Quantum dynamics of excitations and decoherence in many-spin systems detected with Loschmidt echoes: its relation to their spreading through the Hilbert space.

In this work, we overview time-reversal nuclear magnetic resonance (NMR) experiments in many-spin systems evolving under the dipolar Hamiltonian. The Loschmidt echo (LE) in NMR is the signal of excitations which, after evolving with a forward Hamiltonian, is recovered by means of a backward evolution. The presence of non-diagonal terms in the non-equilibrium density matrix of the many-body state is directly monitored experimentally by encoding the multiple quantum coherences. This enables a spin counting procedure, giving information on the spreading of an excitation through the Hilbert space and the formation of clusters of correlated spins. Two samples representing different spin systems with coupled networks were used in the experiments. Protons in polycrystalline ferrocene correspond to an 'infinite' network. By contrast, the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline in the nematic mesophase represents a finite proton system with a hierarchical set of couplings. A close connection was established between the LE decay and the spin counting measurements, confirming the hypothesis that the complexity of the system is driven by the coherent dynamics.

Lysophosphatidylcholine-arbutin complexes form bilayer-like structures.

Arbutin is known to suppress melanin production in murine B16 melanoma cells and inhibit phospholipase action. This encourages the possibility to stabilize it in lipid aggregates for its administration in medical applications. Thus, it was of interest to demonstrate that monomyristoylphosphatidylcholine (14:0 lysoPC) and arbutin may form association complexes. This was studied by Electron Microscopy (EM), 31P Nuclear Magnetic Resonance (31P NMR), Electronic Paramagnetic Resonance (EPR) and Fourier Transform Infrared Spectroscopy (FTIR). EM images show the formation of particles of c.a. 6 nm in diameter. For a 1:1 lysoPC-arbutin molar ratio 31P NMR shows a spectrum with a shoulder that resembles the axially symmetric spectrum characteristic of vesicles. The addition of La3+ ions to the arbutin-lysoPC complex allows one to distinguish two phosphorous populations. These results suggest that arbutin-lysoPC forms vesicles with bilayers stabilized in an interdigitated array. FTIR spectroscopy shows that arbutin interacts with the hydrated population of the carbonyl groups and with the phosphates through the formation of hydrogen bonds. It is interpreted that hydrophobic interactions among the phenol group of arbutin and the acyl chain of lysoPC are responsible for the decrease in acyl chain mobility observed at the 5th C level by EPR. A model proposing the formation of interdigitated bilayers of arbutin-lysoPC could explain the experimental results.

Cholesterol-induced alterations of the packing properties of gangliosides: an EPR study.

The effect of cholesterol (Chol) on two kinds of glycolipid assemblies, one composed of monosialogangliosides (GM1a) and the other formed by a natural mixture of bovine brain gangliosides (TBG), has been analysed. The experimental approach involves spin label electron paramagnetic resonance (EPR) in aqueous lipid dispersions. The employment of a hydrosoluble spin label and a 'quencher' of the EPR signal that is not able to permeate lipid interfaces, allowed us to conclude that GM1a/Chol mixtures give rise to vesicles at Chol proportions for which TBG/Chol mixtures form micelles. The use of different liposoluble spin labels reveals that cholesterol produces a straightening of the hydrocarbon chains in both lipid systems. In GM1a/Chol mixtures, this feature is more pronounced and it is coupled with a decrease in polarity at the chain ends.

Ganglioside hydration study by 2H-NMR: dependence on temperature and water/lipid ratio.

Dynamic properties of 2H2O in samples of ganglioside aggregates hydrated at water/lipid ratios ranging from 25:1 to 8000:1 mole/mole were studied by using deuterium nuclear magnetic resonance (2H-NMR). We present a physical model for the interpretation of the measured spin-spin relaxation times (T2). For all the concentrations studied the model provides evidence for the existence of at least two kinds of water environments: one in which the rotational correlation time is in the range of 10(-9) to 10(-8) s, and a second in which it lies between 10(-11) to 10(-10) s. A detailed study on the temperature dependence was performed for two of the concentrations, one corresponding to the hexagonal phase (100:1 mole/mole) and the other involving a micellar phase (200:1 mole/mole). In the 100:1 2H2O/ganglioside molar ratio sample, most of the water is tightly bound to long cylindrical structures. For the 200: 1 sample, there are on average approximately 30 water molecules tightly bound to the polar head group of each ganglioside molecule. The relative number and dynamics of molecules in this environment are essentially insensitive to temperature variations in the range 220-300K The rest of water molecules are also influenced by the aggregate, having a different mobility from that observed in the free liquid state.