During flash cooling of protein crystals in liquid cryogens, cooling rates are determined by sample size, choice of cooling liquid, and by the thickness of the cold gas layer that forms above the liquid. We describe an experimental protocol for ultra-rapid cooling of protein crystals. This protocol requires no complex apparatus, and yields ice-ring-free diffraction without the use of penetrating cryoprotectants. © 2007 Springer Science+Business Media B.V.

High-transition-temperature (high-Tc) superconductivity is ubiquitous in the cuprates containing CuO2 planes, but each cuprate has its own character. The study of the material dependence of the d-wave superconducting gap (SG) should provide important insights into the mechanism of high-Tc superconductivity. However, because of the pseudogap phenomenon, it is often unclear whether the energy gaps observed by spectroscopic techniques really represent the SG. Here, we use scanning tunnelling spectroscopy to image nearly optimally doped Ca2xNaxCuO2Cl2(Na-CCOC) with Tc≤25-28K.

We performed high-resolution scanning tunneling microscopy/spectroscopy on an optimally-doped Ca2-xNaxCuO2Cl2 crystal with Tc ∼ 25 K. The so-called "checkerboard" local-density-of-state modulation previously found in heavily underdoped regime also manifests in the spectroscopic map of the optimally-doped sample. In addition, spatially-inhomogeneous energy gap with peaks at the gap edges is observed below about 10 meV. The gap tends to be buried at elevated temperatures and correlates with the checkerboard modulation.

We report on new instabilities of the quasistatic equilibrium of water drops pinned by a hydrophobic inclined substrate. The contact line of a statically pinned drop exhibits three transitions of partial depinning: depinning of the advancing and receding parts of the contact line and depinning of the entire contact line leading to the drop's translational motion. We find a region of parameters where the classical Macdougall-Ockrent-Frenkel approach fails to estimate the critical volume of the statically pinned inclined drop. © 2007 The American Physical Society.

Helicases are molecular motors that separate DNA strands for efficient replication of genomes. We probed the kinetics of individual ring-shaped T7 helicase molecules as they unwound double-stranded DNA (dsDNA) or translocated on single-stranded DNA (ssDNA). A distinctive DNA sequence dependence was observed in the unwinding rate that correlated with the local DNA unzipping energy landscape. The unwinding rate increased ∼10-fold (approaching the ssDNA translocation rate) when a destabilizing force on the DNA fork junction was increased from 5 to 11 pN.

Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral-weight transfers from the high-energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction-a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca1.88Na0.12CuO 2Cl2 and Bi2Sr2Dy 0.2Ca0.8Cu2O8+δ.

We designed and created nanofabricated quartz cylinders well suited for torque application and detection in an angular optical trap. We made the cylinder axis perpendicular to the extraordinary axis of the quartz crystal and chemically functionalized only one end of each cylinder for attachment to a DNA molecule. We directly measured the torque on a single DNA molecule as it underwent a phase transition from B-form to supercoiled P-form.

The mechanochemical kinetics of transcription elongation was examined with a combination of theoretical and experimental approaches. The predictive power of a sequence-dependent thermal ratchet model for transcription elongation was tested by establishing model parameters based solely on measurements under chemical perturbations and then directly predicting responses under mechanical perturbations without additional model parameters. Agreement between predicted and measured force-velocity curves provides strong support for a simple mechanochemical coupling mechanism.

Using an atomic force microscope, we measured effective spring constants of stacks of graphene sheets (less than 5) suspended over photolithographically defined trenches in silicon dioxide. Measurements were made on layered graphene sheets of thicknesses between 2 and 8 nm, with measured spring constants scaling as expected with the dimensions of the suspended section, ranging from 1 to 5 Nm.