Protein crystallization underpins structural biology and industrial separation processes, yet predicting and controlling its pathways remains hard. Classical nucleation theory treats crystallization as a random process in which monomers assemble into a critical nucleus, after which the crystal starts to grow. For many proteins, ferritin among them, this model fails to explain the full range of observed behavior. A growing body of evidence points to alternative, non-classical pathways involving metastable intermediate phases, liquid-liquid phase separation, or amorphous precursors. This work sits somewhat off to the side: nearby in the archive are studies of proteins in beams, filaments in flow, crystals with memory, and the elasticity of liquid in a trap, but there are no direct neighbors on ferritin crystallization in levitating droplets. This is the first attempt to combine acoustic levitation with X-ray scattering to map crystallization pathways in real time.
A droplet of ferritin solution with polyethylene glycol (PEG) and salt, about 10 microliters in volume, was injected into an ultrasonic acoustic levitator operating at a frequency of 100 kHz. The droplet evaporated, and its volume shrank, which drove up the concentrations of protein and polymer. At the same time, an X-ray beam was passed through the droplet at the CoSAXS beamline at MAX IV (Lund, Sweden), with an energy of 18 keV and a beam size of 100×100 micrometers. Detectors recorded small-angle (SAXS) and wide-angle (WAXS) scattering with an exposure of 0.1 seconds. An optical microscope imaged the droplet to estimate its volume. The researchers measured the protein structure factor, which shows how proteins are arranged relative to one another, and the intensity of Bragg peaks, which signal crystalline order. They varied the PEG concentration (0, 3, 5, 7 mass percent) and its molecular mass (1000 and 6000 g/mol).
As a droplet with PEG6000 evaporated, the protein first stayed in solution, then at a concentration of about 400–500 mg/ml Bragg peaks appeared, indicating the formation of crystals. With further dehydration, however, the peaks vanished, meaning the crystals lost their order. This process coincided in time with the onset of semi-crystallization of the PEG itself, but the authors showed that the loss of order was not caused by the polymer. The structure factor at small angles showed that attractive forces act between proteins before crystallization, which is typical of a pathway governed by direct interparticle interactions. When PEG6000 was replaced with PEG1000, the picture changed: crystallization began later, when the droplet volume was already nearly constant, and the crystals remained stable until the end of the experiment. Interactions between proteins were predominantly repulsive, and crystallization occurred at high concentration, pointing to a pathway governed by excluded-volume effects, as in a hard-sphere system. The distance between proteins in the crystal was larger for PEG1000 than for PEG6000, which confirms the difference in packing.
The work stands apart from its neighbors in that it is the first to link the molecular mass of a polymer to the choice of crystallization pathway and to the stability of protein crystals under continuous concentration. Neighboring studies examined proteins in beams or the elasticity of liquid in a trap, but did not look at crystallization kinetics in a levitating droplet. Here it is shown that short PEG1000 chains stabilize crystals during dehydration, possibly through adsorption on the protein surface and retention of water. This provides a simple tool for improving crystallization protocols in structural biology, where stable crystals are often needed for X-ray crystallography.
The method of acoustic levitation combined with X-ray scattering can be extended to other proteins and polymers to build maps of crystallization pathways. It would be interesting to test how the adsorption of PEG of different lengths affects the local environment of the protein in the crystal, for instance using molecular modeling or neutron scattering. One could also investigate the role of evaporation rate and initial salt concentration in order to control nucleation and crystal growth.
The results will affect structural biology, where stable crystals are needed to determine protein structures, and pharma, where the crystallization of protein drugs matters for purification and formulation.
The next step will be to test the hypothesis that PEG1000 adsorbs on the ferritin surface, using spectroscopy or calculations, and to try other polymers with different chain lengths.
The work is directly tied to the unsolved problem of predicting non-classical protein crystallization pathways, where classical nucleation theory breaks down. It shows how the molecular parameters of additives can switch the crystallization mechanism, which matters for controlling polymorphism and crystal quality.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The work follows ferritin crystallization in an acoustically levitated evaporating droplet using simultaneous SAXS/WAXS and optical microscopy. By varying the concentration and molecular mass of PEG, the authors showed that PEG1000 slows crystallization but stabilizes the crystals during dehydration, whereas PEG6000 leads to their destruction. An analysis of the structure factor reveals a transition from attractive to repulsive interactions between the proteins.
Where its strength is
The strong point is the combination of acoustic levitation with X-ray scattering for continuous, contact-free tracking of structural evolution within a single droplet, which makes it possible to distinguish crystallization pathways by their interparticle interactions.
Where it could go next
Worth trying
Why this may matter
Understanding how additives govern the stability of protein crystals during dehydration matters for developing methods to store and deliver protein formulations, and also for improving crystallization protocols in structural biology.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
A droplet with a volume of 10 microliters evaporates in the levitator in roughly 20 minutes, and over that time the protein concentration climbs from 50 to 500 mg/ml, that is, by a factor of 10.
In Stanislaw Lem's novel Solaris, the ocean creates structures that cannot be understood by earthly methods; here, by contrast, we see how a simple polymer governs the birth and death of protein crystals in a droplet suspended in midair.