Using the MicroRT™ System for Room- and Low-Temperature Crystallography

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MicroRT capillaries are made of clear thin-wall plastic that gives less background scatter than regular quartz X-ray capillaries. They won’t break or shatter, and can be cut with scissors or a razor blade. They are pre-sealed at one end, so you don't need to use sealing wax. They must be used with Mitegen’s goniometer bases, which are designed to capture the MicroRT capillaries. Our goniometer bases come in a variety of standard styles to provide compatibility with all sample changing hardware. Our MicroRT Aligner makes sliding the capillary past your crystal without bumping it and onto any of our goniometer bases a breeze. If you don't use the Aligner, we recommend our GB-B3/B3A/B3S style bases and their reusuable counterparts.

To prepare a sample, start by injecting your environment-stabilizing solution (e.g., mother liquor or reservoir solution) into the MicroRT capillary tube using a pipette with a gel-loading tip. Try to inject the solution all the way down to the sealed end, so that there's no gas bubble between the end and the liquid plug. This will help the liquid plug to stay put even if the sample is roughly handled, and eliminate plug motion caused by temperature changes.

Be generous with your plug size (say, 10-40 μl), especially if you want the crystal to remain fully hydrated for several days. There is some evaporation (roughly 80 nl/hour) through the very thin wall of the capillary tube.

Mount your crystal on a MicroMount, MicroLoop, MicroMesh or MicroGripper that's been inserted into a Mitegen goniometer base. Remove excess liquid to minimize the chance of sample slippage, but don't let the crystal dry out entirely or it may fall off the mount. Or use a MicroGripper to securely hold your crystal.

Attach your MicroMount+base to a magnetic wand, and place it and the MicroRT capillary in the MicroRT Aligner. Slide the capillary tubing down the channel, past the crystal and onto the base. If you don't have a MicroRT Aligner, work under a magnifier or microscope, and draw the MicroRT capillary tubing down over the crystal and mount and onto the goniometer base, being careful not to bump the crystal. To make a better seal between the capillary and base, apply a tiny amount of grease (e.g., Dow-Corning #976V high vacuum grease) or oil to the capillary-capturing tip of the goniometer base. This generally isn't necessary if you're going to collect data within the next few hours.

Align the crystal manually or using an autoalignment system, just as you would when not using the MicroRT capillary.

After you've collected a desired number of frames, slide the MicroRT capillary off. You can then soak the crystal in any desired solution or replace the liquid plug with another solution (e.g., to vapor diffuse in a small molecule or to dehydrate the crystal), and then draw the capillary back over the crystal and take more frames.

Once you've collected your room temperature frames, just pop off the MicroRT capillary, plunge the mount and crystal into your favorite liquid cryogen, and you're ready to collect a low-temperature data set.

If you want store your crystals for longer than a few days, replace the MicroRT tubing with any 2 mm ID glass or thick-walled plastic tubing.

The MicroRT system can also be used with conventional nylon loop mounts. However, since the loops can have random positions and orientations relative to the pin and tube axis, the chance of bumping the loop and crystal while drawing the tubing is much greater.

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