SUMMARY
The scan speed limit of atomic force microscopes has been calculated. It is determined by the spring constant of the cantilever k, its effective mass m, the damping constant D of the cantilever in the surrounding medium and the stiffness of the sample. Techniques to measure k, k/m and D/m are described. In liquids the damping constant and the effective mass of the cantilever increase. A consequence of this is that the transfer function always depends on the scan speed when imaging in liquids. The practical scan speed limit for atomic resolution in vacuum is 0·1 μm/s while in water it increases to about 2 μm/s due to the additional damping of cantilever movements. Sample stiffness or damping of cantilever movements by the sample increase these limits. For soft biological materials imaged in water at a desired resolution of 1 nm the scan speed should not exceed 2 μm/s.
A new method for the investigation of ion translocating membrane proteins is presented. Protein containing membrane fragments or vesicles are adsorbed to a solid supported membrane. The solid supported membrane consists of a lipid monolayer on a gold evaporated or gold sputtered glass substrate which is coated with a long chained mercaptan (CH3(CH2)mSH, m = 15, 17). Specific conductance and specific capacitance of the solid supported membrane are comparable to those of a black lipid membrane. However, the solid supported membrane has the advantage of a much higher mechanical stability. The electrical activity of bacteriorhodopsin, Na,K-ATPase, H,K-ATPase, and Ca-ATPase on the solid supported membrane is measured and compared to signals obtained on a conventionally prepared black lipid membrane. It is shown that both methods yield similar results. The solid supported membrane therefore represents an alternative method for the investigation of electrical properties of ion translocating transmembrane proteins.
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