Chaperones for wild molecules

Researchers use a technique called high-resolution electron excitation spectroscopy to gain a detailed picture of a molecule's structure. Using this technique they can map the energy barriers present when a molecule switches from one structure to another or, for example, how the photon energy is distributed over the molecule. These measurements can only be performed if the molecules concerned have no interactions with other molecules and are internally cooled to temperatures just above absolute zero. As a result of this the lines in the spectrum measured have a much smaller width than would be observed at room temperature in the solution phase. These lines directly reflect the molecular characteristics and form in effect the molecule's signature. The greater the amount of details contained in this signature, the better the molecule can be characterised. This approach works superbly for small molecules but unfortunately not for larger and more complex systems. One of the reasons for this is that the number of structures that can be adopted by the molecule is so large that the lines of each of these structures can no longer be distinguished from each other. Their signatures merge so to speak and the details of the separate structures are lost.

A molecular template
Researchers from the Molecular Photonics Group at the Universiteit van Amsterdam in cooperation with colleagues from the University of California at Santa Barbara , the University of Edinburgh, and the Università di Bologna - and with financial support from the Foundation for Fundamental Research on Matter - have devised a new method for controlling the flexibility of molecules. The method is based on a molecular architecture named rotaxane. This is a molecular system that can be conceptualised as a thread surround by a ring, which is not bonded to the thread. Under normal conditions the thread is highly flexible and can adopt a large number of structures, but the interactions between the ring and the thread ensure that the ring functions as a mould, which forces the thread into a single structure or a limited number of structures. This 'moulded' thread is then rendered accessible for further spectroscopic experiments by removing the template with a laser pulse. If this would be done at room temperature, the thread would immediately reassume all of the structures that it could have in its free form. What makes this approach so ingenious is that the ring and thread are first internally cooled by means of supersonic expansion before the ring is removed. Consequently the thread remains 'frozen' in the structure imposed upon it by the template.

Countless templates
The shape of the rings can be changed relatively simply, which makes it possible to produce a wide range of moulds. In addition to this, new synthetic methods have been developed at the University of Edinburgh which make it possible to provide virtually every 'linear' molecule with such a template. Therefore the researchers expect that in the future the new technique will play a major role in unravelling and controlling the structural dynamics of complex molecular systems.

Information
For more information, please contact prof.dr. Wybren Jan Buma, Universiteit van Amsterdam, phone +31 (0)20 525 69 73 or dr. Anouk Rijs, FOM Institute for Plasma Physics Rijnhuizen, phone +31 (0)30 609 68 23.

Reference
The article is titled "Shaping of a conformationally flexible molecular structure for spectroscopy". Authors are Anouk M. Rijs, Bridgit O. Crews, Mattanjah S. de Vries, Jeffrey S. Hannam, David A. Leigh, Marianna Fanti, Francesco Zerbetto and Wybren Jan Buma.