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We present a combined theoretical (molecular dynamics, MD) and experimental (differential scanning calorimetry, DSC) study of the effect of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl chain-labeled fluorescent phospholipid analogs (C6-NBD-PC and C12-NBD-PC) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. DSC measurements reveal that <1 mol% of NBD-PC causes elimination of the pre-transition and a large loss of cooperativity of the main transition of DPPC. Labeling with C6-NBD-PC or C12-NBD-PC shifts the main transition temperature to lower or higher values, respectively. Following our recent report on the location and dynamics of these probes (BBA 1768 (2007) 467-478) in fluid phase DPPC, we present a detailed analysis of 100-ns MD simulations of systems containing either C6-NBD-PC or C12-NBD-PC, focused on their influence on several properties of the host bilayer. Whereas most monitored parameters are not severely affected for 1.6 mol% of probe, for the higher concentration studied (6.2 mol%) important differences are evident. In agreement with published reports, we observed that the average area per phospholipid molecule increases, whereas DPPC acyl chain order parameters decrease. Moreover, we predict that incorporation of NBD-PC should increase the electrostatic potential across the bilayer and, especially for C12-NBD-PC, slow lateral diffusion of DPPC molecules and rotational mobility of DPPC acyl chains.

To study the influence of PEO on the relaxation process of PMMA, the real and imaginary parts of the permittivity, ′ and ′′, of PEO/PMMA blends (with PEO contents up to 25 wt %) were measured as a function of frequency and temperature, and the results obtained were compared with those of pure PMMA. It is shown that the presence of PEO broadens the relaxation process of PMMA and increases its relaxation strength. The lower T g component (PEO) has a plasticizing effect on the relaxation of PMMA, for the blends with 10 and 15% PEO. The splitting of the Rβ process of PMMA seems to be affected by the presence of the second component, being the onset of the R relaxation shifted to lower temperatures with the increase of PEO concentration. The DSC thermograms show a single glass transition temperature, which is displaced to lower temperatures with the increase of PEO content, and no evidence of crystallinity was found. The presence of a singular broad transition in the 20% PEO blend, evidenced in both dielectric and calorimetric experiments, may be associated with the merging of the R and β process and to concentration fluctuations, confirming that this blend lies in the border of the miscibility/immiscibility region.