v) as a cosolvent showed the highest total phenolic compounds yield, with 66.60 ± 7.41 mg GAE/g coat (p > 0.05) and 7.30 ± 0.64 mg C3GE/g coat (p < 0.05) of anthocyanins compared to conventional leaching. Nineteen tentative phenolic compounds were identified in leaching crude extract using ESI-QTOF. Quercetin-3-D-galactoside was identified in crude and purified extracts. The purified SFC extract showed IC₅₀ 0.05 ± 0.002 and IC₅₀ 0.21 ± 0.008 mg/mL for DPPH and ABTS, respectively. The lowest IC₅₀ value of tyrosinase inhibition was 0.143 ± 0.02 mg/mL and 0.005 ± 0.003 mg/mL of elastase inhibition for leaching purified extract. Phenolic compounds presented theoretical free energy values ranging from −5.3 to −7.8 kcal/mol for tyrosinase and −2.5 to −6.8 kcal/mol for elastase in molecular docking (in silico) studies. The results suggest that the purified extracts obtained by SFE or conventional leaching extraction could act as antioxidant and antiaging ingredients for cosmeceutical applications.     

Ab initio study of the reaction of CHO+ with H2O and NH3

An MP4(full,SDTQ)/6-311++G(d,p)//MP2(full)/6-311++G(d,p) ab initio study was performed of the reactions of formyl and isoformyl cations with H₂O and NH₃, which play an important role in flame and interstellar chemistries. Two different confluent channels were located leading to CO+H₃O⁺/NH. The first one corresponds to the approach of the neutral molecule to the carbon atom of the cations. The second one leads to the direct proton transfer from the cations to the neutrals. At 900 K the separate products CO+H₃O⁺/NH are the most stable species along the Gibbs energy profiles for the processes. For the reaction with H₂O the reaction channel leading to HC(OH) (protonated formic acid) is disfavored with respect to the two CO+H₃O⁺ channels in agreement with the experimental evidence that H₃O⁺ is the major ion observed in hydrocarbon flames. According to our calculations, NH+H₂O are considerably more stable in Gibbs energy than NH₃+H₃O⁺;NH will predominate in the reaction zone when ammonia is added to CH₄+Ar diffusion flame, as experimentally observed. At 100 K the most stable structures are the intermediate complexes CO…HOH/HNH. Particularly the CO…HOH complex has a lifetime large enough to be detected and, therefore, could play a certain role in interstellar chemistry. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1432–1443, 1999     

On the Randomized Error of Polynomial Methods for Eigenvector and Eigenvalue Estimates

In this paper we consider the problem of estimating the largest eigenvalue and the corresponding eigenvector of a symmetric matrix. In particular, we consider iterative methods, such as the power method and the Lanczos method. These methods need a starting vector which is usually chosen randomly. We analyze the behavior of these methods when the initial vector is chosen with uniform distribution over the unitn-dimensional sphere. We extend and generalize the results reported earlier. In particular, we give upper and lower bounds on the _pnorm of the randomized error, and we improve previously known bounds with a detailed analysis of the role of the multiplicity of the largest eigenvalue.     

Influence of the deacetylation degree on chitosan emulsification properties

The emulsification of sunflower oil by chitosan solutions with deacetylation degrees (DD) between 73 and 95% was studied using different techniques. The droplet size distributions, conductivity, ageing behavior and viscosity of emulsions were studied as functions of the chitosan DD. All DD gave stable polydisperse water-in-oil-in-water emulsions with different viscosities. Two optimum DD values were found, 81 and 88%, giving complete emulsification without residual oil or sedimentation. Chitosans with intermediate DD were less effective emulsifiers. Chitosans with higher DD gave poor emulsification. Received: 11 January 1999 Accepted in revised form: 31 May 1999     

Approximating Rings with Local Units Via Automorphisms

For a ring A with local units we investigate unital overrings T of A, and compare the automorphism groups Aut (A) and Aut (T).     

Charge transfer between a molecule and an infinite electron reservoir in the MO scheme

A quantitative reference for comparing molecules (and sites of molecules) likely to exchange electrons with a molecular or crystalline partner can be obtained by coupling such molecules to a model electron reservoir with a continuous energy spectrum. In this study the required procedure is defined and mathematically formulated, within the standard orbital scheme. The reservoir is modeled as an infinite linear chain of atoms with one atomic orbital whose coupling is larger than the largest half-bandwidth of the molecules to be compared, coupled by a suitable bond parameter to a specific site of the molecule. The resulting level broadening and local density of states (DOS) are determined using Dyson's equation to get a tractable expansion of the Green function of the molecule-reservoir system. An example of application and a brief discussion of the dependence of the results on the type of Hamiltonian utilized are given. © 1996 John Wiley & Sons, Inc.