Atomic Force Microscopy Of Protein Films And Crystals

A new AFM instrument dedicated to protein crystal imaging in solution is here successfully introduced, as proven by its testing on both the crystals and the Langmuir-Blodgett films of two proteins having quite different molecular weight, namely chicken egg white lysozyme and bovine serum albumin. The AFM consists of a custom measuring head with a flexible SPM controller in house produced which effectively drives the head for contact, non-contact and spectroscopy modes. By providing the user with full control and monitoring of signals at the front panel we have achieved an high degree of customisation optimal for the protein crystal measurements here described. This approach allows to study the crystal periodicity and morphology in their mother liquid. In this way, protein crystal structure is not destroyed, as in the case of drying, and one can thereby investigate its periodic structure in its “natural aqueous” environment. Comfortingly it appears to distinguish the protein crystals from the salt crystals, which under the optical microscope are frequently quite similar and often their difference is revealed only during X-ray analysis. Finally the AFM estimates of the given single proteins packing, order and morphology appear quite similar in the LB thin film and in the crystals, thereby allowing routine crystal measurements at high resolution reproducing previous report on membrane protein bovine cytochrome P450scc similarly studied in LB films and crystals. Our intention for the future is to attempt the exploitation of the use of optical tweezers for both AFM and microGISAXS studies of protein crystal growth, considering the recent proven ability to utilize the birefringence of lysozyme crystals to control their orientation and to monitor their growth in optical tweezers.