Scissors that twist, not cut

Michael J. Lander

You might not be able to see them, but they are there — tiny molecular scissors, opening and closing in response to changing light regimes. One of a number of molecular machines developed in past years — from unidirectional motors to tweezers — the devices are the first of their kind to manipulate another compound in a controlled reversible way. They also have potential applications in genetics, drug delivery and other fields.

Photoinduced configurational changes in the handle unit of molecular scissors (right) translate to an opening and closing of the scissors’ blades. A molecule loosely bound to the blades is twisted back and forth as a result.

Researchers led by Kazushi Kinbara and Takuzo Aida at the University of Tokyo created the contraptions and successfully tested their action on a guest molecule. The 11-step synthesis process that they developed yielded a 3-nm device composed of interconnected molecules of three types, each acting as the scissor analogue’s pivot, handle or blades.

For use as a pivot, they chose the compound ferrocene, whose two cyclopentadienyl rings rotated readily about the iron (II) atom sandwiched between them, even at low temperatures. The researchers then bound aniline moieties to the compound’s rings and joined them through oxidative coupling to produce a handle-like azobenzene unit. To an opposing atom on each of the rings, they attached a blade — in this case, a molecule of zinc porphyrin, chosen for its ability to coordinate to nitrogenous bases.

The azobenzene component provided the driving force behind the scissors’ functioning. When exposed to UV light at 350 nm, azobenzene synthesized in a trans configuration (long form) assumed a cis conformation (short form), which pulled the handle’s arms toward one another. At the ferrocene, this movement translated to a clockwise rotation, which brought about an opening of the blades. Visible light of 420-nm wavelength or greater induced a reversal of this condition.

As presented on March 25 at the 223rd meeting of the American Chemical Society, the scientists went on to use the device to contort a bidentate molecular rotor that associated itself with the zinc porphyrin units. When exposed to UV light from a bandpass-filtered xenon arc lamp, the scissors twisted the guest in one direction. Rotation in the opposite sense occurred under illumination with visible light. The researchers verified the movement of the blades and the rotor by analyzing circular dichroism spectral changes.

Establishing that the guest underwent twisting while in a bound state proved difficult, however. Only careful comparison of azobenzene’s photoisomerization time with the rotor’s dissociation rate allowed the researchers to conclude that the latter remained attached to the device. Another challenge lay in uniting the scissors’ components — an issue that the team currently is tackling again as it attempts to produce a machine comprising more than three molecular units.

It may be another five to 10 years before such devices enter the applied realm, the researchers predict. Nonetheless, they envision the technology’s usefulness in precisely controlling protein activity. Because of their small size, the scissors might introduce drugs into cells or manipulate genes and similar molecules. According to the group, scientists also could use the machines to modify the surface of nanoparticles and nanocapsules destined for biosensing and drug release applications.