W0439

Low-resolution Phasing by Se-MAD in the Presence of Radiation Damage. Constantina Bakolitsa*, Ana Gonzalez⁺, Frank von Delft^§, Robert C. Liddington*, *The Burnham Inst., 10901 North Torrey Pines Rd., La Jolla, CA 92037, ⁺SLAC, 2575 Sand Hill Rd., Menlo Park, CA 94025, ^§The Scripps Research Inst., 10550 North Torrey Pines Rd., La Jolla, CA 92037.

Phasing by Se-MAD is currently the most popular technique for de novo structure determination. However, with increasingly powerful synchrotron sources, radiation damage is emerging as a limiting factor in determining the success or failure of a Se-MAD experiment. Collecting data at the inflexion and high energy remote wavelengths was recently suggested as the optimal strategy for minimizing radiation damage but the study was limited to well-diffracting samples and low radiation doses¹.

As a case study, we will present the phasing by two-wavelength Se-MAD of protein crystals affected by significant radiation damage as indicated by a 0.5-0.7% increase in cell dimensions between the initial and final frames collected. Radiation-induced oxidation damage was also the cause for the absence of visible density for 1/3 of expected Se sites. Internal non-isomorphism rendered global scaling of data invalid, and both Patterson and dual-space direct methods failed to locate the anomalous scatterers; they were eventually located by combining local scaling and direct methods².

We will discuss the data collection and phasing strategies employed in this particular case and seek to analyze and extend the applicability of phasing by Se-MAD in the presence of radiation damage.