Microscope detection options for colorless protein crystals grown in lipidic cubic phases

• Published in: Journal of applied crystallography 2003-10, Vol.36 (5), p.1295-1296
• Main Author: Nollert, Peter
• Format: Article
• Language: English
• Subjects: Analytical biochemistry: general aspects, technics, instrumentation; Analytical, structural and metabolic biochemistry; Biological and medical sciences; Conventional optical microscopes; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; lipidic cubic phase; membrane protein crystallization; microscopy; Optical instruments, equipment and techniques; Physics
• ISSN: 1600-5767; 0021-8898; 1600-5767
• DOI: 10.1107/S0021889803013724

The use of lipidic cubic phases as crystal nucleation and growth matrices is becoming popular and has yielded crystals of soluble and membrane proteins. So far, all of the membrane proteins crystallized by this method have been colored. This feature has facilitated the detection of the often encountered microcrystals in initial screening rounds. Indeed, small colorless protein crystals have poor optical contrast as a result of the small differences in refractive index of the protein crystal and the surrounding lipidic cubic phase. While a perfect preparation of a lipidic cubic phase is transparent and optically isotropic, in a crystallization setup it frequently disguises crystals due to cracks, inclusions, surface distortions and phase boundaries. Here, several specialized microscopic techniques and illumination conditions are compared and it is found that sufficient contrast is generated by cross polarization microscopy and by Hoffman modulation contrast microscopy for the detection of colorless protein crystals.