Gravity determines handedness of molecular screws
For the first time, researchers have demonstrated that gravity plays a role in the formation of molecular aggregates, and that it can even be used to make them right-handed or left-handed. Many chemical and biochemical substances exist in the form of one of two structures that are the exact mirror-image of one another but which, from a biological point of view, have very different properties. However, no-one could explain exactly how these images originally formed. It now seems that a combination of gravitational and rotational forces may play a role, as demonstrated in an experiment carried out in the Nijmegen High Field Magnet Laboratory (HFML). The results were published in Nature Chemistry on 12 February.
The researchers carried out a growth experiment in which they allowed flat dye molecules (porphyrins) to aggregate. By varying the gravitational force using a strong magnetic field, and by rotating the vials containing the molecules in solution, they were able to produce left-handed or right-handed aggregates as desired. The aggregates look a little like screws – or their mirror image. Interestingly, rotation alone will not produce a single form of the screw, and neither will the application of just a magnetic field. Apparently something special happens when the two forces are combined.
Inverted gravitational force
The super magnets at HFML (the magnet laboratory run jointly by Radboud University Nijmegen and the Foundation for Fundamental Research on Matter [FOM]) are so strong that they can cause non-magnetic materials to levitate, such as the famous frog. The gravitational force is in fact then being counterbalanced.
It is also possible to invert the gravitational force, as the magnetic force varies in strength at different places within the magnet – a property that the researchers made use of, placing a whole series of vials in the magnet. Therefore in one vial the gravitational force was normal, in another higher than normal, and there were other vials in which the gravitational force was less than normal or even inverted. All vials were rotated clockwise in exactly the same manner – only where the gravitational force was inverted was the rotation reversed compared with the gravitational force, and therefore anti-clockwise. The screws created all wound in the direction in which the liquid had moved in relation to the gravitational field. The researchers are now trying to work out the exact mechanism behind this discovery. This is important because they believe that they can also produce other types of chiral structures on demand in their magnet.
Left hand into right glove
The production of many substances results in a mixture of left-handed and right-handed variants of the same substance. Chemists call this 'chirality', the property that substances can have two variants that are an exact mirror-image of one another, but which do not fit together, rather like a right hand and a left hand. And, just as a left hand does not fit into a right glove, it is also possible that a left-handed molecule behaves very differently from its right-handed mirror image in the body. One variant of the notorious drug Thalidomide had the useful property that it prevented morning sickness during pregnancy; the other variant produced malformed babies. Unsurprisingly, all chiral variants of a drug must now be tested.
For experts: true and false chirality
The article in Nature Chemistry will be accompanied by a News & Views article, an indication that the journal itself considers this a highly interesting discovery. The chiral screws are produced using a combination of forces that is 'falsely chiral'. If the whole process was reversed in time, the chirality of the combination of forces would also be reversed. The influence of these forces is therefore not chiral at all and should not have a chiral effect. It was however predicted several decades ago that this kind of false chirality would nevertheless, under certain conditions, be capable of producing chiral structures, but this had never been proven. Now, to the delight of scientists, it has.
Reference
'Selection of supramolecular chirality by application of rotational and magnetic forces', N. Micali, H. Engelkamp, P.G. van Rhee, P.C.M. Christianen, L. Monsu-Scolaro and J.C. Maan
Contact
Hans Engelkamp +31 (0)24 365 33 67
Peter Christianen +31 (0)24 365 22 45
You can watch a movie about the research here.