Activation of the complement system generates potent chemoattractants and opsonizes cells for immune clearance. Short-lived protease complexes cleave complement component C3 into anaphylatoxin C3a and opsonin C3b. Here we report the crystal structure of the C3 convertase formed by C3b and the protease fragment Bb, which was stabilized by the bacterial immune-evasion protein SCIN. The data suggest that the proteolytic specificity and activity depends on dimerization of C3 with C3b of the convertase. SCIN blocked the formation of a productive enzyme-substrate complex. Irreversible dissociation of C3bBb is crucial to complement regulation and was determined by slow binding kinetics of the Mg2+-adhesion site in Bb. Understanding the mechanistic basis of the central complement activation step and microbial immune evasion strategies targeting this step will aid the development of complement therapeutics.

In this contribution the use of Analytical Ultracentrifugation (AUC) for the modern analysis of colloids is reviewed. Since AUC is a fractionation technique, distributions of the sedimentation coefficient, particle size and shape, molar mass and density can be obtained for particle sizes spanning the entire colloidal range. The Ångström resolution and the reliable statistics with which particle size distributions can be obtained from analytical ultracentrifugation makes this a high resolution analysis technique for the characterization of nanoparticles in solution or suspension. Several examples showing successful applications of AUC to complex problems in colloid science are given to illustrate the broad range and versatility of questions that can be answered by AUC experiments.

We report a method to determine the particle size distribution of small colloidal silica spheres via analytical ultracentrifugation and show that the average particle size, variance, standard deviation, and relative polydispersity can be obtained from a single sedimentation velocity (SV) analytical ultracentrifugation (AUC) experiment. The particle size distribution (psd) from the enhanced van HoldeWeischet (vHW) analysis accounts for the dynamic light scattering results quite well. In addition, the vHW psd equals the psd from a continuous distribution of sedimentation coefficients analysis where whole sedimentation velocity boundaries are fitted. The SV AUC interference optical data also yield the specific particle volume such that distributions of sedimentation coefficients for colloidal spheres can be converted directly to particle size distributions. Our results show that SV AUC experiments may yield a quantitative particle size distribution without a priori knowledge of the particle size and the shape of the size distribution.Synthetic macromolecules as well as colloids are always to a certain degree heterogeneous with respect to size and shape.For the size and shape determination, as well as the overall particle size distribution (psd), several techniques are available 1-3 such as transmission electron microscopy (TEM, or variations on this technique such as scanning electron microscopy and cryo-TEM) and atomic force microscopy (AFM). These microscopic techniques, except cryo-TEM, 4,5 do not image in situ. Drying of TEM samples may lead to particle shrinkage and distortion of particle structures, including aggregation due to capillary forces. Moreover, the electron beam may seriously damage the colloids via the melting and sintering of nanoparticles, as is the case for the small silica particles that are subject of the present study. Even if TEM images are fairly representative for the colloids in solution, many counts are needed for reliable statistics. For example, in the case of a polydispersity around 30%, typically at least 1000 counts are needed for a representative size distribution (see the case studies for various colloids in ref 1).Techniques that sample colloids in situ are static light scattering (SLS) and dynamic light scattering 6 (DLS). For sufficiently narrow size distributions, SLS data in the Guinier region can be analyzed with a momentum-expansion, to obtain an effective radius that is independent of the detailed shape of the distribution function. 1 For a broad size distribution, however, it is hardly possible to extract in any a priori fashion reliable distribution parameters from static light scattering profiles. DLS may be conducted on polydisperse colloids for which the particle shape is known, to obtain an apparent average particle size and standard deviation. The shape of the particle size distribution from DLS, however, may be highly questionable because of the ill-conditioned inversion problem encountered in DLS. 6 Since the scattered light intensity scales with the particle vo...

We report an analytical ultracentrifugation study on sedimentation in dilute stable dispersions of uniform, magnetic iron oxide (Fe3O4) colloids. On increase of the dipolar coupling constant, tuned by the average particle size, the linear concentration dependence of the sedimentation velocity shows an abrupt transition from the hindered sedimentation expected for hard spheres to a marked acceleration already for weak dipolar interactions. This transition is not reproduced by sedimentation theory derived from an effective, isotropic pair correlation function of the type proposed by De Gennes and Pincus, for reasons which are made clear. Accelerated settling, instead, follows a scaling based on the mass action law for dimer formation with dipoles in head-to-tail configuration. Our work illustrates that orientational averaging of dipole interactions of ferromagnetic colloids in the dilute regime is inapplicable, with an obvious implication for the possible existence of isotropic gas-liquid criticality for such colloids.

We demonstrate the existence of discrete single molecular [ All of these clusters are colored and hence can be monitored quantitatively by using AUC equipped with absorbance optics. Each cluster type has a distinct attenuance spectrum, and because the optical density at the wavelengths used here is linear with concentration, the absolute amounts can be determined from the attenuance. AUC has been used extensively for, among others, the analysis of protein-protein and protein-DNA/RNA interactions, 12,13 polymers, 14 oxo- † S.R. and K.L.P. contributed equally. * To whom correspondence may be addressed: s.roy@isis.u-strasbg.fr. (S.R.), w.k.kegel@uu.nl (W.K.K.