Probabilistic Estimate of |Foa| from FEL Data

Carmelo Giacovazzo,Giovanni Cascarano,Benedetta Carrozzini

doi:10.3390/cryst8040175

Carmelo Giacovazzo, Giovanni Cascarano + Show 1 more

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https://doi.org/10.3390/cryst8040175

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Abstract

The method of the joint probability distribution function was applied in order to estimate the normal structure factor amplitudes of the anomalous scatterer substructure in a FEL experiment. The two-wavelength case was examined. In this, the prior knowledge of the moduli | F 1 + | , | F 1 − | , | F 2 + | , | F 2 − | was used to predict the value of | F 0 a | , which is the structure factor amplitude arising from the normal scattering of the heavy atom anomalous scatterers. The mathematical treatment provides a solid theoretical basis for the RIP (Radiation-damage Induced Phasing) method, which was originally proposed in order to take the radiation damage induced by synchrotron radiation sources into account. This was further adapted to exploit FEL data, where the crystal damage is usually more massive.

Highlights

The properties of large tunability, high monochromaticity, small beam divergence and large power of the modern synchrotron beamlines have made MIR, MAD and MIRAS popular tools for determining the crystal structure solution of proteins
The amplitudes of the structure factors corresponding to the anomalous scatterers or to the heavy atom substructure are estimated
The fact that many protein structures have been solved using free-electron lasers (FEL) data obtained by Molecular Replacement techniques suggests that data collected from an undamaged crystal and data collected from a damaged crystal have sufficient internal consistency to be useful in attempts to determine the protein crystal structure

Summary

Introduction

The properties of large tunability, high monochromaticity, small beam divergence and large power of the modern synchrotron beamlines have made MIR, MAD and MIRAS popular tools for determining the crystal structure solution of proteins. The fact that many protein structures have been solved using FEL data obtained by Molecular Replacement techniques suggests that data collected from an undamaged crystal (low fluence) and data collected from a damaged crystal (high fluence) have sufficient internal consistency to be useful in attempts to determine the protein crystal structure. This was the main reason that encouraged us to describe a probabilistic theory integrating FEL damage into the framework of the anomalous dispersion techniques [12]. This work focuses on the first step of a phasing procedure, which involves finding the heavy atom positions according to the diffraction data

The Mathematical Model

Conclusions