Molecular propeller is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers:〔J. Vacek and J. Michl, ''A molecular "Tinkertoy" construction kit: Computer simulation of molecular propellers'', New J. Chem. 21, 1259 1997.〕〔C. D. Simpson, G. Mattersteig, K. Martin, L. Gherghel, R. E. Bauer, H. J. Rader and K. Mullen, ''Nanosized molecular propellers by cyclodehydrogenation of polyphenylene dendrimers'', J. Am. Chem. Soc. 126, 3139 2004. (Abstract )〕 it has several molecular-scale blades attached at a certain pitch angle around the circumference of a shaft, aligned along the rotational axis.
The molecular propellers designed in the group of Prof. Petr Král from the University of Illinois at Chicago have their blades formed by planar aromatic molecules and the shaft is a carbon nanotube.〔B. Wang and P. Král, ''Chemically Tunable Nanoscale Propellers of Liquids'', Phys. Rev. Let. 98, 266102 2007.
〕 Molecular dynamics simulations show that these propellers can serve as efficient pumps in the bulk and at the surfaces of liquids. Their pumping efficiency depends on the chemistry of the interface between the blades and the liquid. For example, if the blades are hydrophobic, water molecules do not bind to them, due to their little bond polarity, and the propellers can pump them well. If the blades are hydrophilic, water molecules form hydrogen bonds with the atoms in the polar blades. This can largely block the flow of other water molecules around the blades and significantly slow down their pumping.
Molecular propellers can be rotated by molecular motors that can be driven by chemical, biological, optical and electrical means,〔T. R. Kelly, H. De Silva and R. A. Silva, ''Unidirectional rotary motion in a molecular system'', Nature 401, 150 1999. (Abstract )〕〔N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada and B. L. Feringa, ''Light-driven monodirectional molecular rotor'', Nature 401, 152 1999. (Abstract )〕〔C. Bustamante, Y. R. Chemla, N. R. Forde and D. Izhaky, ''Mechanical processes in biology'', Annual Review of Biochemistry, 73, 705 2004. (Abstract )〕 or various ratchet-like mechanisms.〔R. D. Astumian, ''Thermodynamics and kinetics of a Brownian motor'', Science 276, 917 1997. (Abstract )〕 Nature realizes most biological activities with a large number of highly sophisticated molecular motors, such as myosin, kinesin, and ATP synthase.〔S. P. Tsunoda, R. Aggeler, M. Yoshida, and R. A. Capaldi, ''Rotation of the c subunit oligomer in fully functional F1Fo ATP synthase'', Proc. Natl. Acad. Sci. USA, 98, 898 2001.
(Abstract )〕 For example, rotary molecular motors attached to protein-based tails called flagella can propel bacteria.
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