Agneta Caragheorgheopol

Spin-labelled Au nanoparticles

A series of Au nanoparticles functionalised with nitroxide spin labels has been prepared and studied by EPR spectroscopy. Samples with low coverage of the spin label were used to investigate the dynamics of the surface-attached labels at different distances from the Au surface. The rotational correlation times of spin labels vary from 10(-10) s to more than 3 x 10(-9) s, depending on the chain length of the label and the surrounding ligand. The samples with higher coverage of the spin label show an increasing contribution of the exchange interaction between nitroxides adsorbed in a close proximity to each other on the same nanoparticle. Quantitative analysis of the EPR spectra of these samples suggests the presence of non-equivalent binding sites on the surface of Au nanoparticles. Additionally, EPR signals of isolated radical pairs were observed at intermediate coverage.

Aggregation numbers and microstructure characterization of self-assembled aggregates of poly(ethylene oxide) surfactants and related block-copolymers, studied by spectroscopic methods.

This review presents typical examples of micellar size measurements and structural characterization of self-assembled systems as determined with spectroscopic methods, selected from our representative results accumulated in recent years of systematic studies. The choice of examples has aimed at emphasizing the potentiality of the methods used in the study of poly(ethylene oxide) (PEO) surfactants. By using the time-resolved fluorescence quenching method aggregation numbers have been determined for direct and reverse micelles as well as for micelle-like clusters formed in (bio)polymer-surfactant solutions. By using specific spectral parameters of a variety of properly chosen molecular probes [fluorescence, UV-VIS and electron spin resonance (ESR) spin probes], problems such as hydration degree and profile of the PEO chains, local viscosity, ordering and packing of surfactant chains have been addressed. The intention was to correlate results obtained from different spectroscopic methods and to refer all polarity data to a common scale, such as Kosowers polarity factor.

Structured mesoporous organosilicas from an acetonitrile–water template system

The synthesis of templated mesoporous silicas with a narrow pore size distribution can be achieved in a water/acetonitrile/n-dodecylamine system. The benefits of using this system compared to aqueous ethanol to prepare silicas are the much enhanced wall thickness, leading to greater structural stability, as well as a lack of co-surfactancy from acetonitrile, which means that the pore size remains constant over a wide range of solvent compositions. The particles obtained from this route are cylindrical, with partially aligned mesopores, in contrast to those obtained from aqueous ethanol, which are spherical and have a wormhole structure. A further major advantage is that the amine-functionalised materials, formed by direct co-condensation, are nucleophilic, in contrast to those prepared from aqueous ethanol. This latter feature allows for the elaboration of these amine functions to attach a range of more complex functionality, examples of which are given.

NMR and ESR investigations of the interaction between a carboxylic acid and an amine at the focal point of L-lysine based dendritic branches

This paper reports the characterisation of supramolecular complexes formed between the carboxylic acid group at the focal point of host dendritic branches based on l-lysine building blocks and an amine group on an appropriate guest molecule. (1)H NMR titration investigations indicate that the interaction is relatively weak. Interestingly the dendritic generation appears to have no effect on the thermodynamics of benzylamine recognition - in contrast to previous studies in which charged guests have been bound to dendritic hosts. Control experiments using dendritic branches in which the carboxylic acid is protected as a methyl ester indicate that there is only a small amount of non-specific binding of the amine functionalised guest molecule within the dendritic framework itself. ESR investigations clearly show the binding between the dendritic branch and amine functionalised TEMPO radicals. Most interestingly, rotational correlation times can be determined from the ESR studies and they indicate that the mobility of the TEMPO radical is diminished on binding to the dendritic branch. Notably this effect is generation dependent, with larger dendritic branches having a more dramatic effect on the tumbling of the radical. Control experiments clearly prove the importance of the acid-base interaction and also demonstrate that effective binding only occurs in non-polar solvents. These results therefore illustrate that using host-guest chemistry at the focal point of a dendritic structure is an effective way to control and modify the solution phase properties and mobility of active species such as radicals.

Polarity profiles in reverse micelles of Triton X-100, as studied by spin probe and absorption probe techniques

The microenvironment characteristics — polarity, viscosity and order degree - in Triton X-100 reverse micelles (RM) in cyclohexane and benzene — n-hexane 30 : 70 (v/v), were investigated with the aid of a homologous series of cationic spin probes, CAT n, of 5- and 16-doxyl stearic acids and of a new absorption probe, 4-nitropyridine-N-oxide (NP). The spectral parameters were related to local hydration values by means of a series of poly(oxyethylene) (PEO)/water calibration mixtures. All results regarding the polarities in RM and calibration mixtures have been expressed in terms of Kosowers Z values, by determining the linear dependence of the transition energies of NP, ENP, on Z, for a series of polar solvents: Z = 3.008ENP − 178.4 (in kcal mol−1). This way the relative radial positioning of all probes was established and could be compared with those of 1-methyl-8-oxyquinolinium betaine (QB) and methyl orange (MO), previously used in the same systems. A continuous variation of the polarity in the RM was evidenced with different probes, from the most polar region in the center, with the polarity of ethanol/water mixtures (CAT 1, CAT 4 and QB) to a region corresponding to tetraethylene glycol (TG)/water mixtures and triethylene glycol monoethyl ether (TGME)/water mixtures (NP and CAT 8), to a region at the limits of the core, with the polarities of the triethylene glycol dimethyl ether (TGDE)/water mixtures (MO) and to still lower values, found with the 16-doxyl probe, in the corona. At the same time, the different hydration dynamics at various depths in the micelles were followed showing earlier saturation in the outer regions of the core as compared to the central regions. The spin probes evidenced the compacting and ordering effects of water on the surfactant chains in the polar core and corona. From this point of view the RM in benzene-hexane appear to have a much lower viscosity in the polar core (over the whole range of water contents) as compared to those in cyclohexane. The order degree of the surfactant chains is lower too and the corona appears to be penetrable by an oil-soluble Cu complex, at variance with the cyclohexane system. All this data can be consistently explained by the loosening effect of benzene, solvating the surfactant chains in the corona.

Nitroxide spin probes for magnetic resonance characterization of ordered systems

Abstract Stable free radicals belonging to different classes have been synthesized in view of potential applications in studies of organized systems. Among them a new pyrroline nitroxide with a high fluorine content, 2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy-3-[(1H,1H-perfluoro-1-octoxy)carbonyl] ( 3 ), apt to probe the hydrophobic perfluoro-rich domains inside macroaggregates, has been prepared by a rarely encountered approach namely the spin-labelling of the very cosurfactant participating in the self-assembling process. Emphasis has then been laid upon the synthesis of further stable free radicals from the group of substituted nitronyl nitroxides. A general experimental procedure has been adapted according to the nature of the 2-substituent, resulting in a series of nitronyl nitroxides with a wide range of hydrophobicity and showing spectral parameters in good agreement with literature values. Using the spin probe-electron spin resonance technique, 3 and the 1- H -imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-ethyl-3-oxide ( 7 , R =C 2 H 5 ) or the 1- H -imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-undecyl-3-oxide ( 7 , R =C 11 H 23 ) have been applied in ESR investigations on vesicle phases of a zwitterionic surfactant (TDMAO)/perfluorocosurfactant (PFC) system and on Triton X-100 reverse micelles, respectively. While spin probe 3 proved to be insensitive to differences between the charged and uncharged phases, probes 7 were able to detect changes in the micelle micropolarity induced by hydration. The results corroborate well with previous UV measurements in the Triton X-100 system using the same probes 7 .

Factors affecting the stability and equilibria of free radicals. XIII. N-alkoxy- and N-aralkoxypicrylamines and ESR spectra of the corresponding capto-dative persistent aminyls

Abstract Five O -alkylhydroxylamines and three aralkylhydroxylamines have been picrylated to give O -alkyl- N -picrylhydroxylamines. These were converted to the corresponding N -(ar)alkoxy-picryl-aminyl radicals in toluene solution, and the ESR spectra were recorded. Simulations of the spectra with reasonable parameters and g values confirm the expected radical structures. Hyperfine coupling constants for nuclei in the picryl (acceptor) ring are smaller than those for the (ar)alkoxy group. This indication of competitive electron pair delocalization to the picryl ring, together with the long lifetimes of these radicals (compared with the symmetrically substituted diphenylaminyls), both support the concept of captodative stabilization.

Chapter 5:Getting an Inside View of Nanomaterials with Spin Labels and Spin Probes

The world as we know it depends heavily on materials chemistry. From the dawn of civilisation, synthetic materials have been crucial for all industrial development. Until recently, materials research was mainly focussed on the bulk properties of materials; this proved very successful and many new ma...

Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-assembled gel-phase materials

We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel-sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools-as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.

New amino-nitroxide spin labels.

Stable free mono- and diradicals containing reactive primary or secondary amino groups in the side-chain have been synthesized by transesterification of amino-substituted esters with paramagnetic alcohols or from spin-labeled acid derivatives and amines. In the second approach the new radical 18 (1-oxyl-3-(2-bromoethoxycarbonyl)-2,2,5,5-tetramethylpyrroline) is proposed as an efficient alkylating species. The nitroxides described are pH-sensitive spin probes and spin labels potentially useful for a diversity of ESR applications in chemistry and biology. New spin-labeled tyramine 16 (N-(1-oxyl-3-carbonyl-2,2,5,5-tetramethyl-pyrroline)tyramine) was successfully employed in a novel assay of protein oxidative damage.