Horia Caldararu

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.

The structure of gelatin–water/oil microemulsion sols and gels. An EPR spin-probe and spin-labelling study

The microenvironments of sols and gels of gelatin in water-in-oil microemulsions formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane have been characterized by EPR spin-labelling and spin-probe techniques. Using spin-labelled gelatin, incorporated both in sol and gel, the changes in the microstructure(s) around the gelatin have been monitored, at different water and gelatin compositions. Specific locations of the gelatin have been found: some of the gelatin is located at the water/surfactant interface and part in pools of water. Gelatin is not found in the apolar phase. The spin-probe technique, with amphiphilic probes, provides information on changes in the structure of the aggregates as a result of gelatin incorporation. Increased microviscosity, and a tighter packing of the AOT chains is observed, confirming the location of gelatin at the water/surfactant interface. Our results favour a model for the gel structure in which all the gelatin is covered by water and surfactant, with large cavities of polar and apolar solvent.

Structural aspects in self-assembled systems of polyoxyethylene surfactants, as studied by the spin probe technique

Abstract Typical examples of structural characterization of self-assembled systems by the spin probes technique, selected from our representative results accumulated in the last years of systematic studies, are presented. The choice of the examples has aimed at emphasizing the potentiality of this technique in the study of self-assembled systems, in general, and of those of PEO surfactants, in particular. By using specific ESR parameters (the nitrogen hyperfine splitting (hfs), a N , the rotational correlation time, τ c , the order parameter, S ) of a variety of properly chosen nitroxides, problems such hydration degree and profile of the PEO chains, ordering and order profile along these chains, their penetrability by the oil solvent, role of the terminal OH in the micellization, as well as differences in these quantities vs . the nature of the aggregate (micelle, reverse micelle, lamellar phase, etc.), nature of the surfactants (conventional or triblock copolymer), solubilizates (water in reverse micelles or various alcohols in micelles) and temperature have been discussed.

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.

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.

EPR spin-labelling and spin-trapping study of proteins in reverse micelles

EPR spectroscopy has been used to study the motions of several spin-labelled and spin-trapped proteins (α-chymotrypsin, cytochrome c and myoglobin) enclosed within reverse micelles formed by sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) in isooctane. In several cases the spectra obtained from the encapsulated protein are significantly different from those observed in bulk solution. The motions of the labelled proteins, inferred from the anisotropy of the EPR spectra (A‖ values), vary with the amount of solubilized water (W0) and hence the physical size of the water-pool in the reverse micelles; it is suggested that the level of solvation and the solvent structure in the water-pool of the reverse micelle cause the observed changes in motion. These results support the previously postulated ‘water-shell’ model of proteins contained in reverse micelles.

The strength of one-electron acceptor sites of some X and Y zeolites: An ESR study

In the present study the one-electron acceptor sites of some X and Y zeolites and of a silica—alumina catalyst have been investigated by adsorption of a series of electron donor compounds (anthracene (An), dibenzo-p-dioxin (Dx), phenoxathiin (Phx) and phenothiazine (Pht)) which have different ionization potentials but similar molecular “diameters”. The donor compounds yielded on the studied surfaces adsorbed cation radicals of which the electron spin resonance (ESR) spectra have been measured. For the zeolite samples, using the above donors, the one-electron acceptor sites could be distinguished according to their oxidizing strength. The ratio between the concentrations of the sites measured with Pht and An as donors is proposed as a parameter for evaluating the strength distribution of oxidizing sites on the studied samples. Some differences between the results obtained on zeolites and on silica—alumina are underlined. The ratio between the concentrations of “inner” and “outer” surface acceptor sites could be evaluated by using Pht and perylene, the ionization potential values of which are close but the molecular “diameters” are different.

Dynamics of spin-labelled α-chymotrypsin in reverse micelles of differently charged surfactants

Analysis and simulation of the EPR spectra of α-chymotrypsin spin-labelled at two sites (methionine-192 and serine-195) in water and sodium bis(2-ethylhexyl) sulfosuccinate (AOT)–isooctane reverse micelles has provided information on the rate and nature of label motion in these media. The correlation time of methionine-labelled chymotrypsin, and the value of A∥ for serine-labelled chymotrypsin in reverse micelles have been studied as functions of surfactant charge [AOT, negative, and cetyltrimethylammonium bromide (CTAB), positive], of the net protein charge above and below its isoelectric point, and of the addition of neutral co-surfactants. The results obtained are consistent with the ‘water-shell’ model of protein solvation in these systems, with no evidence for any ionic significant interactions between protein and surfactant headgroups.

Hydrophobic supramolecular complexes of various cations with 18-crown-6 as the ligand and N-methoxy-picramide as the anion pair in a water/methylene chloride two-phase system: complexes with α-amino acids, their characteristics and conditions for formation

Abstract α-Amino acids (arginine, valine, glycine, isoleucine, alanine, leucine, and phenylalanine), as amphionic species, were extracted from water into methylene chloride as neutral, hydrophobic, red supramolecular complexes of the type (CE-AmAc) + A − where CE is 18-crown-6, AmAc is RCH(NH 3 ) + COOH, and A − is the N -methoxy-2,4,6-trinitroanilinium anion, where the latter gives rise to the red colour of the complexes (λ max = 462 nm). The extraction constants ( K ex s ) of the α-amino acids from water to methylene chloride as supramolecular complexes were determined in relationship to the aqueous ionic strength ( I ).