Polarization effects on peptide conformations at water–membrane interface by molecular dynamics simulation

The electrostatic image method was applied to investigate the conformation of peptides characterized by different hydrophobicities in a water–membrane interface model. The interface was represented by a surface of discontinuity between two media with different dielectric constants, taking into account the difference between the polarizabilities of the aqueous medium and the hydrocarbon one. The method consists of a substitution of the real problem, which involves the charges and the induced polarization at the surface of discontinuity, by a simpler problem formed with charges and their images. The electric field due to the polarization induced at the surface by charge q was calculated using a hypothetical charge q′ (image of q), symmetrically located on the opposite side of the surface. The value of q′ was determined using the appropriate electrostatic boundary conditions at the surface. By means of this procedure, the effect of the interface can be introduced easily in the usual force field. We included this extension in the computational package that we are developing for molecular dynamics simulations (Thor ). The peptides studied included hydrophilic tetraaspartic acid (Asp–Asp–Asp–Asp), tetralysine (Lys–Lys–Lys–Lys), hydrophobic tetrapeptide (His–Phe–Arg–Trp), an amphiphilic fragment of β-endorphin, and the signal sequence of the E. coli λ-receptor. The simulation results are in agreement with known experimental data regarding the behavior of peptides at the water–membrane interface. An analysis of the conformational dynamics of the signal sequence peptide at the interface was performed over the course of a few nanoseconds. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 971–982, 1999     

Goldbug variations

This column is a place for those bits of contagious mathematics that travel from person to person in the community, because they are so elegant, suprising, or appealing that one has an urge to pass them on. Contributions are most welcome.     

Synthesis of new amphiphilic chlorin derivatives from protoporphyrin-IX dimethyl ester

A simple synthesis of new amphiphilic chlorin derivatives from protoporphyrin-IX dimethyl ester is reported. The preparation of such compounds is based in a straightforward methodology, which involves the Diels-Alder reaction of protoporphyrin-IX dimethyl ester with maleic anhydride followed by addition of nucleophilic species to the initially formed cycloadducts, a transformation, which is highly regioselective. Preliminary photophysical studies with the new compounds show that they meet adequate features for PDT applications.     

Baryon resonances and polarization transfer in hyperon photoproduction

A partial-wave analysis of data on photoproduction of hyperons including single and double polarization observables is presented. The large spin transfer probability reported by the CLAS Collaboration can be successfully described within an isobar partial-wave analysis.     

Josephson effect in nuclear reactions

The transfer of Cooper pairs during the scattering of two superfluid nuclei just below the Coulomb barrier is discussed on a phenomenological and a microscopic level. This transfer is analogous to the Josephson current between two superconductors. Candidates for the observation of such an effect are heavy ions with a small difference in mass number. Numerical estimates are presented.     

On the validity of the random phase approximation for pairing vibrations of closed shell nuclei

Energy loss measurements on solid xenon applying 51 keV-electrons are reported. The spectrum shows excitonic structure and a plasma loss. From the energy loss function —Im1/g3 optical constants are calculated by means of a Kramers-Kronig transform. They are compared with optical absorption spectra and found to be in good agreement. An appendix gives detailed information about the calculation of —Im 1/g3 from energy loss measurements.     

Definition of a multicenter bond index

The tensor character of the first-order density matrix leads to the definition of an MO multicenter bond index for closed-shell systems. It is here applied to three-center bonds. Satisfactory results are obtained for compounds involving secondary bonds, strong and normal hydrogen bonds; the index for the peptide bond is found to be similar to that of strong hydrogen bonds.On leave of absence from Inst. de Física, UFBA, 40000 Salvador, BA, Brasil.     

Dynamical treatment of binding,polarization and recoil in asymmetric ion-atom collisions

The cross sections for quenching the lowestn ² P states of the alkali atoms Li, Na, K., and Rb by the inert gases He, Ne, Ar, Kr, and Xe are presented for 5 eV≦E _c.m.≦ 100 eV. These cross sections are derived from the corresponding cross sections for collisional excitation by applying the principle of microreversibility. Upper estimates for the quenching cross sections at thermal energies are given; in all studied cases the quenching cross sections are <8·10^?3Ų. These new upper limits are in most cases much lower than those obtained from other methods previously.     

Validation of Photodynamic Action via Photobleaching of a New Curcumin-Based Composite with Enhanced Water Solubility

Motivated by the photochemical and photophysical properties of curcumin-based composites, the characteristics of a new curcumin-based water-soluble salt were investigated via absorption and fluorescence spectroscopy. Photobleaching was investigated using a set of LEDs in three different wavelengths (405 nm, 450 nm and 470 nm) to illuminate an aqueous solution of curcumin, evaluating its degradation for five different exposure times (0, 5, 15, 45 and 105 minutes). The results were compared with equivalent measurements of dark degradation and illumination in the presence of a singlet-oxygen quencher. Three solution concentrations (50, 100 and 150 μg/ml) were studied. To measure the fluorescence, it was used low power 405 nm excitation laser source. Time dependent photodegradation of curcumin was observed, as compared to the natural degradation of samples maintained on a dark environment. Two main absorption peaks were detected and their relation responded to both concentration and wavelength of the illumination source. A spectral correlation between absorption of curcumin and the emission bands of the sources showed an optimal spectral overlap for the 450 nm LED. For this source, photobleaching showed a less intense degradation on the presence of singlet oxygen quencher. This last result confirmed singlet oxygen production in vitro, indicating a strong potential of this composite to be used as a blue-light-activated photosensitizer.