Starburst dendrimers (SBDs) of different generations (size) and level of protonations of their surface amino groups and solutions of dimyristoylphosphatidylcholine (DMPC) vesicles were analyzed, both separately and in mixtures of the two components, by negative-staining transmission electron microscopy (TEM), dynamic light scattering (DLS), and, mainly, by computer-aided continuous wave (CW)-and pulsed-electron paramagnetic resonance (EPR). For the EPR study, the SBDs were labeled with a nitroxide radical (SBD-T). TE micrographs showed the vesicles as multilamellar structures of spherical shape with diameters ranging from 0.2 to 1.2 m. DLS measurements provided the mean vesicle diameter (d) at ca. 400 nm, whereas the diameter of generation 6 was 7 nm. No large-sized permanent supramolecular structures (d > 400 nm) were formed. EPR measurements at room temperature were poorly informative, since (1) a fraction of the dendrimers was not interacting with the vesicles, and (b) the labels that were interacting with the vesicles were rotating quickly around the main axis. Interactions between the dendrimers and the vesicles (tested by a decrease in rotational mobility of the label) became EPR-observable and quantifiable below the freezing transition of a portion of the solution, which could not be detected by EPR analysis. The fraction of the dendrimers interacting with the vesicles underwent a glass transition. Dendrimer-vesicle interactions modified the direction of the fast-rotation axis of the radical, and the interaction was more effective for protonated dendrimers of a larger size, i.e., later generation. A "complex" was formed between one solvent molecule and the nitroxide radical. Interactions between the SBD-T and the vesicle partially compressed the hydration layer of the N-O group, and/or the hydration layer of the vesicle headgroups was compressed onto the unpaired-electron site. This study provides information on the possible utilization of starburst dendrimers as gene carriers.

This report describes a computer-aided CW- and pulsed-electron paramagnetic resonance (EPR) investigation on the structural modification of dimyristoylphosphatidylcoline (DMPC) vesicles, which occur upon interaction with starburst dendrimers (SBDs). Probes used for this study included doxyl-functionalized stearic acids, with the doxyl group attached at different positions of the stearic chain (5DXSA, 12DXSA, and 16DXSA). Mainly mobility and polarity parameters were evaluated from the analysis of the CW-EPR spectra, whereas the analysis of the decay and modulation of the electron spin−echo (ESE) signal provided information on the structural environment of the paramagnetic center. Due to the interaction with the SBD surface, the vesicle structure became more rigid and ordered. The enhanced rigidity of the structure also caused the tilting of the chains of about 50° with respect to the surface line. The permeability of water at the chain beginning level increased, thus increasing the rotational mobility of the probe. The perturbing effects lessened toward the end of the chains. A fraction of 16DXSA (15%) was in the bent conformation, with the chain inserted into the lipid layer and the two polar heads at the external surface. The interaction with protonated dendrimers caused the swelling of the vesicle structure. This study indicated that the bilayer structure is modified but only partially perturbed by the addition of the dendrimers, and the integrity of the vesicle, as a model cell membrane, is preserved after the interaction with the dendrimers. This is encouraging for the use of the SBDs as drug and gene carriers.

We studied in the temperature range 4-300 K the dynamics and the electron spin echo decay of the radical 4-oxo-2,2,6,6,-tetramethyl-1-piperidinyloxyl (tempone) doping substitutionally a single crystal of the diketone 2,2,4,4-tetramethyl-cyclobutan-1,3-dione. Electron nuclear double resonance (ENDOR) and high field electron paramagnetic resonance (EPR) spectra (110 GHz) displayed three types of motion: interconversion between twisted-crossover conformations, rotation of the methyl groups and libration of the radical in the crystal matrix. At room temperature all the motions are fast, and they reach the slow motion regime each at a different temperature, being all slow below 80 K. An approximate value of 9 kJ mol(-1) for the energy barrier hindering the rotation of the tempone methyl groups is estimated. At low temperature their motion is too slow to give any contribution to the echo decay. The echo decays as a linear exponential in the range 100-300 K, due to the radical motions. At T<100 K the interaction with the methyl protons of the matrix is the dominant dephasing mechanism, and the echo decays as a gaussian. At T < 20 K it depends on the square root of the time. Echo decays and electron spin echo envelope modulations (ESEEM) are simulated by using the available theoretical models. A damping of ESEEM superimposed to the echo decay is discussed

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Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Abstract. The photo-fading of crystal violet (CV), one of the earliest synthetic dyes and an ink component, is examined both in solution and on paper. Aqueous solutions of CV were exposed to UV light (365nm) and samples were taken at constant time intervals and analysed with a High Performance Liquid Chromatography-Photo Diode Array (HPLC-PDA) and Liquid Chromatography-Mass Spectroscopy (LC-MS). Demethylation products were positively identified. Also, deamination probably occurred. The oxidation at the central carbon likely generates Michler's ketone (MK) or its derivatives, but still needs confirmation. To study CV on paper, Whatman paper was immersed in CV and exposed to UV light. Before and after different irradiation periods, reflectance spectra were recorded with Fibre Optic Reflectance Spectrophotometry (FORS). A decrease in CV concentration and a change in aggregation type for CV molecules upon irradiation was observed. Colorimetric L*a*b* values before and during irradiation were also measured. Also, CV was extracted from paper before and after different irradiation periods and analysed with HPLC-PDA. Photo-fading of CV on paper produced the same products as in solution, at least within the first 100 hours of irradiation. Finally, a photo-fading of CV in the presence of MK on Whatman paper was performed. It was demonstrated that MK both accelerates CV degradation and is consumed during the reaction. The degradation pathway identified in this work is suitable for explaining the photo/fading of other dyes belonging to the triarylmethane group.

A computer aided analysis of both cw-and pulsed-Electron Paramagnetic Resonance (EPR) spectra of 5doxylstearic acid (5DXSA) as a probe was carried out to compare the aggregation process of Sodium Dodecyl Sulfate (SDS) surfactants in the absence and in the presence of Starburst Dendrimers (SBDs), and to provide information on the interactions between SDS and SBDs. Mobility and polarity parameters were extracted from the cw-EPR analysis, whereas the analysis of the Eleetron Spin Echo Envelope Modulation (ESEEM) signal provided details about the doxyl environment in the SDS mieeUes. In the absence of SBDs, the formation of SDS mieelles was revealed by the decrease in mobility of the probes inserted in the micelles. The high packing of SDS chains in the rniceUes prevented the water permeability at the doxyl site. In the presence of the dend¡ the analysis of the EPR spectra suggested the formation of SDS aggregates at the dendfimer surface (cooperative interaction). The larger the size of the dendrimers and the protonation of their surface, the stronger the interactions resulted between the SDS surfactants and the SBD surface. The analysis of the ESEEM pattern indicated that the cooperative interaction of the surfaetant with the SBD surface led to a less packed structure of the aggregates. A schematic view was proposed to describe the local structure of the doxyl group and its environment in the absenee and in the presence of the dendrimers.

In this work we address the interpretation, via an ab initio integrated computational approach, of continuous wave electron spin resonance (cw-ESR) spectra of p-(methylthio)phenyl nitronylnitroxide (MTPNN) dissolved in toluene. Our approach is based on the determination of the spin Hamiltonian, averaged with respect to fast vibrational motions, with magnetic tensor parameters (Zeeman and hyperfine tensors) characterized by quantum mechanical density functional calculations. The system is then described by a stochastic Liouville equation, with inclusion of diffusive rotational dynamics. Parametrization of diffusion rotational tensor is provided by a hydrodynamic model. Cw-ESR spectra of MTPNN are simulated for a wide range of temperatures (155-292 K) with minimal resorting to fitting procedures, proving that the combination of sensitive ESR spectroscopy and sophisticated modeling can be highly helpful in providing structural and dynamic information on molecular systems.

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