electronic reprint Journal of Applied Crystallography ISSN 0021-8898 The development and application of a method to quantify the quality of cryoprotectant solutions using standard area-detector X-ray images Michael B. McFerrin and Edward H. Snell Copyright © International Union of Crystallography Author(s) of this paper may load this reprint on their own web site provided that this cover page is retained. Republication of this article or its storage in electronic databases or the like is not permitted without prior permission in writing from the IUCr. J. Appl. Cryst. (2002). 35, 538–545 McFerrin and Snell ¯ Cryoprotectant quantification research papers Journal of Applied The development and application of a method to Crystallography quantify the quality of cryoprotectant solutions ISSN 0021-8898 using standard area-detector X-ray images Received 21 March 2002 Accepted 21 May 2002 Michael B. McFerrin and Edward H. Snell* NASA Laboratory for Structural Biology, Code SD46, MSFC, Huntsville, AL 35812, USA. Correspondence e-mail: [email protected] An X-ray based method for determining initial cryoprotectant concentrations necessary to protect solutions from crystalline ice formation on ¯ash cooling was developed. X-ray images from a charge-coupled device (CCD) area detector were integrated as powder patterns and quanti®ed by determining the standard deviation of the slope of the normalized intensity curve in the resolution range where ice rings are known to occur. The method was tested by determining the concentrations of glycerol, PEG400, ethylene glycol and 1,2-propanediol necessary to form an amorphous glass at 100 K with each of the 98 crystallization solutions of Crystal Screens I and II (Hampton Research, Laguna Hills, California, USA). For conditions that required glycerol concentrations of 35% or above, cryoprotectant conditions using (2R,3R)-( )- 2,3-butanediol were determined. The method proved to be remarkably reliable. The results build on previous work [Garman & Mitchell (1996). J. Appl. Cryst. # 2002 International Union of Crystallography 29, 584±587] and extend the number of suitable starting conditions to alternative Printed in Great Britain ± all rights reserved cryoprotectants. 1. Introduction with PEG400, ethylene glycol and 1,2-propanediol have been Macromolecular crystal X-ray data collection at cryogenic determined using the method. temperatures ( 100 K) has become routine in the home laboratory and is especially important at synchrotron sources. Cryoprotection of crystals greatly reduces radiation damage 2. Experimental from the X-ray beam and improves data quality as a result of A video microscope system was set up off-line consisting of a reduced thermal motion (atomic displacement factors). This COHU CCD, NAVITAR optical telescope, front and side has enhanced many studies, such as ultra-high resolution data illumination, and a Bandit frame grabber board. The system collection and MAD phasing methods. Cryogenic techniques was focused on an alignment pin positioned on a rotatable have been well reviewed (Garman, 1999; Garman & goniometer illuminated from both front and back. An Oxford Schneider, 1997; Rodgers, 1994, 1997). The key to the tech- 600 Cryostream (Cosier & Glazer, 1986) was positioned such nique is preventing ice crystal nucleation by forming an that its tip was 5 mm from the alignment pin at an angle of 45. amorphous glass upon cooling of the sample. Typically cryo- The cryostream was operated at 100 K and the alignment pin protectants are added that impede nucleation and growth of replaced with a magnetic mount. The goniometer head was ice crystals, thus enabling glass formation (Angell & Choi, translated vertically such that the cryoloops used in the 1986; Echlin, 1992; Steinbrecht & Zierold, 1987). experiment were in the center of the ®eld of view when Several commonly used cryoprotectants include glycerol, mounted on the magnetic mount. ethylene glycol, MPD, PEGs, sucrose, erythritol and xylitol. Using a similar method to that of Garman & Mitchell Garman & Mitchell (1996) published the glycerol concentra- (1996), glycerol was heated in a water bath to 343 K and tions required to form an amorphous glass during cryocooling measured out in 30 ml quantities using a positive displacement of the 50 Hampton Research Crystal Screen I solutions pipette into 600 mL microfuge tubes. Heating the glycerol (Jancarik & Kim, 1991). In that study, X-ray images were reduced its viscosity and so allowed accurate repeated collected for 5% increments in glycerol concentration until ice dispensing of the required volume. Six Hampton Screen I rings vanished and a cross section through the diffuse scat- solutions, conditions 2, 6, 33, 36, 38 and 47, were used tering ring had similar slopes on both the high- and the low- sequentially to make up concentrations with cryoprotectant resolution side. We have developed a quantitative analysis increasing in 5% increments. These conditions were chosen to technique and used it here to expand the work of Garman & represent a sample of the conditions originally described by Mitchell (1996). A further 48 cryoprotectant conditions with Garman & Mitchell (1996). Testing these conditions was glycerol and 98 conditions of the Hampton Screens I and II carried out to ensure our results and methodologies were 538 McFerrin and Snell Cryoprotectant quantification J. Appl. Cryst. (2002). 35, 538±545 electronic reprint research papers unsuccessful cryocooling attempt was de®ned as one that had any evidence of opaqueness, features such as lines, or clear ice formation, e.g. Fig. 1(b). In the event of an unsuccessful cooling attempt, the glycerol concentration was increased by 5% and the experiment repeated. A similar procedure was repeated for glycerol with each of the conditions in Hampton Screen II. The percentage of glycerol found from visual observation of successful cooling was used as a starting point for X-ray analysis. X-ray data were collected using a Nonius Kappa2000 CCD detector and an FR591 rotating anode source. A similar cryostat arrangement was used as in the off-line experiments, Figure 1 with an Oxford 600 Cryostream, operating at 100 K, an angle Digital video microscope image showing (a) a clear drop indicating a visually successful cryoprotectant condition and (b) a failure, an opaque of incidence of 45 and a distance of 5 mm from the end of the drop with surface ice features. nozzle to the sample loop. X-ray images were taken at 5% below the concentration percentage that had been visually determined to be successful. Successive images were taken at comparable with those of Garman & Mitchell. A 20 mm nylon 5% intervals until no features were seen in the diffraction cryoloop of diameter 0.7±1.0 mm was used to pick up a sample pattern. If the lowest-concentration image did not show clear of the cryoprotected solution, with the loop nearly parallel to rings (ice or strong diffuse scatter) images were recorded in the surface of the solution, and position it on the goniometer decreasing steps of 5% until rings were seen. The X-ray head in the cryostream. The video microscope provided a clear generator operated at 47 kV and 100 mA with a 0.3 mm image of the loop in the 100 K nitrogen gas stream. Successful diameter collimator and a sample to detector distance of cryocooling was de®ned as that which gave a transparent 75 mm, giving a resolution of 2.8 AÊ at the edge and 2.3 AÊ at smooth glass in the loop under ideal illumination (Fig. 1a). The the corners of the detector. The exposure time of 15 s was sample was warmed and allowed to recool to ensure the determined by imaging of several imperfect conditions conditions formed a smooth transparent glass repeatedly. An showing opaqueness or ice in the video microscope images. These conditions clearly showed diffraction features in this exposure time. The loop was positioned such that it was perpendicular to the X-ray beam. An image of a blank loop perpendicular to the beam was taken as a reference image and an image of the loop parallel to the beam was recorded for comparison. Each X-ray image was visually examined. It was then integrated as a powder diffraction pattern using the POWDERIZE program of the Nonius COLLECT data processing software. A least-squares ®t line was determined based on the regions of data that repre- sented background scatter, where the peaks resulting from ice formation were not observed (from 7 to 17, and 31 to 40 in 2; 12.7 to 5.3 AÊ , and 3.0 to 2.4 AÊ resolution). The reference image of the blank loop was then treated in the same manner to establish a trend- line for the basal level of background scatter. The trend-line from the sample data was then divided by the blank-loop trend-line to produce a scale factor for each point in the sample data. The sample data were then divided by this scale factor at each point, effec- tively normalizing the data to that of the blank loop. An approximation of the derivative was taken at each point of the sample data. The derivative was very sensitive to changes in intensity caused by ice rings. To reduce noise, the standard deviation of the derivative was determined for the `signal' region of the data Figure 2 (between 17 and 31 in 2; between 5.3 and 3.0 AÊ X-ray images of Hampton Screen II condition 22, 0.1 M MES pH 6.5, 12% w/v PEG 20000, showing one quarter of the imaged area for (a) 10%, (b) 15%, (c) 20% resolution). This standard deviation value was used as and (d) 25% 1,2-propanediol. a quantitative measure of the quality of the cryo- J. Appl. Cryst. (2002). 35, 538±545 McFerrin and Snell Cryoprotectant quantification 539 electronic reprint research papers cooling.