Device That Sorts Through Structural Isomers of Neutral Molecules

The Max Planck Society is reporting about research by a group of German investigators from the Fritz Haber Institute in Berlin that created a novel biomolecule sorting device, currently in an experimental stage, but with a possible future applicability for a variety of clinical or experimental tasks:

"Our filter for conformers works like a quadrupole mass filter," explains Frank Filsinger, who, as a doctoral student, carried out most of the work. Quadrupole mass filters are used in many laboratories to separate molecules by their mass-to-charge ratio. The apparatus used by the researchers in Berlin sorts the particles in a very similar way, with the difference that they isolate them on the basis of their mass and their dipole moment. Dipole moment is a measure of the strength of a dipole.
The scientists tested their new method on an aminophenol – on two conformers in which the hydroxide group of the molecule is oriented differently. This group consists of an oxygen and a hydrogen atom, and is characteristic of alcohols. Their different orientations in the aminophenol are called cis and trans positions. In the cis version, the hydroxide group points to one side of the molecule, in the trans variant it points precisely to the other side. For this reason, the dipole moment of the cis-aminophenol is approximately three times greater than that of its trans counterpart.
In order to isolate the two conformers with the hydroxide "arm" in different positions, the researchers vaporized a small quantity of the substance and bundled it into a molecular beam. The beam travels exactly one metre in the Berlin researchers’ equipment. In order that the cis and trans versions separate over this distance, Küpper and his colleagues apply electrical fields that exert forces on the molecules: they group four electrodes – live metal rods that form a sort of tube – around the molecular beam. The beam moves through this tube. Alternating voltage runs through two electrodes, causing the positive and negative poles to repeatedly jump backwards and forwards. The direction in which the force of the electrical field acts on the molecules changes accordingly.
The frequency of the alternating field is decisive; that is, the speed at which the poles change places. Different dipoles vary in their response to the alternating field. Finally, at a certain frequency of the alternating field only molecules with a certain dipole moment, or more precisely, only those with a certain mass-to-dipole-moment ratio, reach the end of the apparatus. All the others gradually drift out of the trajectory of the beam.
The researchers working with Frank Filsinger in Berlin not only isolated only one specific conformer in this way. They can even sort the conformers by the amount they rotate. Molecules rotate constantly, but not always at the same speed. There is a measure of the speed of rotation – the rotation quantum number, which increases with the rotational speed of the molecule. However, the dipole of the particle becomes thereby increasingly weaker and the electric field has a weaker effect on it. "We also filter out the molecules in the lowest rotation quantum states," says Küpper. This allows the molecules to be oriented in space particularly well. The researchers hope that, in future, they will be able to get all the particles with arms facing in the right direction moving.

Press release: Casting for molecules