New data reduction protocol for Bragg reflections observed by TOF single-crystal neutron diffractometry for protein crystals with large unit cells
In protein crystallography, high backgrounds are caused by incoherent scattering from the hydrogen atoms of protein molecules and hydration water. In addition, the scattering intensity from large unit-cell crystals is very small, which makes it difficult to improve the signal-to-noise ratio. In the case of time-of-flight (TOF) single-crystal neutron diffractometry, the measured spectra cover four-dimensional space including X, Y, and TOF in addition to intensity. When estimating the integrated intensity, 3D background domains in the vicinity of peaks should be clearly classified. In conventional 1D or 2D background evaluation, the evaluation is applied for individual peaks assigned using peak searches; however, it is quite difficult to classify the 3D background domain in TOF protein single-crystal neutron diffraction experiments. We undertook the development of a data reduction protocol for measurements involving large biomacromolecules. At the initial stage of the reduction protocol, appropriate 3D background estimation and eliminations were applied over the entire range of X, Y, and TOF bins. The histograms were then searched for peaks and indexed, and the individually assigned peaks were finally integrated with an effective profile function in the TOF direction. Three-dimensional deconvolution procedures for overlapping peaks associated with large unit cells were implemented as necessary. This data reduction protocol may lead to the improvement of signal-to-noise ratios to enable TOF spectral analysis of large unit-cell protein crystals.