Molecular engines are molecular machines capable of spinning when energy is applied to them. Traditionally, the term "molecular engine" is used when it comes to organic protein compounds , however, at present it is also used to refer to inorganic molecular engines [1] and is used as a general concept. The possibility of creating molecular motors was first voiced by Richard Feynman in 1959 .
The main feature of molecular rotors is the repeated unidirectional rotational movements that occur when energy is applied. In the future, this direction was developed due to two scientific reports published in 1999 that describe the nature of molecular rotors. However, the reports did not indicate the reasons due to which the molecules were able to generate torque. It is expected that in the near future a significant amount of research will be carried out in this field and an understanding of the chemistry and physics of nanoscale rotors will appear.
Content
Overview of rotation methods
Chemical Method
For the first time, the creation of a molecular rotation engine was reported by Ross Kelly in his work in 1999 [2] . His system consisted of three trypticin rotors and a chelicin part and was able to perform unidirectional rotations in the 120 ° plane.
Rotation takes place in 5 stages. First, the amine group on the trypticin part of the molecule is converted to the isocyanic group by condensation of the phosgene molecules (a). The rotation around the central axis is due to the passage of the isocyanine group in the immediate vicinity of the hydroxyl group located on the chelicin part of the molecule (b), due to which these two groups react with each other (c). This reaction creates a trap for the urethane group , which increases its tension and provides the beginning of rotational motion with a sufficient level of incoming thermal energy. After the molecular rotor is set in motion, only a small amount of energy is required to carry out the rotation cycle (d). Finally, cleavage of the urethane group restores the amine group and provides further functionality of the molecule (e).
The result of this reaction is the unidirectional rotation of the trypticin portion by 120 ° with respect to the chelicin portion . The additional forward movement is prevented by the chelicin part of the molecule, which plays a role similar to the role of ratchet in the clockwork. Unidirectional movement is the result of asymmetry of the chelicin moiety, as well as the appearance of the urethane group (c). The rotation can be carried out only clockwise, for the process of rotation in the other direction requires much greater energy costs (d).
The Kelly engine is a great example of how chemical energy can be used to create unidirectional rotational motion, a process that resembles the consumption of ATP (adenosine triphosphoric acid) in living organisms. Nevertheless, this model is not without serious flaws: the sequence of events that leads to a rotation of 120 ° is not repeated. Therefore, Ross Kelly and his colleagues looked for different ways to ensure the repetition of this sequence. Attempts to achieve the goal were unsuccessful and the project was closed [3] .
Light Method
In 1999, a report was received from the laboratory of Dr. Ben Feringa at the University of Groningen ( Netherlands ) about the creation of a unidirectional molecular rotor [4] . Their 360 ° molecular rotation engine consists of bishelicin linked by a double axial bond and having two stereo centers.
One cycle of unidirectional rotation takes 4 stages. At the first stage, a low temperature causes an endothermic reaction in the trans isomer (P, P), transforming it into the cis isomer (M, M), where P is a right-handed helix and M is a left-handed helix (1, 2). In this process, two axial methyl groups are converted to equatorial.
By raising the temperature to 20 ° C, methyl groups are converted back to exothermal (P, P) cis-axial groups (3). Since the axial isomers are more stable than the equatorial isomers , the reverse rotation process is not possible. Photoisomerization converts the cis isomer (P, P) to the trans isomer (M, M), again with the formation of equatorial melyl groups (3, 4). The thermal isomerization process at 60 ° C closes the 360 ° rotation cycle with respect to the initial position.
A serious obstacle to the implementation of this reaction is the low rotation speed, which is not even comparable with biological molecular rotors existing in nature. In the fastest systems with fluorine groups today, half of the thermal inversion of the helix of a molecule takes 0.005 seconds [5] . This process occurs using the Barton-Kellogg reaction. The slow rotation step, as suggested, can be significantly accelerated due to a larger number of tert- butyl groups , which make the isomer even less stable than methyl groups . Since the instability of the isomers increases, then the inversion of the helix of the molecule is accelerated.
The operating principles of the Feringa molecular rotor were included in the prototype nanorobot [6] . The prototype has synthetic helicin engines with an oligo chassis and 4 carbon [ unknown term ] wheels and is expected to be able to move on a hard surface under the control of a scanning tunneling microscope . However, while the engine does not work on the basis of fullerene wheels, because they reduce the photochemical reaction of the parts of the rotor.
Electron Tunneling
By analogy with a traditional electric motor, nanoscale molecular motors can be set in motion by resonant or non-resonant electron tunneling [7] . Based on these principles, nanoscale rotating machines were developed by Peter Krall and his associates at the University of Illinois at Chicago [8] .
As shown in the right part of the figure, one of the types of motors has an axis formed on the basis of carbon nanotubes, which can be mounted on CNT bearings. The motor has three (six) blades formed from polymerized acetone. The blades are oriented at an angle of 120 ° (60 °) to each other and have a length of 2 nm to prevent non-resonant tunneling of electrons from the blades to the shaft (axis). Energy is supplied to the system through electron transfer along the blades by resonant tunneling. The blades form molecules conjugated to fullerenes covalently joined in the upper part of the blades. In principle, such hybrid molecular rotors can be synthesized in cycloaddition reactions.
In a uniform electrostatic field E oriented along the vertical direction, periodic charging and discharging of the motor blade using electron tunneling from two neutral metal electrodes are used. Each fullerene switch changes the sign of the charge with the help of two electrons from positive (+ q ) to negative (- q ) through the tunnel between the neutral electrode and fullerene. To rotate the motor blade, the electrode loses two electrons (as a result of which the charge changes on it), and the blade performs half the rotation cycle in the electric field E. The other half of the rotation cycle occurs similarly (only the electrode receives two electrons). Thus there is a continuous rotation of three (six) blades with fullerenes. The molecular engine drives its dipole P , which is located in the middle orthogonal [ unknown term ] towards the electric field E , generating a constant torque.
The efficiency of the electron tunneling method is comparable to the similar efficiency of the drive of macroscopic electric motors, but it can decrease due to noise and structural defects.
Links
- ↑ Synthetic Molecular Motors Jordan Quinn Online article Archived April 16, 2007.
- ↑ Unidirectional rotational movements of molecular systems . Ross Kelly, Harshani, and Richard Silva. Journal of Nature 1999 , 401 , 150-152. General information (unavailable link from 09/22/2014 [1768 days])
- ↑ Progress towards rationally engineered chemical molecular rotation rotors . Ross Kelly, Zaolu Kai, Fehmi Damkatsi, Sreleza Paniker, Bean Ti, Simon Bushel, Ivan Cornella, Matthew Piggio, Richard Silives, Marta Kavero, Yagin Zao and Sergey Yasmin 2007 , 129 , 376–386. General information .
- ↑ Light-driven unidirectional molecular rotors . Nagatoshi Koimura, Robert Zijlstra, Richard Van Delden, Nobiyuki Harada, Ben Feriga Nature Journal 1999 , 401 , 152-155. General information (unavailable link from 09/22/2014 [1768 days]) .
- ↑ Adjustment of rotational movements in light-controlled unidirectional molecular rotors . Yavin Vicario, Martin Velko, Ike Miitsma and Ben Feringa, 2006 , 128 , 5127-5135. General information .
- ↑ Movement of motorized nanomachines . Ian Francis Mirin, Yasishiro Shirai and James; 2006 , 8 , 1713-1716. Graphical General .
- ↑ P. Král and T. Seideman, Current-induced rotation of helical molecular wires , J. Chem. Phys. 2005 , 123 , 184702. Abstract (link not available) .
- ↑ B. Wang, L. Vukovic and P. Král, Nanoscale rotary motors driven by electron tunneling , Phys. Rev. Lett. 2008 , 101 , 186808. Abstract (unreachable link from 09/22/2014 [1768 days]) .
See also
- Molecular propellers
- Nanotechnology
- Nanorobot