Hydrophile-lipophile balance of alkyl ethoxylated surfactants as a function of intermolecular energies

Summary The semiempirical MNDO method has been used in order to examine the variation of the molecular properties of hydrocarbons C_nH_2n+2 (with 1n19) and ethylene oxide chains CH₃(CH₂CH₂O)_mCH₃ (with 1m19) as a function of their molecular length. Least-square fits of those properties have been calculated, along with two mathematical relations between the hydrophile-lipophile balance of alkyl-phenol ethoxylated surfactants and (1) the ratio of molecular lengths between their lipophilic and hydrophilic branches; (2) the intermolecular energies between the molecules of surfactant, water and hexane.     

Cyclodextrins as hosts for poorly water-soluble compounds in enzyme catalysis

The capability of cyclodextrins to enhance greatly the solubility in water of poorly water-soluble substances makes them an ideal alter native for investigating the expression of enzyme activity with such substrates in aqueous solution. This capability is demonstrated by using soybean lipoxygenase with linoleic acid/β-cyclodextrin and di ethylstilbestrol/γ-cyclodextrin, and cholesterol oxidase with choles terol/methyl-β-cyclodextrin.     

Flavonoid separation by capillary electrophoresis. Effect of temperature and pH

Summary The use of capillary electrophoresis for the analysis of selected flavonols present in fruit juices and wines (kaempferol-3-rutinoside, rutin, avicularin, quercitrin, isoquercitrin, isorhamnetin, kaempferol and quercetin) was explored, and the effect of pH and temperature on the separation studied. The method had good reproducibility and analyses were carried out in less than 10 minutes.     

MEPSIM: A computational package for analysis and comparison of molecular electrostatic potentials

Summary MEPSIM is a computational system which allows an integrated computation, analysis, and comparison of molecular electrostatic potential (MEP) distributions. It includes several modules. Module MEPPLA supplies MEP values for the points of a grid defined on a plane which is specified by a set of three points. The results of this program can easily be converted into MEP maps using third-parties graphical software. Module MEPMIN allows to find automatically the MEP minima of a molecular system. It supplies the cartesian coordinates of these minima, their values, and all the geometrical relationships between them (distances, angles, and dihedral angles). Module MEPCOMP computes a similarity coefficient between the MEP distributions of two molecules and finds their relative position that maximizes the similarity. Module MEPCONF performs the same process as MEPCOMP, considering not only the relative position of both molecules but also a conformational degree of freedom of one of them. The most recently developed module, MEPPAR, is another modification of MEPCOMP in order to compute the MEP similarity between two molecules, but only taking into account a particular plane. The latter module is particularly useful to compare MEP distributions generated by systems of aromatic rings. MEPSIM can use several wavefunction computation approaches to obtain MEP distributions. MEPSIM has a menu type interface to simplify the following tasks: creation of input files from output files of external programs (GAUSSIAN and AMPAC/MOPAC), setting the parameters for the current computation, and submitting jobs to the batch queues of the computer. MEPSIM has been coded in FORTRAN and its current version runs on VMS/VAX computers.     

Natural lycopene from Blakeslea trispora: all-trans lycopene thermochemical and structural properties

Structural and thermal properties of all-trans lycopene are described in the present paper. Different crystalline lycopene samples obtained from fermentation process and recrystallized lycopene were analyzed. Structural properties (NMR, mass spectrometry and powder X-ray diffraction) of lycopene were clarified with recent techniques. High purity sample analysis by differential scanning calorimetry was used to study thermal behavior of pure lycopene and with traces of isomers. But also this was correlated with HPLC method for determine lycopene purity and isomers in low proportion.     

Reaction’s mechanism of fresh apple juice enzymatic browning in the presence of maltosyl-β-cyclodextrin

Polyphenol oxidase causes enzymatic browning by catalyzing natural phenolic compounds to quinones. Subsequent nonenzymatic oxidative polymerization reactions form brown, black, or red pigments (melanins). Many methods can be shown experimentally to diminish or prevent browning but are not all applicable for food use. In this work, the colour of fresh apple juice was evaluated in the presence of maltosyl-β-CD, compound that complex polyphenol oxidases substrates. The effectiveness of CD as browning inhibitor was determined by the difference between the colours observed in the CD-treated sample and the controls, using the colour space CIE- L, a*, b* system. We have kinetically modelled apple juice enzymatic browning in the absence and presence of maltosyl-β-CD. The complexation constant between the mixtures of diphenols present in apple juice and maltosyl-β-CD was calculated (K_c = 4.09 mM⁻¹).     

Electrostatic strain and concerted motions in the transient complex between plastocyanin and cytochrome f from the cyanobacterium Phormidium laminosum

Many fleeting macromolecular interactions, like those being involved in electron transport, are essential in biology. However, little is known about the behaviour of the partners and their dynamics within their short-lived complex. To tackle such issue, we have performed molecular dynamics simulations on an electron transfer complex formed by plastocyanin and cytochrome f from the cyanobacterium Phormidium laminosum.Besides simulations of the isolated partners, two independent trajectories of the complex were calculated, starting from the two different conformations in the NMR ensemble. The first one leads to a more stable ensemble with a shorter distance between the metal sites of the two partners. The second experiences a significant drift of the complex conformation. Analyses of the distinct calculations show that the conformation of cytochrome f is strained upon binding of its partner, and relaxes upon its release. Interestingly, the principal component analysis of the trajectories indicates that plastocyanin displays a concerted motion with the small domain of cytochrome f that can be attributed to electrostatic interactions between the two proteins.     

Theoretical approach to microwave-radiation-induced zero-resistance states in 2D electron systems

We present a theoretical model in which the existence of radiation-induced zero-resistance states is analyzed. An exact solution for the harmonic oscillator wave function in the presence of radiation, and a perturbation treatment for elastic scattering due to randomly distributed charged impurities, form the foundations of our model. Following this model most experimental results are reproduced, including the formation of resistivity oscillations, their dependence on the intensity and frequency of the radiation, temperature effects, and the locations of the resistivity minima. The existence of zero-resistance states is thus explained in terms of the interplay of the electron microwave-driven orbit dynamics and the Pauli exclusion principle.