title: The hidden treasure in your data: phasing with unexpected weak anomalous scatterers from routine data sets.
abstract: Single-wavelength anomalous dispersion (SAD) utilizing anomalous signal from native S atoms, or other atoms with Z ≤ 20, generally requires highly redundant data collected using relatively long-wavelength X-rays. Here, the results from two proteins are presented where the anomalous signal from serendipitously acquired surface-bound Ca atoms with an anomalous data multiplicity of around 10 was utilized to drive de novo structure determination. In both cases, the Ca atoms were acquired from the crystallization solution, and the data-collection strategy was not optimized to exploit the anomalous signal from these scatterers. The X-ray data were collected at 0.98 Å wavelength in one case and at 1.74 Å in the other (the wavelength was optimized for sulfur, but the anomalous signal from calcium was exploited for structure solution). Similarly, using a test case, it is shown that data collected at ∼1.0 Å wavelength, where the f'' value for sulfur is 0.28 e, are sufficient for structure determination using intrinsic S atoms from a strongly diffracting crystal. Interestingly, it was also observed that SHELXD was capable of generating a substructure solution from high-exposure data with a completeness of 70% for low-resolution reflections extending to 3.5 Å resolution with relatively low anomalous multiplicity. Considering the fact that many crystallization conditions contain anomalous scatterers such as Cl, Ca, Mn etc., checking for the presence of fortuitous anomalous signal in data from well diffracting crystals could prove useful in either determining the structure de novo or in accurately assigning surface-bound atoms.
authors.affiliation: Division of Biological Sciences, Poornaprajna Institute of Scientific Research, #4, 16th Cross, Sadashivnagar, Bangalore 560 080, India.
authors.forename: Raghurama P
authors.initials: RP
authors.lastname: Hegde
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine, Ullmann Building, Jack and Pearl Resnick Campus, 1300 Morris Park Avenue, Bronx, New York, NY 10461, USA.
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Fedorov
authors.affiliation: Lilly Biotechnology Center, Eli Lilly and Company, 10290 Campus Point Drive, San Diego, CA 92121, USA.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Sauder
authors.affiliation: RCSB Protein Data Bank, Center for Integrative Proteomics Research, Rutgers University, The State University of New Jersey, 174 Frelinghuysen Road, Piscataway, NJ 08854, USA.
authors.forename: Stephen K
authors.initials: SK
authors.lastname: Burley
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine, Ullmann Building, Jack and Pearl Resnick Campus, 1300 Morris Park Avenue, Bronx, New York, NY 10461, USA.
authors.forename: Steven C
authors.initials: SC
authors.lastname: Almo
authors.affiliation: Division of Biological Sciences, Poornaprajna Institute of Scientific Research, #4, 16th Cross, Sadashivnagar, Bangalore 560 080, India.
authors.forename: Udupi A
authors.initials: UA
authors.lastname: Ramagopal
mesh_terms: D000818:Animals; D019843:Archaeal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002118:Calcium / Q000737:chemistry*; D002413:Cations, Divalent; D002645:Chickens / Q000378:metabolism; D003001:Cloning, Molecular; D003460:Crystallization; D018360:Crystallography, X-Ray; D004527:Egg Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D005822:Genetic Vectors / Q000737:chemistry / Q000378:metabolism; D009113:Muramidase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D044224:Pseudomonas syringae / Q000737:chemistry; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013455:Sulfur / Q000737:chemistry*; D019603:Thermoplasmales / Q000737:chemistry; D014961:X-Ray Diffraction; D014965:X-Rays
chemical_list: D019843:Archaeal Proteins; D001426:Bacterial Proteins; D002413:Cations, Divalent; D004527:Egg Proteins; D011994:Recombinant Proteins; D013455:Sulfur; D009113:Muramidase; D002118:Calcium
keywords: SAD phasing; weak anomalous signal