Synchrotron x-ray-diffraction studies of actin structure during polymerization

Synchrotron x-ray diffraction was used to identify the oligomers that formed during the earliest stages of actin polymerization. Solution diffraction patterns from G-actin (monomer) and from F-actin (polymer) contain information about the size and shape of actin monomers and the length, width, and subunit organization of filaments. Comparison of patterns collected during polymerization reveals an increase in scatter at spacings greater than 9.0 nm; formation of scattering bands at 5.4,4.9, and 3.4 nm; formation of a scattering minimum at 6.5 nm; and the presence of an isosbestic point at 9.0 nm. These scattering bands arise from the formation of, and organization of subunits in, filaments. At various actin concentrations (0.37-5 mg/ml), the change in scatter in these regions follows simple exponential kinetics with no detectable lag. Our analysis of the x-ray patterns shows that by 0.4 sec after mixing, most of the actin has formed dimers, which then rapidly incorporate into oligomers. At 4 mg/ml the early oligomers increase in length to greater than 30.0 nm within 10 sec. These results suggest that under our conditions actin molecules condense into filaments without the rate-limiting formation of nuclei

The crystallization and melting of linear polyethylene studied by temperature-modulated SAXS and WAXD

Temperature-reversible and -irreversible morphological events could be separated in the case of linear polyethylene during quasi-isothermal crystallization by using simultaneous temperature-modulated synchrotron SAXS and WAXD. Crystallization and subsequent annealing was followed at 126 degreesC for 90 min while applying a temperature modulation with an amplitude of 1 degreesC and a period of 2 min. The crystal growth rate associated with the irreversible part of the crystallization decreases with increasing temperature in a cycle, The crystalline lamellae irreversibly thicken with time. The actual crystallite thickness, however, exhibits a superimposed modulation out of phase with that of the temperature modulation. Melting was studied during heating at 1 degreesC/min after cooling at 10 degreesC/min. A temperature modulation was superimposed with an amplitude of 2 degreesC and a period of 3 min. Once again temperature-reversible crystal thickness changes and irreversible crystal thickening could be observed.status: publishe

Solutions of hyperbranched aromatic polyamides

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The overall conformation of conventional kinesins studied by small angle X-ray and neutron scattering

The quaternary structures of several monomeric and dimeric kinesin constructs from Homo sapiens and Drosophila melanogaster were analyzed using small angle K-ray and neutron scattering. The experimental scattering curves of these proteins were compared with simulated scattering curves calculated from available crystallographic coordinates. These comparisons indicate that the overall conformations of the solution structures of D. melanogaster and H. sapiens kinesin heavy chain dimers are compatible with the crystal structure of dimeric kinesin from Rattus norvegicus. This suggests that the unusual asymmetric conformation of dimeric kinesin in the microtubule-independent ADP state is likely to be a general feature of the kinesin heavy chain subfamily. An intermediate length Drosophila construct (365 residues) is mostly monomeric at low protein concentration whereas at higher concentrations it is dimeric with a tendency to form higher oligomers