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

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 surprisin g to see in recent years the development of a number of physico-chemical te chniques, all aiming at giving us clues on the origin of crystals: determin ation 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 var iations in the size and interactions of protein molecules in solution. They are useful for monitoring the pre-nucleation/nucleation slate of the syste m, although the interpretation of the resulting measurements can differ. Dy namic light scattering measures the intensity fluctuation caused by Brownia n motion of the particles, within the detection volume, over time using cor relation methods. It allows measurement of the nuclei until they reach a si ze suitable for visual observation by microscope. Howver, elaborate imstrum entation and optical alignment problems have made in-situ applicatins diffi cult, particularly under microgravity environment. We have developed a new fibre optic probe based on the turbidity technique and have applied to vari ous earth (and later microgravity) protein crystallisation systems to test its capabilities. Turbidity is st 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-compa ct system exploits the Mie theory for optical particle sizing and offers a fast means of quantitatively and non-invasively monitoring in-situ the vari ous growth stages of protein crystallisation. The turbidity system consists of a miniature tungsten halogen source, a temperature controlled crystalli zation chamber and a miniature spectrometer connected by fiber optics. Newl y 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 too l will permit the exploration of new ways to grow good quality crystals and also provide some scientific basis for understanding the process of crysta llization. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.