Pegylation – in search of balance and enhanced bioavailability

In the process of finding better therapeutics, thousands of new molecules are synthesised every day. Many of these can be poorly soluble in water, leading to a potentially promising drug being rejected during testing due to its poor solubility. Polyethylene glycol (PEG) has become known as an excellent modification to remedy this and was initially used to increase circulation time and reduce the immunogenicity of therapeutic proteins. Thus significantly increasing their safety and range of use. Another group of compounds in which significant benefits of pegylation have been seen are photosensitisers. Used in photodynamic therapy, they are often characterised by very high hydrophobicity. Pegylation of their structure significantly increases their affinity for cancer cells and facilitates their penetration through cell membranes. Classical small-molecule drugs can benefit from temporary combinations hydrolysed in the body or very short PEG chains. This approach allows a significant increase in the bioavailability of the drug while avoiding the disadvantages of small molecule pegylation. However, the most common motive for pegylation recently is the creation of drug carriers. Liposomes and nanoparticles make it possible to exploit the advantages of PEG to stabilise their structure and increase circulation time while not modifying the structure of the active compound. Unfortunately, PEGs also have their drawbacks. The first is their high molecular weight range, especially for longer chains, which poses difficulties in purification. Another is the emergence of antibodies directed against PEG. Nevertheless, pegylation is still an up-and-coming method for modifying pharmaceutically active molecules

{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure

EXAFS Studies of Zn1-xMnxS Ternary Compounds

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Anharmonicity and structural phase transition in the Mott insulator Cu2_2P2_2O7_7

Ab initio investigations of structural, electronic, and dynamical properties of the high-temperature $\beta$ phase of copper pyrophosphate were performed using density functional theory. The electronic band structure shows the Mott insulating state due to electron correlations in copper ions. By calculating phonon dispersion relations, the soft mode at the A point of the Brillouin zone was revealed, showing the dynamical instability of the $\beta$ phase at low temperatures. The double-well potential connected with the soft mode is derived and the mechanism of the structural phase transition to the $\alpha$ phase is discussed. The self-consistent phonon calculations based on the temperature-dependent effective potential show the stabilization of the $\beta$ phase at high temperatures, due to the anharmonic effects. The pronounced temperature dependence and the large line width of the soft mode indicate an essential role of anharmonicity in the structural phase transition

Ab initio and nuclear inelastic scattering studies of Fe3_3Si/GaAs heterostructures

The structure and dynamical properties of the Fe$_3$Si/GaAs(001) interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the Fe$_3$Si/GaAs multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the Fe$_3$Si crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk Fe$_3$Si. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the Fe$_3$Si layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meVComment: 14 pages, 9 figures, 4 table

Structure and elastic properties of Mg(OH)2_2 from density functional theory

The structure, lattice dynamics and mechanical properties of the magnesium hydroxide have been investigated with static density functional theory calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to the temperature of at least 150 K and show that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30K

Influence of anharmonicity on the negative thermal expansion of α−Sn\alpha-Sn

The lattice vibrational properties of $\alpha-Sn$ (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, $\alpha-Sn$ exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at $\sim 27K$ and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in $\alpha-Sn$ is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of $\alpha-Sn$