W0208

Making Drops Stay Put: Fixing the Contact Line in Hanging Drop Protein Crystallization. Kalinin Y., Berejnov V., Thorne R. E., Physics Dept., Cornell Univ., Ithaca, NY, 14853

Hanging drop vapor diffusion is among the most popular methods for protein crystallization. One advantage is that crystals sediment down to the gas-liquid interface. They are thus less likely to adhere to the glass slide than in a sitting drop, and can be easily removed for diffraction analysis. One disadvantage is that drops can move relatively freely over the hydrophobic siliconized glass cover slips used to minimize drop spreading. The drop usually distorts, spreading laterally over the cover slip, and sometimes slides off when the glass is flipped for well sealing and subsequent crystal retrieval. This motion is due to weak pinning and instability of the contact line of the droplet-glass-air interface.

We have developed a method to fix the contact line and thus fix the droplet position. Treatment using either 1M NaOH, 0.1% HF or ozone of a circular area roughly corresponding to the desired drop size yields large improvements in contact line pinning, as illustrated in the Figure at left. Drops can be flipped without motion, and the drop size and curvature can be customized. Most importantly, the drop size and shape for a given drop volume - which determines the equilibration time with the well - is now much more reproducible. This eliminates an important variable factor affecting nucleation rates and subsequent growth in both hanging and sitting drop vapor diffusion growth.