大型単位格子を持つタンパク質結晶の測定を可能にするJ-PARC中性子回折装置の設計

Designing neutron difractometer for protein crystal with large unit cell at J-PARC

友寄 克亮; 栗原 和男; 玉田 太郎; 黒木 良太

Tomoyori, Katsuaki; Kurihara, Kazuo; Tamada, Taro; Kuroki, Ryota

J-PARCの物質生命科学施設(MLF)に建設予定の生体高分子専用高分解能単結晶中性子回折装置では、生命現象の基盤となる役割を担っている膜蛋白質などの大型蛋白質の中性子構造解析を対象とする。したがって反射スポットの分離が鍵となるため線源にはパルス幅が短い非結合型減速材(DM)を選択する。しかしDMから得られる中性子強度が比較的弱いことに加えて、対象とする蛋白質の単位格子体積が大きいため反射強度が弱く、水素原子からの非干渉性散乱の効果が大きいため十分なS/Nが得られない。さらに入射中性子パルス形状が時間方向に非対称な分布を持つためこの問題が顕在化する。本内容は、時間方向のパルス形状を忠実に再現するための関数と精度の高いバックグラウンド除去法を組み合わせた反射積分方法の開発に関する提案である。

The energy struggling functions such as Vavilov and Landau distributions used to describe the energy loss of charged particles traversing a thin absorber were found to be in excellent agreement with the observed time-of-flight (TOF) profile observed by a TOF single-crystal diffractometer installed at a coupled moderator (CM) pulsed neutron source in J-PARC. It might be expected that the functions are also applicable in the case of decoupled moderator (DM) because the asymmetrical tail of the emission pulse is generated by the thermalized delayed neutron as with the CM while the pulse width of the DM is small compared with that of the CM. In the case of DM with small tail, the problem regarding the overlap of reflections might not be so serious even with large unit cell. On the other hand, it should be noted that the weak signals from biomarcromolecular crystals with the large unit cell are buried under the high level background caused by the incoherent scattering of hydrogen atoms. The profile functions especially might be useful to evaluate precisely the weak signal due to large unit cell of crystals. We will propose the profile-fitting integration method with the combined use of the energy struggling functions and background subtraction method by the Statistics-sensitive Nonlinear Iterative Peak-Clipping (SNIP) algorithm.