Spin-selective recombination kinetics of a model chemical magnetoreceptor

We determine the spin-selective kinetics of a carotenoid-porphyrin-fullerene triad that has previously been used to establish the principle that a photochemical reaction could form the basis of the magnetic compass sensor of migratory birds and show that its magnetic sensitivity can be understood without invoking quantum Zeno effects.     

Determining a 'safe' high-mass limit in matrix-assisted laser desorption/ionisation time-of-flight mass spectra of coal derived materials with reference to instrument noise

Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Polynomially scaling spin dynamics simulation algorithm based on adaptive state-space restriction

We report progress with an old problem in magnetic resonance -- that of the exponential scaling of simulation complexity with the number of spins. It is demonstrated below that a polynomially scaling algorithm can be obtained (and accurate simulations performed for over 200 coupled spins) if the dimension of the Liouville state space is reduced by excluding unimportant and unpopulated spin states. We found the class of such states to be surprisingly wide. It actually appears that a majority of states in large spin systems are not essential in magnetic resonance simulations and can safely be dropped from the state space. In restricted state spaces the spin dynamics simulations scale polynomially. In cases of favourable interaction topologies (sparse graphs, e.g. in protein NMR) the asymptotic scaling is linear, opening the way to direct fitting of molecular structures to experimental spectra.     

Spin relaxation effects in photochemically induced dynamic nuclear polarization spectroscopy of nuclei with strongly anisotropic hyperfine couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal DeltaHFC-Deltag (hyperfine coupling anisotropy--g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse DeltaHFC-Deltag cross-correlation, which is shown to lead to an inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.     

Multiple decoherence-free states in multi-spin systems

A numerical procedure is presented for mapping the vicinity of the null-space of the spin relaxation superoperator. The states populating this space, i.e. those with near-zero eigenvalues, of which the two-spin singlet is a well-studied example, are long-lived compared to the conventional T(1) and T(2) spin-relaxation times. The analysis of larger spin systems described herein reveals the presence of a significant number of other slowly relaxing states. A study of coupling topologies for n-spin systems (4≤n≤8) suggests the symmetry requirements for maximising the number of long-lived states.     

Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.     

Chemically amplified 19F-1H nuclear Overhauser effects

Chemically induced dynamic nuclear polarisation (CIDNP) is explored as a source of nuclear hyperpolarisation in heteronuclear Overhauser effect experiments. A photochemical reaction proceeding through a radical pair intermediate is used to enhance (19)F nuclear magnetisation in 3-fluorotyrosine by more than an order of magnitude with a corresponding increase in the amplitudes of (19)F-(1)H cross-relaxation and cross-correlation effects. The reactions employed are cyclic and leave the sample chemically unchanged. The potential for enhancing the sensitivity of heteronuclear NOEs in (19)F-labelled proteins is discussed.     

Intramolecular electron transfer in lysozyme studied by time-resolved chemically induced dynamic nuclear polarization

The kinetics of the chemically induced dynamic nuclear polarization (CIDNP) produced in reactions of hen lysozyme with photosensitizers have been studied for the native state of the protein at pH 3.8 and for two denatured states. The latter were generated by raising the temperature to 80 degrees C or by combining a temperature rise (to 50 degrees C) with the addition of chemical denaturant (10 M urea). Detailed analysis of the CIDNP time dependence on a microsecond time scale revealed that, in both denatured states, intramolecular electron transfer (IET) from a tyrosine residue to the cation radical of a tryptophan residue (rate constant k(f)) is highly efficient and plays a decisive role in the evolution of the nuclear polarization. To describe the observed CIDNP kinetics with a self-consistent set of parameters, IET in the reverse direction, from a tryptophan residue to a tyrosine residue radical (rate constant k(r)), has also to be taken into account. The IET rate constants determined by analysis of the CIDNP kinetics are, at 80 degrees C: k(f) = 1 x 10(5) s(-1) and k(r) = 1 x 10(4) s(-1); at 50 degrees C in the presence of 10 M urea: k(f) = 7 x 10(4) s(-1), k(r) = 1 x 10(4) s(-1). IET does not appear to influence the CIDNP kinetics of the native state.     

Time-resolved CIDNP study of non-native states of bovine and human alpha-lactalbumins

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of the aromatic dye 2,2'-dipyridyl with non-native states of bovine and human alpha-lactalbumins (BLA and HLA) in aqueous solution have been studied using the time-resolved CIDNP technique. Non-native states have been obtained at pH 2 in the presence of 0, 8, and 10 M urea-d(4) and at pH 6.7 in the presence of 10 M urea-d(4). The dependence of the geminate CIDNP spectra of the two proteins on the denaturant concentration is shown to be determined by the intrinsic reactivity of the amino acid residues toward the triplet excited dye rather than by structural changes in the proteins. Values of the proton paramagnetic relaxation times (T(1)) have been obtained from an analysis of the CIDNP kinetics. For tryptophan and tyrosine residues, the T(1) values change in opposite directions when the proteins are progressively denatured, reflecting the different internal mobilities of the two types of residues. It has been found that for both BLA and HLA the CIDNP kinetics of the non-native states formed at pH 6.7 in the presence of 10 M urea are almost identical to those at pH 2 with no urea, suggesting that the polarizable amino acid side chains have closely similar solvent accessibilities and motional properties in the two non-native states.