Sample damage by X-rays and other radiation limits the resolution of structural studies on non-repetitive and non-reproducible structures such as individual biomolecules or cells. Cooling can slow sample deterioration, but cannot eliminate damage-induced sample movement during the time needed for conventional measurements. Analyses of the dynamics of damage formation suggest that the conventional damage barrier (about 200 X-ray photons per A2 with X-rays of 12 keV energy or 1 A wavelength) may be extended at very high dose rates and very short exposure times. Here we have used computer simulations to investigate the structural information that can be recovered from the scattering of intense femtosecond X-ray pulses by single protein molecules and small assemblies. Estimations of radiation damage as a function of photon energy, pulse length, integrated pulse intensity and sample size show that experiments using very high X-ray dose rates and ultrashort exposures may provide useful structural information before radiation damage destroys the sample. We predict that such ultrashort, high-intensity X-ray pulses from free-electron lasers that are currently under development, in combination with container-free sample handling methods based on spraying techniques, will provide a new approach to structural determinations with X-rays.

Yeast fatty acid synthase (FAS) is a 2.6-MDa barrel-shaped multienzyme complex, which carries out cyclic synthesis of fatty acids. By electron cryomicroscopy of single particles we obtained a threedimensional map of yeast FAS at 5.9-Å resolution. Compared to the crystal structures of fungal FAS, the EM map reveals major differences and new features that indicate a considerably different arrangement of the complex in solution compared to the crystal structures, as well as a high degree of variance inside the barrel. Distinct density regions in the reaction chambers next to each of the catalytic domains fitted the substrate-binding acyl carrier protein (ACP) domain. In each case, this resulted in the expected distance of ∼18 Å from the ACP substrate-binding site to the active site of the catalytic domains. The multiple, partially occupied positions of the ACP within the reaction chamber provide direct structural insight into the substrate-shuttling mechanism of fatty acid synthesis in this large cellular machine.cryoelectron microscopy | fatty acid synthesis | acyl carrier protein | molecular architecture

We present laboratory measurements of scattering properties of 15 different types of coastal and inland water phytoplankton species and two types of estuarine sediments. These properties are the scattering function as well as the angular distribution of a ratio of scattering matrix elements, which in practice equals the degree of linear polarization of the scattered light if the incident light is unpolarized. Laser light with a wavelength of 633 nm was used, and a scattering angle range from 20° to 60° was covered. The results can be used in the context of waterquality studies and to test results of theoretical models. The measured scattering functions are all strongly peaked in forward directions, but not equally so. For the covered angles, they vary significantly as a function of scattering angle. The measured angular distributions of the degree of linear polarization are mostly bell shaped, showing a maximum near 90°, whose magnitude is clearly different for the phytoplankton compared to the silt particles. We find that the morphology and structural features of the particles studied play an important and complex role in their light-scattering behavior. In particular, internal cell structures such as gas vacuoles alter the scattering patterns of the phytoplankton species considerably. The external shape of the cells appears to have a much smaller influence. The experimental results are compared with results of Mie calculations and with the "standard scattering function" of San Diego Harbor water. In most cases, Mie calculations cannot provide an adequate approximation of the measured scattering behavior, which indicates that more sophisticated models are required. Only 3 of the 17 measured scattering functions resemble the San Diego Harbor standard scattering function. One of these pertains to small silt particles, showing that this function is representative for water dominated by these particles.Diverse particles occur in coastal and inland waters, such as phytoplankton particles (i.e., cyanobacteria and algae), detritus (organic nonliving particles), and mineral particles (Dekker 1993; Mobley 1994). The scattering and absorption Acknowledgments

Ultrafast time-resolved spectroscopic studies of complex chemical reactions in solution are frequently hindered by difficulties in recovering accurate structural models for transient photochemical species. Time-resolved x-ray and electron diffraction have recently emerged as techniques for probing the structural dynamics of short lived photointermediates. Here we determine the structure of a transient isomer of photoexcited CH 2 I 2 in solution and observe the downstream reactions of the initial photoproducts. Our results illustrate how geminate recombination proceeds via the formation of a transient covalent bond onto the iodine atom remaining with the parent molecule. Further intramolecular rearrangements are thus required for the CH 2 I-I isomer to return to CH 2 I 2 . The generation of I 3 ÿ from those iodine radicals escaping the solvent cage is also followed with time.

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