ブックタイトル日本結晶学会誌Vol60No5-6

概要

日本結晶学会誌Vol60No5-6

The Long-wavelength Macromolecular Crystallography I23 at Diamond Light SourceFig.1 a）Transmission for different path lengths through lysozyme crystals. b）Resolution as function of diffraction angle（2θ）for different wavelengths.while VMXi was designed for in-situ screening experimentsand VMXm will open new opportunities for diffractionexperiments from sub-micron sized crystals. I23 is a beamlinewhich is radically novel in its design, operating in vacuum,with a custom-made detector, allowing experiments towavelengths as long as 5.9 A. In the following this uniquefacility will be introduced.2．The in-vacuum long-wavelength Beamline I23The motivation to extend the accessible wavelength rangebeyondλ＝2 A is to get access to the absorption edgesof calcium, potassium, chlorine, sulphur and phosphorus,elements of high relevance in biology. In Table 1 the energiesand wavelengths for these absorption edges are given andtheir anomalous contributions to the scattering factor f”arepresented. For sulphur f”increases by about a factor of twowhen increasing the wavelength fromλ＝2 A toλ＝3 A andeven a factor three toλ＝4 A. This significant increase ofthe anomalous signal can open new opportunities for nativephasing experiments, towards structure solutions at lowerresolutions or with lower sulphur content.However, such experiments are not trivial as two mainchallenges have to be taken into consideration, the increasingdiffraction angles and absorption effects（Fig.1）.2.1 In-vacuum crystallographyPerforming experiments in vacuum addresses twoimportant aspects, air absorption and air scattering. DiffractedX-rays are not attenuated between the crystal and thedetector surface, making the experiments efficient in termsof radiation damage. Similarly, elimination of air scatteringallows high signal-to-noise ratios by reduction of the noise.The long-wavelength MX beamline is the first beamline for日本結晶学会誌第60巻第5･6号（2018）macromolecular crystallography operated entirely in vacuum.The complete sample environment, including sample hotelfor storage, goniometer and detector share the same vacuumspace. The entire beamline is windowless, directly connectedto the storage ring vacuum. The pressure difference from highvacuum（2×10－8 mbar）inside the I23 end station to the ultrahighvacuum of the storage ring is realised by differentialpumping.2.2 Sample holders & transferSample cooling in vacuum has to be realised by thermalconduction. Heat is conducted from the sample through thesample mount, holder and goniometer to a pulse tube cooler（PTC）which is mounted on top of the vacuum vessel. Thiscooling path consists mainly of copper, with the flexible partsof the goniometer being realised by stacks of copper sheets.The temperature of the second stage of the PTC is 14 K underfull thermal load, rising to around 40 K in the goniometerhead. A good thermal contact between the goniometer headand the sample holders is realised by magnets providing acontact force of around 10 N. 12）Various materials were testedto support the crystals. A good compromise had to be foundbetween thermal conductance, low X-ray absorption andscattering and mechanical properties for crystal harvesting.Litholoops（Molecular Dimensions）and laser cut mounts /loops from black kapton（DuPont）have been successfullyused for experiments on I23. Both materials can beconsidered as thermal insulators, but due to the short distance（＜1 mm）from the tip of the copper pins to the crystal on thesample holders, cooling is sufficient to keep samples belowthe glass transition temperature during the experiments. 13）Figure 2 a and b shows details of the sample holders.Samples are transferred into the vacuum end station235