New expression of bimodal phase distributions in direct-method phasing of protein single-wavelength anomalous diffraction data

doi:10.1088/1674-1056/19/8/086102

Abstract One of the essential points of the direct-method single-wavelength anomalous diffraction (SAD) phasing for proteins is to express the bimodal SAD phase distribution by the sum of two Gaussian functions peaked respectively at $\varphi''_h+|\Delta\varphi_h|$ and $\varphi''_h-|\Delta\varphi_h|$. The probability for $\Delta\varphi$ being positive (P₊) can be derived based on the Cochran distribution in direct methods. Hence the SAD phase ambiguity can be resolved by multiplying the Gaussian function peaked at $\varphi''_h+|\Delta\varphi_h|$ with P₊ and multiplying the Gaussian function peaked at $\varphi''_h-|\Delta\varphi_h|$ with P_- (=1-P₊).The direct-method SAD phasing has been proved powerful in breaking SAD phase ambiguities, in particular when anomalous-scattering signals are weak.However, the approximation of bimodal phase distributions by the sum of two Gaussian functions introduces considerable errors. In this paper we show that a much better approximation can be achieved by replacing the two Gaussian functions with two von Mises distributions. Test results showed that this leads to significant improvement on the efficiency of direct-method SAD-phasing.

Keywords: direct methods single-wavelength anomalous diffraction OASIS program proteins

Received: 21 April 2010
Revised: 29 April 2010
Accepted manuscript online:

PACS:	87.64.Cc	(Scattering of visible, uv, and infrared radiation)
	02.50.Ng	(Distribution theory and Monte Carlo studies)
	87.14.E-	(Proteins)
	87.15.B-	(Structure of biomolecules)

Fund: Project supported by the Innovation Foundation of the Chinese Academy of Sciences and by the National Basic Research Program of China (Grant No. 2002CB713801).

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Zhang Tao (张涛), Gu Yuan-Xin (古元新), Zheng Chao-De (郑朝德), Fan Hai-Fu (范海福) New expression of bimodal phase distributions in direct-method phasing of protein single-wavelength anomalous diffraction data 2010 Chin. Phys. B 19 086102

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New expression of bimodal phase distributions in direct-method phasing of protein single-wavelength anomalous diffraction data

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