DIRECT METHODS PHASE IMPROVEMENT FOR MACROMOLECULAR STRUCTURES USING EITHER SIR OR SAS DATA. D. A. Langs, R. H. Blessing, G. D. Smith, D. Y. Guo. Hauptman-Woodward Medical Research Institute, 73 High St., Buffalo, NY 14203, and Roswell Park Cancer Institute, Buffalo, NY 14263, U.S.A.
Successful efforts at SIR phasing1 using real data measured for threeseparate derivatives of cytochrome c550 at 2.5Å resolution2 (mean phase errors 40-47deg.) were first reported at the 1994 Atlanta ACA meeting. However, phasing methods employing 2.5Å error-free SAS data for the PtCl4 derivative of the same structure, although effective, proved more problematic in adjusting the optimal conditions for successful refinement.3
We subsequently showed to our surprise that a modified tangent formula refinement based on Hauptman's SAS triples invariant estimates4 could produce acceptable solutions which were readily identified by their phase invariant consistency in about 15% of all randomly seeded trials. An easily interpretable map was obtained using 2.5 Å SAS data measured for the K2Pt(NO2)4 derivative of Macromomycin5 (mean phase error 47deg.).
Structural applications to larger proteins, however, may require improved SIR/SAS triples estimates in order to achieve sufficient phasing power to produce similar results. Two new methods will be described which have the capability of reducing the phase error in Hauptman's initial SIR and SAS triples estimates. These analyses should allow us to apply existing SIR and SAS phasing methodologies to more complex structures.
1. Langs, Guo & Hauptman (1995). Acta Cryst. A51. 535-542.
2. Timkovich & Dickerson (1976). J. Biol. Chem. 251, 4033-4046.
3. Langs & Han (1995). Acta Cryst. A51, 542-547.
4. Hauptman (1982). Acta Cryst. A38, 289-294, 632-641.
5. VanRoey & Beerman (1989). Proc. Natl. Acad. Sci. 86, 6587-6591.
Support from NIH grant GM-46733 is gratefully acknowledged.