Illustration of a tetragonal lysozyme crystal showing the shape and orientation of 2D islands which form on the (110) face during growth.
Tetragonal lysozyme crystals grow principally by 2D nucleation growth. These nucleii form on the crystal face and then grow by the addition of growth units. The aim of this work was to determine the growth units involved for the growth of the (110) face. This would facilitate the verification of predictions of the molecular growth mechanism of these crystals from Periodic Bond Chain theory. However, in order to determine the growth unit the crystal growth process has to be observed at the molecular level.
The 2D islands that form on the (110) face are oval in shape and oriented along the two growth directions as shown in figure 1. The traditional AFM approach to following the growth process along a step formed by an island is by performing area scans at regular time intervals. An example is shown in figure 2. The growth rate can be determined from the growth velocity of a step.
|
Figure 1:
Illustration of a tetragonal lysozyme crystal showing the shape and orientation of 2D islands which form on the (110) face during growth. |
 |
Figure 2:
Nucleation and growth of a 2D island on the (110) face of a tetragonal lysozyme crystal observed by AFM area scans. The images were collected at ~30 sec intervals. Each image has a width of 15 mm. |
The AFM area scan approach described above suffers from the drawback of requiring a significant amount of time to perform each area scan. The time intervals between scans is typically in the tens of seconds. Such intervals are too long to observe molecular events.
Our solution to overcoming this drawback is to use AFM linescans instead. In the linescan mode the AFM scan is performed repeatedly along a single line. In the resulting image the horizontal direction represents the line morphology, but the vertical direction now represents the time-evolution of this morphology. By performing a linescan across a growth step, its motion in the scan direction can be captured as shown in figure 3. This approach allows the growth process to be observed on the millisecond time scale.
 |
Figure 3:
AFM linescan image obtained by scanning across a growth step on the (110) face under normal growth conditions. The growth step moves continuously over time producing a slope in the linescan image. The gradient of this slope at any point gives the step velocity at that time. |
As shown in figure 3, under normal growth conditions even AFM linescans only show the growth process as a continuous one. In order to observe individual growth events the growth process must be slowed down considerably. This can be accomplished by lowering the supersaturation of the protein solution. In our study the linescans were performed near the saturation limit of tetragonal lysozyme crystals. At these conditions the step could be observed to remain steady for several seconds, punctuated by the addition of individual growth units which caused the step to move a finite distance, as shown in figure 4.
 |
Figure 4:
AFM linescan image obtained by scanning across a growth step at very low supersaturations. The growth step is initially stationary. After about 10s the step jumps discontinuously by 12.1 ± 1.8 nm, following which it remains stationary for the remainder of the linescan. |
By measuring the distance moved by the step in figure 4 the size of the growth unit in that direction for this growth event can be obtained. Many such linescans were carried out in the two principal growth directions on the (110) face (see figure 1). Click here to see the statistics from these experiments and what they tell us about the growth process on this crystal face.
Return to the homepages of:
|
|
|
|
|
|
 |
 |
 |
 |
 |