Henry Rzepa, talks and Presentations

Scientific ideas can take a long time to gestate, and their fusion sometimes emerges from the most unexpected of directions. The talk will illustrate a chemist’s attempt to merge ideas from different areas of mathematics, molecular biology, physics and indeed chemistry, and which together span some 314 years of scientific (and indeed musical) development. The central theme concerns twisted objects known as Mobius bands, their molecular analogues, and the consequences of exploring what happens when additional twists are added. The talk will also address the theme of how to express the emerging three dimensional concepts in a visually comprehensible and semantically harvestable manner suitable for both a (re-invention) of the scientific journal for an Internet era, and in presentations to an audience, with the help of some scissors, paper and hopefully the audience itself.

Professor Henry Rzepa has had research interests in both computational chemistry and chemical information for thirty years, and was a pioneer of implementing chemistry on the Web at its start in 1993. He is the author of around 250 scientific articles in the former and 50 in the latter category. The talk today will be a fusion of these interests, and in particular how modern informatics practices can suubstantially enhance the scientific perception of published articles, taking the "paper" well beyond its old cellulose based limits.

Both chirality and aromaticity are cornerstone concepts for organic chemistry. Both had their origins in the 1840s or thereafter in the work of Pasteur, van't Hoff and LeBel for the former and Faraday, Loschmidt, Kekule, Armstrong for the latter, this reaching its first stage of theoretical maturity with Huckel's quantum mechanical analysis in the 20th Century (the famous 4n+2 rule).

For a long period, these two concepts were thought to be exclusive; after all aromaticity manifested almost entirely in flat (achiral) benzenoid rings!

Another concept, topology, also originated in the 1840s, having been coined by the mathematician Johann Listing, who also proposed fascinating topological objects such as trefoil knots, and rings now better known by their co-discoverer, Mobius. In the 1960s, the concepts of Mobius topologies and aromaticity started merging. The chemist Heilbronner proposed aromaticity rules for Mobius cycles, although he did not identify such cycles as being chiral (this property appears to have been gradually realised only years later, although its difficult to find this expressed in print). The first such Mobius molecule was only synthesized in 2003; it was not however particularly aromatic! Meanwhile, in 1978 molecular biologists had discovered the fascinating twists and knots in cyclic DNA, via James Wang's topoisomerase enzymes. This was expressed using a concept known as supercoiling, and a new generation of mathematicians formalised this into an equation expressing a so-called linking number, which is comprised of twist and writhe;

Lk = T + W ...(1)

Applied extensively to the properties of cyclic DNA, these concepts did not migrate at all to organic chemists, who by and large dealt with much smaller molecules. Listing in 1847 had also introduced the concept of paradromic winding, which in modern language maps to imparting further twists to the basic Mobius topology. In 2005, we fused these various concepts from chemistry, topology and molecular biology, recognising that a new form of aromaticity based on double- and higher twisted conjugated, and importantly chiral, rings could be possible. We identified various interesting candidate molecules, but were surprised by how relatively stable they appeared (by computation), given they were at least twice as twisted as the classical Mobius rings.

We found a resolution to this paradox in equation (1). The (quantum mechanical) instability we realised is associated with T and not with W. We have now computed values of T and W for a range of topologically interesting (and chiral!) systems, and approximately, those that appear the most synthetically interesting have large values of W compared to T. So W (the writhe) can be regarded as a fundamentally new property of cyclic conjugated molecules, and one moreover that might be associated with stability. This has led to our proposal that eqn (1) and the Huckel 4n+2 rule can be combined as follows;

If Lk is even (measured in units of π), aromaticity is implied for 4n+2 cyclic conjugated electrons

If Lk is odd, aromaticity is implied for 4n cyclic conjugated electrons

Intriguingly both T and W are chiral indices, and they can act together or oppose to create some fascinating novel chiral isomerisms. In a general sense, this type of aromaticity is chiral, and benzene like systems are very much the achiral exceptions (having Lk = 0).

At the end of the talk, I will speculate on some potential real world applications of this fascinating new form of chiral aromaticity, particularly to the design of new chiral metal ligands, and perhaps even mention another interest of ours, the Semantic Web, and how this might in the future enable more efficient fusion of diverse ideas and concepts (linking is a fundamental concept there as well!).

The Semantic wiki as a model for an intelligent chemistry journal, where a case is made for investigating Wikis as an authoring tool for the scientific publication process.

A presentation given at the 233rd ACS Meeting in Chicago, as part of the session on using social networking tools to teach chemistry, and on the theme of Wikipedia - a holistic model for the communal creation of chemical course content

An exploration of unexpected links between different subjects in chemistry, and how new insights into chiral aromaticity came to be revealed.