Probing molecular free energy landscapes by periodic loading

Single molecule pulling experiments provide information about interactions in biomolecules that cannot be obtained by any other method. However, the reconstruction of the molecule's free energy profile from the experimental data is still a challenge, in particular, for the unstable barrier regions. We propose a new method for obtaining the full profile by introducing a periodic ramp and using Jarzynski's relation for obtaining equilibrium quantities from nonequilibrium data. Our simulated experiments show that this method delivers significant more accurate data than previous methods, under the constraint of equal experimental effort.     

Two-dimensional coarsening kinetics of reconstruction domains: GaAs(001)-beta(2 x 4)

We study the nonconserved coarsening kinetics of a reconstructed semiconductor surface. The domain size evolution is obtained in situ by time-resolved surface x-ray diffraction. The system exhibits four equivalent domain types with two nonequivalent types of domain boundaries. Small domains are prepared by molecular beam epitaxy deposition of one GaAs layer. We find the correlation lengths of the domain size distribution to depend on time as l is proportional to t(0.42+/-0.05) in the half-order reflections and l is proportional to t(0.22+/-0.05) in the quarter-order reflections. The fraction of the higher energy domain boundaries increases as lnt.     

Dissection of heteronuclear NMR experiments for studies of magnetization transfer efficiencies

Modern NMR experiments for applications with biological macromolecules in solution typically include multiple magnetization transfer steps. When working with large structures, a significant fraction of the magnetization is lost during these transfers. For the design and optimization of complex experimental schemes, the magnetization transfer efficiencies have therefore commonly been calculated from the spin relaxation times. This paper now suggests a new method for measurement of individual transfer efficiencies directly with the system of interest, using short, reliable experiments. Initial applications of this approach with a 110,000 Da protein indicate that there is a wide range of transfer efficiencies among individual spin pairs in a structure of this size, which leads to a correspondingly large variation of the individual signal intensities and the need for techniques to enhance the weak signals.     

Fast voltage transients in capacitive silicon-to-cell stimulation detected with a luminescent molecular electronic probe

The capacitive stimulation of nerve cells from semiconductor chips is a prerequisite for the development of neuroelectronic devices. We report on the primary response of a cell membrane to a voltage step applied to oxidized silicon. It is observed with a luminescent voltage-sensitive dye. We find exponential voltage transients with a time constant of 1-5 micros. We assign the short response to an electrical decoupling by a thin film of electrolyte between oxide and membrane. The high-pass filtering of stimulation is a crucial constraint for the development of silicon-to-neuron interfaces.     

Homoclinic bifurcation in a Hodgkin-Huxley model of thermally sensitive neurons

We study global bifurcations of the chaotic attractor in a modified Hodgkin-Huxley model of thermally sensitive neurons. The control parameter for this model is the temperature. The chaotic behavior is realized over a wide range of temperatures and is visualized using interspike intervals. We observe an abrupt increase of the interspike intervals in a certain temperature region. We identify this as a homoclinic bifurcation of a saddle-focus fixed point which is embedded in the chaotic attractors. The transition is accompanied by intermittency, which obeys a universal scaling law for the average length of trajectory segments exhibiting only short interspike intervals with the distance from the onset of intermittency. We also present experimental results of interspike interval measurements taken from the crayfish caudal photoreceptor, which qualitatively demonstrate the same bifurcation structure. (c) 2000 American Institute of Physics.     

Radiative decay ofJ/ψ into γ π+ π−

The radiative decayJ/ψ → γ π⁺ π^? has been studied using the 8.6 millionJ/ψ produced in the DM2 experiment at the DCIe ⁺e^? storage rings at Orsay. The π⁺ π^? mass spectrum shows a cleanf ₂ (1270) signal, and the possible presence of two other states at thef ₂ (1720) andf ₄ (2030) masses. For thef ₂ (1270), the branching ratio BR(J/ψ →γf)xBR(f→π⁺ π^?) is measured to be (7.50±0.30±1.12)×10^?4, and the spin analysis prefers theJ=2 assignment, with helicity parametersx=0.83±0.06 andy=0.01±0.06. The existence of higher mass states is discussed.     

QCD sum rules in exclusive kinematics and pion wave function

We suggest a modification of the QCD sum rules for three-point correlation functions that relates hadron expectation values of an operator of interest to properties of the QCD vacuum in alternating external fields. A new sum rule is obtained for the nucleon magnetic moments. Relations are found between the couplings g_πNN , g_ρωπ and the value of the pion wave function at the point with equal momentum carried by the quark and the antiquark. Our results seem to exclude the possibility of having a pronounced dip in the pion wave function in the middle point, as has been assumed on the evidence of a large value of the second moment.