A. Meents, P. Roedig, I. Vartiainen, R. Duman, S. Panneerselvam, N. Stuebe, O. Lorbeer, M. Warmer, G. Sutton, D. I. Stu art, E. Weckert, C. Da vid, and A. Wag ner Deutsches Elektronen Syn chro tron DESY, Pho ton Sci ence, Notkestraße 86, 22607 Ham burg Paul Scherrer Institut, Villigen PSI, 5232, Swit zer land Di a mond Light Source Ltd., Di a mond House, Harwell Sci ence & In no va tion Cam pus, Didcot, Oxfordshire, OX11 0DE, United King dom Di vi sion of Struc tural Bi ol ogy, Wellcome Trust Cen tre for Hu man Ge net ics, Uni ver sity of Ox ford, Ox ford, OX3 7BN, United King dom alke.meents@cfel.de

At low emittance synchrotron sources it has become possible to perform structure determinations from the measurement of multiple microcrystals which were previously considered too small for diffraction experiments. Conventional mounting techniques do not fulfill the requirements of these new experiments. They significantly contribute to background scattering and it is difficult to locate the crystals, making them incompatible with automated serial crystallography. We have developed a micro-fabricated sample holder from single crystalline silicon with micropores, which carries up to thousands of crystals and significantly reduces the background scattering level. For loading, the suspended microcrystals are pipetted onto the chip and excess mother liquor is subsequently soaked off through the micropores. Crystals larger than the pore size are retained and arrange themselves according to the micropore pattern. Using our chip we were able to collect 1.5 Å high resolution diffraction data from protein microcrystals with sizes of 4 micrometers and smaller.

Single-wavelength anomalous dispersion of sulfur atoms (S-SAD) is an elegant phasing method to determine crystal structures that does not require heavy atom incorporation or selenomethionine derivatization. Nevertheless this technique has been limited by the paucity of the signal at usual X-ray wavelengths, requiring very accurate measurement of the anomalous differences. Here we report the data collection and structure solution of the N-terminal domain of the ectodomain of Hepatitis C virus (HCV) E1, from crystals that diffracted very weakly. By combining the data from 32 crystals it was possible to solve the sulfur substructure and calculate initial maps at 7Å resolution, and after density modification and phase extension, using a higher resolution native dataset, to 3.5Å resolution, model building was achievable. The crystal structure of the N-terminal domain of reveals a complex network of covalently linked intertwined homodimers that do not harbor the expected truncated class II fusion protein fold.