Christian Rischel

Conformational characterization of oligomeric intermediates and aggregates in β-lactoglobulin heat aggregation

In one of the first studies of isolated intermediates in protein aggregation, we have used circular dichroism and fluorescence spectroscopy to characterize metastable oligomers that are formed in the early steps of β‐lactoglobulin heat aggregation. The intermediates show typical molten globule characteristics (secondary structure content similar to the native and less tight packing of the side chains), in agreement with the belief that partly folded states play a key role in protein aggregation. The far‐UV CD signal bears strong resemblance to that of a known folding intermediate. Cryo‐transmission electron microscopy of the aggregates reveals spherical particles with a diameter of about 50 nm and an internal threadlike structure. Isolated oligomers as well as larger aggregates bind the dye thioflavin T, usually a signature of the amyloid superstructures found in many protein aggregates. This result suggests that the structural motif recognized by thioflavin T can be formed in small oligomers.

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k(B)T/J< or =0.025 for H=0 and H=8.7 T, where the system is approximately 30% magnetized. At H=0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H=8.7 T, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe ansatz calculations.

Modification of a specific tyrosine enables tracing of the end-to-end distance during apomyoglobin folding

In order to follow the overall geometry of the apomyoglobin molecule during folding, we have converted a specific tyrosine residue into 3‐nitro‐tyrosine. The specificity of the modification was verified by proteolytic cleavage of the modified protein and mass spectroscopy of the resulting fragments. By measuring the energy transfer from the tryptophanyl side‐chains to the modified residue the average end‐to‐end distance can be followed. The experiment shows that after initiation of folding the N‐ and C‐termini are rapidly brought into proximity, possibly to a near‐native distance.

Microsecond structural fluctuations in denatured cytochrome c and the mechanism of rapid chain contraction

In order to improve the understanding of the diffusive chain movements leading to protein folding, we have studied microsecond conformational fluctuations in denatured yeast cytochrome c by fluorescence correlation spectroscopy (FCS). We show that emitted fluorescence from the dye Alexa-488 chemically attached to the protein depends on the extension of the chain, such that fluctuations in chain length will give fluctuations in fluorescence intensity. Exposure to chemical denaturants leads to an increase in diffusion times, indicating expansion of the molecule. Structural fluctuations of the unfolded protein give rise to fluctuations in the emitted fluorescence. However, FCS measurements fail to show conformational fluctuations of chain segments, establishing an upper bound of 4 µs on the timescale of chain fluctuations in the denatured state. This clearly shows that an early process with a time constant of 50 µs observed in folding experiments must involve passage of a free energy barrier, and cannot be barrierless chain collapse as has been proposed.

Mean first-passage time analysis reveals rate-limiting steps, parallel pathways and dead ends in a simple model of protein folding

We have analyzed dynamics on the complex free-energy landscape of protein folding in the FOLD-X model, by calculating for each state of the system the mean first-passage time to the folded state. The resulting kinetic map of the folding process shows that it proceeds in jumps between well-defined, local free-energy minima. Closer analysis of the different local minima allows us to reveal secondary, parallel pathways as well as dead ends.

Effective Hamiltonian and low-lying eigenenergy clustering patterns of four-sublattice antiferromagnets

We study the low-lying eigenenergy clustering patterns of quantum antiferromagnets with p sublattices ~in particular p54). We treat each sublattice as a large spin, and using second-order degenerate perturbation theory, we derive the effective ~biquadratic! Hamiltonian coupling the p large spins. In order to compare with exact diagonalizations, the Hamiltonian is explicitly written for a finite-size lattice, and it contains information on energies of excited states as well as the ground state. The result is applied to the face-centered-cubic Type-I antiferromagnet of spin 1/2, including second-neighbor interactions. A 32-site system is exactly diagonalized, and the energy spectrum of the low-lying singlets follows the analytically predicted clustering pattern.

Magnetic order of s = 12 spins on the FCC lattice at zero temperature calculated by exact diagonalization

Abstract The magnetic order of s = 1 2 spins with Heisenberg couplings on the FCC lattice at T = 0 has been probed by examining the ground state of finite clusters with periodic boundary conditions, for different ratios of the nearest and next-nearest-neighbour couplings, J1 and J2. Results for 32 spins show that the type of antiferromagnetic order is independent of the external field, in agreement with recent experimental results on nuclear magnetic order in silver. Results of calculations on 64 spins in a high field implies that the zero-field phase diagram in the (J1, J2) plane is close to the classical predictions.