262 References

6.20 Using scanning conductance microscopy, images of a purified enzyme of ATP syn­

thase (see Chapter 2) could just be discerned when the enzymes were stuck to a flat

microscope coverslip. But, when potassium channels were overexpressed in a cell

membrane and a patch excised and imaged on a flat surface, no clear images of the

channels could be obtained. Why?

6.21 In an AFM imaging experiment, the maximum vertical displacement of the cantilever

was limited by the height of the silicon nitride tip of 10 μm, giving a full-​scale deflec­

tion of the photodiode output of 5 V. At room temperature with the tip far away from

the sample the rms photodiode output was 2.6 mV with the laser reflecting onto the

back of the cantilever switch on, and 0.9 mV when the laser was switched off. The

machine was used to image single “myosin” molecules on a flat surface (molecular

motors found in muscle tissue), whose head regions generated forces of 5 pN each

when performing a “power stroke.” When the tip is just in contact with a head region,

what offset voltage should be applied to just cause a power stroke to stall?

6.22 In an ABEL trap experiment, a fluorescently labeled protein could be recentered when

experiments were performed at “low” salt in NaCl (equivalent to an ionic strength

of ~10 mM) but was not able to recenter the protein at “high” salt (ionic strength

>0.2M). What could be the explanation?

REFERENCES

KEY REFERENCE

Rief, M., Gautel, M., Oesterhelt, F., Fernandez, J.M., and Gaub, H.E. (1997). Reversible unfolding of

individual titin immunoglobulin domains by AFM. Science 276:1109–​1112.

MORE NICHE REFERENCES

Abbondanzieri, E.A. et al. (2005). Direct observation of base-​pair stepping by RNA polymerase.

Nature 438:460–​465.

Arkawa, H., Umemura, K., and Ikai, A. (1992). Protein images obtained by STM, AFM and TEM.

Nature 358:171–​173.

Ashkin, A. (1970). Acceleration and trapping of particles by radiation pressure. Phys. Rev. Lett.

24:156–​159.

Ashkin, A., Dziedzic, J.M., Bjorkholm, J.E., and Chu, S. (1986). Observation of a single-​beam gradient

force optical trap for dielectric particles. Opt. Lett. 11:288–​290.

Bailey, S.T. et al. (2010). Light-​scattering study of the normal human eye lens: Elastic properties and

age dependence. IEEE Trans. Biomed. Eng. 57:2910–​2917.

Bayley, H. (2009). Piercing insights. Nature 459:651–​652.

Binnig, G. et al. (1982). Tunneling through a controllable vacuum gap. Appl. Phys. Lett. 40:178–​180.

Binnig, G., Quate, C.F., and Gerber, C. (1986). Atomic force microscope. Phys. Rev. Lett. 56:930–​933.

Cohen, A.E. and Moerner, W.E. (2005). Method for trapping and manipulating nanoscale objects in

solution. Appl. Phys. Lett. 86:093109.

Cohen, J.A., Chaudhuri, A., and Golestanian, R. (2011). Active polymer translocation through

flickering pores. Phys. Rev. Lett. 107:238102.

De Souza, N. (2014). Tiny tools to measure force. Nat. Methods 11:29.

Dufresne, E.R. and Grier, D.G. (1998). Optical tweezer arrays and optical substrates created with dif­

fractive optical elements. Rev. Sci. Instrum. 69:1974–​1977.

Durnin, J., Miceli, J.J., and Erberly, J.H. (1987). Diffraction-​free beams. Phys. Rev. Lett. 58:1499–​1501.

Fields, A.P. and Cohen, A.E. (2011). Electrokinetic trapping at the one nanometer limit. Proc. Natl.

Acad. Sci. USA 108:8937–​8942.

Finer, J.T., Simmons, R.M., and Spudich, J.A. (1994). Single myosin molecule mechanics: Piconewton

forces and nanometre steps. Nature 368:113–​119.