RESUMO
The effects of urocanic acid (UA) on thermodynamic parameters of model dipalmitoylphosphatidylcholine (DPPC) lipid membrane have been studied by means of differential scanning calorimetry (DSC). The observed ordering effect of UA on the lipid bilayer is reflected in the increase in both the main phase transition temperature and cooperative unit size of the lipid membrane. Analysis of FTIR spectra suggests localization of UA molecules in the vicinity of the polar heads and carbonyl groups of DPPC due to electrostatic interactions and H-bonds. On the basis of experimental data obtained and geometry parameters of UA and DPPC molecules, some variants of the UA localization in DPPC bilayer were discussed.
Assuntos
1,2-Dipalmitoilfosfatidilcolina/química , Ácido Urocânico/química , Varredura Diferencial de Calorimetria , Ligação de Hidrogênio , Interações Hidrofóbicas e Hidrofílicas , Cinética , Membranas Artificiais , Modelos Moleculares , Transição de Fase , Espectroscopia de Infravermelho com Transformada de Fourier , Eletricidade Estática , Termodinâmica , Temperatura de TransiçãoRESUMO
Crystalline Lu(2)O(3):Eu(3+) nanolayers with a europium content from 1 to 10 at.% were immobilized onto amorphous SiO(2) submicrospheres by a sol-gel technique, forming monodisperse SiO(2)/Lu(2)O(3):Eu(3+) core-shell structured nanophosphors. The nanostructure, morphology and composition of the core-shell particles obtained were investigated by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and x-ray diffraction (XRD) techniques. The Lu(2)O(3) nanolayers are found to begin to crystallize on the SiO(2) cores at 500 degrees C and a solid-phase reaction between the Lu(2)O(3) and SiO(2) components occurs at 1000 degrees C. Lu(2)O(3) --> Lu(2)SiO(5) --> Lu(2)Si(2)O(7) phase transitions in the core-shell particles were observed at temperatures ranging from 1000 to 1200 degrees C. Under x-ray excitation, europium-doped core-shell nanophosphors are characterized by effective luminescence in the lambda = 575-725 nm range corresponding to (5)D(0) --> (7)F(J) transitions (J = 0-4) of Eu(3+) ions. It has been shown that the radioluminescence intensity of the heterostructures strongly depends on the annealing temperature, the number of coated layers and the dopant concentration. The radioluminescence of the SiO(2)/Lu(2)O(3):Eu(3+) heterostructures coated with a protective undoped Lu(2)O(3) shell is higher than in the case of the unprotected SiO(2)/Lu(2)O(3):Eu(3+) core-shell phosphors. The possible reasons for such a phenomenon are also discussed.