IS THE ATOMIC RESOLUTION NECESSARY FOR PROTEIN CRYSTAL STRUCTURE SOLUTION?

A new probabilistic formula estimating triplet invariants [1], and capable of exploiting, as prior information, a model electron density map which is gradually created during the ab initio phasing process, has been recently tested on a set of protein structures with data at non atomic resolution [2]. All the test structures contain heavy atom larger than Ca, and show a structural complexity which may attain the value of about 8000 non-hydrogen atoms in the asymmetric unit: their data resolution varies in the interval (1.5, 2.1 Å). It has been shown that most of such structures were solved by our new Direct Methods procedure MDM [3], against the traditional common believe that atomic resolution is a necessary ingredient for the success of direct ab initio phasing. The aim of this work is to refine and to solve those test structures for which MDM was not able to provide sufficiently good models. The new technique involves the massive use of three supplementary tools: a)the so called hybrid (1-2)-Fourier synthesis ?Q = ? - 2?p according to the theory described by [4]. b)the VLD (Vive La Difference) algorithm [5] [6] [7] [8]; c)the Free Lunch algorithm [9] [10]. We show that most of the structures resistant to MDM are now solved by such combined procedure . [1] M.C. Burla, B. Carrozzini, G.L. Cascarano, G. Comunale, C. Giacovazzo, A. Mazzone, G. Polidori, (2012), Acta. Cryst. A68, 513-520. [2] M.C. Burla, B. Carrozzini, G.L. Cascarano, C. Giacovazzo, G. Polidori, (2015), J. Appl. Cryst. 48, 1692-1698. [3] M.C. Burla, C. Giacovazzo, G. Polidori, (2013), J. Appl. Cryst. 46,1592-1602. [4] M.C. Burla, B. Carrozzini, G.L. Cascarano, C. Giacovazzo, G. Polidori, (2011), Acta Cryst. A67,447-455. [5 M.C. Burla, R. Caliandro, C. Giacovazzo, G. Polidori, (2010) Acta Cryst. A66, 347-361. [6] M.C. Burla, C. Giacovazzo, G. Polidori, (2010), J. Appl. Cryst. 43,825-836. [7] M.C. Burla, C. Giacovazzo, G. Polidori, (2011), J. Appl. Cryst. 44,193-199. [8] M.C. Burla,R. Caliandro, B. Carrozzini, G.L. Cascarano, C. Giacovazzo, G. Polidori (2011), J. Appl. Cryst. 44,1143-1151 [9] R. Caliandro, B. Carrozzini, G.L. Cascarano, L. De Caro, C. Giacovazzo, D. Siliqi, (2005) Acta Cryst. D61, 556-565. [10] R. Caliandro, B. Carrozzini, G.L. Cascarano, L. De Caro, C. Giacovazzo, D. Siliqi, (2005) Acta Cryst. D61, 1080-1087.