Mad Phasing at Argonne's Advanced Photon Source. E. M. Westbrook, Center for Mechanistic Biology and Biotechnology, Argonne National Laboratory, Argonne, IL 60439 USA.

Argonne's Structural Biology Center (SBC) Collaborative Access Team (CAT) has just completed construction of two new beamlines designed explicitly with MAD phasing capability for macromolecular crystallography. These two new beamlines: 19ID and 19BM, have essentially identical x-ray optics and experimental equipment, although they have different sources. The 19ID source is an undulator; the 19BM source is an APS dipole magnet. Each beamline is equipped with x-ray optics designed to deliver a stable, intense, highly focused, physically small x-ray beam with low angular divergence onto protein crystal samples. Both beamlines can reach all of the absorption edge energies routinely used in macromolecular crystallography. Currently, 19ID delivers 1.3 x 10¹³ x-ray photons/s onto its sample stage; at its tightest focus- .083mm x .038mm- this yields a flux density of 4.1 x 10¹⁵ x-ray photons/s/mm2. 19BM delivers 2.8x10¹¹ photons/s onto a 0.12mm x 0.040mm focus, which is a flux density of 5.8x10¹³ photons/s/mm2. Both of these beamlines can be rapidly tuned for MAD phasing studies (the undulator gap for 19ID is tuned simultaneously) and, with Si(111) monochromator crystals, energy resolution of about 1 eV can be obtained with slight detuning of the monochromator.

Crystals at SBC beamlines are oriented with a miniature kappa goniostat, permitting alignment of crystal axes for data collection. Data are recorded on a CCD mosaic detector with a square active area 21cm x 21cm. Data frames are transferred to disk at 10 MB/s, permitting rapid frame rates with good duty cycles. Data collection and processing are carried out by J. W. Pflugrath's D*TREK. Although these beamlines are still being commissioned, one new structure has already been determined at 19ID by MAD phasing. We anticipate that these two beamlines will rapidly become important resources for the community for new structure determination by MAD phasing methods.

This work was supported by the U. S. Department of Energy, Office of Health and Environmental Research, under Contract W-31-109-ENG-38.