The mechanisms by which macromolecular impurities degrade the diffraction properties of protein crystals have been investigated using X-ray topography, high-resolution diffraction line shape measurements, crystallographic data collection, chemical analysis, and two-photon excitation fluorescence microscopy. Hen egg-white lysozyme crystals grown from solutions containing a structurally unrelated protein (ovotransferrin) and a related protein (turkey egg-white lysozyme) can exhibit significantly broadened mosaicity due to formation of cracks and dislocations but have overall B factors and diffraction resolutions comparable to those of crystals grown from uncontaminated lysozyme. Direct fluorescence imaging of the three-dimensional impurity distribution shows that impurities incorporate with different densities in sectors formed by growth on different crystal faces, and that impurity densities in the crystal core and along boundaries between growth sectors can be much larger than in other parts of the crystal. These nonuniformities create stresses that drive formation of the defects responsible for the mosaic broadening. Our results provide a rationale for the use of seeding to obtain high-quality crystals from heavily contaminated solutions and have implications for the use of crystallization for protein purification. Proteins 1999;36:270-281.

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We have observed an unusual form of creep at low temperatures in the charge-density-wave (CDW) conductor NbSe3. This creep develops when CDW motion becomes limited by thermally-activated phase advance past individual impurities, demonstrating the importance of local pinning and related short-length-scale dynamics. Unlike in vortex lattices, elastic collective dynamics on longer length scales results in temporally ordered motion and a finite threshold field. A first-order dynamic phase transition from creep to high-velocity sliding produces "switching" in the velocity-field characteristic.

We present a detailed experimental study of current oscillation phenomena in charge-density-wave (CDW) transport. The amplitude and harmonic content of the quasiperiodic component of the current oscillations remain large and approximately constant even at very high electric fields, suggesting that the form and magnitude of the impurity pinning potential are approximately independent of applied field. A simple single-coordinate model of CDW motion in a nonsinusoidal pinning potential, motivated by the quantum tunneling theory of CDW depinning, accounts for all the qualitative features of our results. Nearly perfect velocity coherence seems to be characteristic of weakimpurity-pinned CDW's in high-quality NbSe3. However, other types of crystal defects together with contact effects act to broaden the observed distribution of CDW velocities. This velocity distribution plays a crucial role in the "ringing" observed in the CDW response to a current pulse, and also in generating the broadband noise.

We present a detailed experimental study of ac-dc interference phenomena in charge-density-wave (CDW) transport. Harmonic and subharmonic "steps" of constant CDW current are observed when ac and dc voltages are applied together, and the widths of these steps exhibit Bessel-like oscillations as a function of both ac amplitude and frequency. We now observe similar oscillations in the dc threshold field, and thereby complete the analogy between these phenomena and similar effects in Josephson junctions. By minimizing crystal defects and contact effects, we have obtained complete harmonic and subharmonic phase locking in very long NbSe3 crystals under a broad range of conditions, including cases where the total time-dependent voltage remains above the dc threshold VT. We analyze our experimental results with a single-coordinate model used previously to account for subharrnonic steps. It is based on the tunneling theory and assumes CDW motion in a highly nonsinusoidal pinning potential. This model provides an excellent and semiquantitative account of all our results; it also provides a detailed interpretation of interference features observed in the ac conductance, electric-field-induced anomalies observed in the CDW elastic response, and the inductive loops observed in phase-space plots of the CDW*s response to ac fields. The quality of the agreement provides strong evidence both for an effective pinning potential of the proposed form and for a single-degree-of-freedom description of CDW transport.