One of the problems in chemistry is that the stuff we’re looking at is inherently small, and hence nearly impossible to see (although with an appropriately modified microscope you can get pretty close). To make things more tangible, we have to rely on models – be that physical ball-and-stick molecules or intricate computer simulations.
The beads-on-a-string model for folding polymers is a cornerstone of our theoretical understanding of how large molecules fold up. But in a paper in PNAS this week, George Whitesides of Harvard University argues that it, like all theoretical models, is incomplete. Whitesides proposes that a simple macroscopic physical model may be just as useful in understanding the problem, without worrying so much about all the complex theory.
The physical model is based on electrostatic forces between oppositely charged plastic beads fixed on a cord, with smaller ‘spacer’ beads in between to determine how flexible the string is. The beads are charged up by shaking them around on a sheet of paper – just like rubbing a balloon on your jumper to stick it to the wall. They then attract and repel each other, and fold up to form all sorts of intricate patterns.
By changing the sequence, the shape of the beads or the spacing between them, the group could model different kinds of polymers. Some simple sequences consistently formed the same final structures, whereas longer or more flexible sequences were less predictable and folded differently every time – a selection of the final structures of a 40-bead sequence is shown in the pictures.
The next stage, according to the paper, is to shake the bead strings in a bucket full of electrostatically neutral beads to simulate a solvent and see how that affects folding in three dimensions.
The real question is, is this a valid model for the bead-on-a-string theory? And even if it is, does that make it a realistic model of the real behaviour of polymers at the molecular scale?
I’m a massive fan of Whitesides for the way he challenges how people think about all sorts of things (see here and here for more interesting ideas), but I’d love to hear what all of you out in the blogosphere think – is this a stroke of genius or just an interesting plaything?