DLS under Oil

45 A comparison with DLS measurements in a MRC plate and a cuvette shows that the quality of DLS within the NextalQia1 plate at well A3 and A4 is a bit better as in a MRC plate and not as good as in an optical cuvette. Figure 25 shows – based on the DLS measurements carried out – in which of the wells of a NextalQia-1 plate DLS measurements are possible (green). In the other wells DLS measurements are restricted (E and F9, G6) or impossible. DLS measurements are impossible when laser or detector is blocked by the high walls separating the 2W1R compartments within this type of plate. When using smaller volumes (< 1.2 µL) DLS will become impossible in nearly all wells. The distribution of red on the plate indicates a problem stemming from the polar stage. Since the relative position of laser and detector to the walls varies due to the way positioning is carried out by the stage not all wells are affected from blocking of the DLS optics. To overcome this problem the polar stage – being beneficial in the case of capillaries – should be coupled with a x-y-stage that would allow better measurements in the case of plates such as the NextalQia1.

46 be achieved by replacing paraffin oil with other oils that are permeable for water vapor [112]. Most known is the commercially available “Al´s Oil”, a 1:1 mixture of paraffin oil and silicone oil. In this chapter the application of in situ DLS towards both methods will be described. So far no DLS monitoring of droplets in microbatch plates had been reported.

Interesting about the DLS monitoring within microbatch plates under oil is the fact that the droplets are contained in a stable environment. If pure paraffin oil is used the drop size will be stable for weeks and thus a once adjusted DLS signal will be stable for a very long time. Any change of the recorded ACF can hence be attributed to changes in the drop making DLS results very reliable. Furthermore the oil layer forms a flat surface that should cause no distortions of the laser light (as it is the case with the laser on the round surface of sitting drop vapor diffusion droplets). The path of light through the oil towards onto the round drop bears not such a strong risk of distortions and flares that can deteriorate the DLS signal since the refractive index of paraffin oil (1.47, [116]) and water (1.33, [117]) differ not as much as it is the case between water and air (1.00). Another reason for expected good results of light scattering experiments within droplets under oil is that most of the dust that always resides on a plate is adsorbed in the oil layer, since the oil is poured onto the plate prior to the protein droplets. Moreover the remaining dust in the system will be enriched over time in the viscous oil rather than in water.

3.5.2. Materials and Methods

For initial experiments lysozyme (Merck, Germany, 2 µL, 45 mg/mL in NaOAc/HOAc buffer pH 4.75) was centrifuged at 16’100 x g for 30 minutes and pipetted manually into wells of a Terazaki plate (Nunc, Denmark). The plate was treated previously with paraffin oil (Applichem, Germany) in a way that all wells were filled with oil. Measurements were carried out using a custom designed adaptor for the SpectroLIGHT 500. The optical head was moved relatively to the plate in order to obtain a valid DLS signal in a similar manner as described for the adjustment of measurements in the case of 96 well plates. Furthermore PfGST [102] (Glutathione s-transferase from Plasmodium falciparum, MW: 25 kDa, PDB accession code: 1PA3) in its buffer (0.1 M Tris pH 8.0, 10 mM glutathione) was obtained from RAPHAEL

EBERLE, who had expressed and purified the protein, in terms of a collaborative work to analyze protein quality and monitor crystallization. Droplets were prepared as

47 described for lysozyme. All non-protein solutions were filtered through a 0.22 µm syringe filter (VWR, Germany) prior to use.

3.5.3. Results and Discussion

From the initial DLS measurements in lysozyme solutions within Terazaki plates an ACF typical for monomodal solutions with an intercept of 1.7 could be obtained. Radius distribution analysis showed one particle species in solution with a RH of 2.2 nm (Figure 26).

Figure 26: Results of initial DLS measurements within a small droplet under oil in a Terazaki plate. A 45 mg/mL solution of lysozyme was used as sample.

After the possibility to obtain valid DLS measurements within small droplets under oil was assessed, a crystallization experiment was monitored by DLS. 2 µL Lysozyme were pipetted into the well of a pre-oiled Terazaki plate and small volumes of 1 M NaCl solution were added stepwise. Between the additions DLS measurements were recorded.

As can be seen in Figure 26 the first addition of NaCl (1 µL) led to an increase of protein radius from approx. 2 to 4 nm. After another 1 µL of 1 M NaCl solution was added the monomer radius increases further to approx. 6-7 nm. Moreover a second particle species appears in solution at roughly 90 nm. This second species represents either nuclei or – following the two step nucleation hypothesis [47] – the dense mesoscopic phase from which then macroscopic nuclei emerge. After nucleation appeared the drop was diluted by 2 µL of buffer. The second particle

48 species immediately starts to vanish. However the monomer RH shows only slow decrease.

Figure 27: Contour plot of the development of radius distribution over time derived by in situ DLS within a Terazaki plate. Stepwise precipitant addition led to stepwise growth of protein monomer RH. The colors of the contour plot indicate the relative intensity of the particle, with blue being zero and dark red being maximum.

After these initial measurements, automation of DLS within Terazaki plates was achieved (see chapter 3.6). Moreover it could be shown that droplets under oil bear ideal conditions for DLS measurements thus this method was used to replace the conventional DLS measurements in optical cuvettes in the case of e.g. buffer optimization. The advantage over MRC plates is that the wells within a Terazaki plate are automatically sealed by the oil. Additionally the straight oil surface reduces flares and laser reflections to a minimum (see Figure 28). The crystallization of PfGST [102] under batch conditions was optimized for crystallization of the protein in a containerless environment in levitated drops [118] in Xi’an, China. In the course of this optimization in situ DLS was applied to optimize conditions rationally. In Figure 29 the results of such an experiment are presented. The clear difference in ACF and radius distribution between pure PfGST and PfGST at crystallizing conditions showed that even if the pure protein is not monomodal in solution the crystallization process could be investigated by in situ DLS.

49

Figure 28: DLS within Terazaki microbatch plates under oil. A) and B) DLS laser within a normal crystallization droplet. C) Laser trace within such a drop without additional illumination, D) ACF of an in situ DLS measurement within the drop displayed under C), E) and F) Laser trace in non-perfect droplets. Nearly no flares distortions can be observed, DLS measurements are possible. G) and H) at the end of an in situ DLS monitored crystallization experiment of the protein PfGST. DLS does not influence the crystal growth when applying microbatch under oil conditions.

A)

H) G)

E) F) C) D)

B)

50

Figure 29: Comparison of ACF (A) and radius distribution (B) of PfGST (the cartoon plot in the upper right side shows the structure of PfGST, ) in its buffer (blue, 0.1 M Tris pH 8.0, 10 mM Glutathion) and during crystallization (violet) after mixing 2 µL of ammonium sulfate solution (2.6 M, 0.1 M Tris pH 8.0, 10 mM Glutathion) with 1 µL of PfGST. From this set up crystals grew after 12 hours (see Figure 28 G).

3.5.4. Conclusions

It could be shown for the first time that DLS measurements within small droplets under oil in Terazaki plate are possible and can deliver information to optimize the crystallization process. Moreover the quality of DLS measurements in droplets under oil proved to be good enough to replace optical cuvettes or MRC plates in the case of pre-crystallization DLS (e.g. buffer optimization). An example for the application of DLS towards droplets under oil can be found in chapter 5.3.7.

3.6. Automation of Measurements