A fiber optic turbidity system for in-situ monitoring protein aggregation, nucleation and crystallisation

Abstract

The growth of good crystals for diffraction or device application is still largely relevant of art than of science. It is generally admitted, however, that if we were able to learn more about the early steps of nucleation and growth, we would better know how to proceed. Therefore it is not surprising to see in recent years the development of a number of physico-chemical techniques, all aiming at giving us clues on the origin of crystals: determination of solubility curves and phase diagram, scattering techniques with a special emphasis on light scattering and more recently neutrons and X-rays. Static and dynamic light scattering are sensitive techniques to detect variations in the size and interactions of protein molecules in solution. They are useful for monitoring the pre-nucleation/nucleation state of the system, although the interpretation of the resulting measurements can differ. Dynamic light scattering measures the intensity fluctuation caused by Brownian motion of the particles, within the detection volume, over time using correlation methods. It allows measurement of the nuclei until they reach a size suitable for visual observation by microscope. However, elaborate instrumentation and optical alignment problems have made in-situ applications difficult, particularly under microgravity environment. We have developed a new fibre optic probe based on the turbidity technique and have applied to various earth (and later microgravity) protein crystallisation systems to test its capabilities. Turbidity is a scattering technique that is practically insensitive to multiple scattering and is easy to perform. Therefore it is well suited to study concentrated, strongly turbid systems. Our ultra-compact system exploits the Mie theory for optical particle sizing and offers a fast means of quantitatively and non-invasively monitoring in-situ the various growth stages of protein crystallisation. The turbidity system consists of a miniature tungsten halogen source, a temperature controlled crystallization chamber and a miniature spectrometer connected by fiber optics. Newly developed software permit to plot the crystal size and growth in function of the time. A miniaturized microscope with camera is added to the chamber for visualization of macroscopic protein crystals. This new diagnostic tool will permit the exploration of new ways to grow good quality crystals and also provide some scientific basis for understanding the process of crystallization.