Weighing single molecules with light

The modern-day equivalent of a scale pan for molecules
22 June 2019


Weighing molecules with light


The entire point of research is to find out things that we don’t already know, make things we don’t yet have, do things we cannot do and fix things we cannot yet fix... 

Ultimately, all of this is driven by curiosity and an inability to accept that we don’t understand how stuff works. About 50 years ago, a few theoretical physicists proposed the existence of another elementary particle: the Higgs boson. And then humanity went on a decade-long journey, investing billions and billions, employing thousands of scientists, digging a 27 km long tunnel and cooling a good chunk of it to below the temperature of outer space. Why? Because we wanted to know if this notional entity actually exists, and if so, what are its properties, and what can it tell us about the Universe? The key challenge was: detecting it in the first place, attempting to see a particle that nobody had seen before.

The Higgs boson and the story of its discovery are symptomatic of a lot of scientific discovery. We build new machines that allow us to see something that nobody has seen before, often just because of the challenge. Even more frequently, once we have built it, we have no idea what it is good for apart from completing the pure challenge that was there at the outset. Is the only outcome of CERN that we have detected the Higgs Boson? Of course not - its discovery was accompanied by the development of new detectors, software and hardware, all of which did not exist before and are now being used to make breakthroughs all over the place in the chemistry, physics, life sciences and biomedicine. The lesson seems to be that, if you look hard enough, not only will you find something interesting, but the processes of looking harder than anyone has previously will result in unintended and - often - unenvisioned consequences.

As a chemist, you spend your existence drawing molecules, contemplating how bonds form and break, and how reactants turn into products. The biological equivalent is a diagram containing different-coloured blobs with acronyms on them coming together and separating in what appears to be a well-choreographed manner, eventually yielding a function. The desire not just to draw but also to see these molecules, has been the driving force behind the development of technologies that give us information on the atomic structure of the species of interest. Most often, however, this information is static: molecules are literally frozen, either by forming a crystal or by being cooled rapidly. This means that, while we can see structure, the dynamics - which is what we imply with our little synchronised arrows - are much harder to follow.

What I am fascinated by is asking the question "how close can I get to actually seeing what I draw on a piece of paper?" After all, the reason why we make such drawings is that visualising a process in this way helps us understand as well as explain better to others what is going on. Traditionally, however, we extract this information indirectly: we make measurements that produce some curves; we fit these curves to different models; and from these models we infer what must be happening on a molecular level. What we are trying to do is to see these processes one molecule at a time. Because if we could, then we would not need to interpret data or come up with mechanisms, we would literally see what is going on.

Not only do we need to see these blobs, which represent different biomolecules, we need to be able to differentiate between different blobs, as well as blobs that are stuck together, rather than those that are by themselves. To give you an idea how important this simple concept of lonely versus sticky blobs is: 8 out of the top 10 highest-grossing drugs in the world - with a staggering $250 billion annual turnover - are based on exactly this principle: one blob (an antibody) binding to another blob (its target). Binding either enhances a certain response that is suppressed in disease or vice versa.  The result is a drug that makes us feel better, removes symptoms and, in an ideal case, even cures us of disease. This concept of molecules interacting is not just specific to drug function, you can trace down essentially every process in nature to molecules coming together and falling apart. In other words, molecular interactions are the basis of how nature works.

The difficulty in visualising and quantifying these interactions by seeing them is one of size. Molecules are small, much smaller than the wavelength of visible light, which is what we usually use to see things. Until about 5 years ago we could not see these blobs at all without first attaching some sort of tag to them first. And even then, once we could see them, we could still not tell the difference between two blobs stuck together versus two individual blobs, which is what we really needed. So just seeing was not going to cut it. If you look at two blobs stuck together vs two individuals, what universal property of matter could you use to distinguish them apart from seeing it? The answer is mass: the mass of the complex is the sum of the masses of the individuals. So even if I cannot see the two molecules stuck together, I could know that they are, or are not, if I could "weigh" them. This is what we do and what we are so excited about. We found out that, not only can you see single proteins by shining a light at them and looking at the light that comes back, but by measuring very accurately how much light comes back, we can "weigh" the molecules. How well does it work? Our accuracy is on the order of a few percent. What I am saying is that we have developed a microscope whose raison d'etre is not making images of small things. Rather, what it does is to weigh molecules, one by one, by just looking at them. The everyday equivalent would be looking at a loaf of bread and being able to tell how much it weighs to within a few grams, just by looking at it!

The reason why weighing by looking is so important when it comes to molecules rather than bread, is that we can build a scale for bread. That’s easy, we’ve been doing it for 5000 years with mechanical scales, whose development contributed significantly to the concept of trade and thus to society as we know it today. Building a scale for a single molecule, on the other hand, is much, much harder. But weighing by looking, once you can see molecules, actually turns out to be quite straightforward.

So what’s next?  It’s simple. We have a new, immensely powerful technology that works in ways people have never really considered: combining the concepts of taking images with weighing things.  Most of the people that will benefit from it often don’t really use - or even much care - about microscopes: they use lots of other methods to find out what their blobs look like, how they change and how they interact with themselves and others.  So, what we need to do is to give people access to the technology, make it easy to use and show all the things it can be used for and provide information about.  Our scale made of light works - for us. What we need to make sure is that it works for others too. That what it measures helps us understand better how nature works and what goes wrong in disease, and helps to fix it by developing better, more effective drugs.  And who knows, if we can achieve that, maybe the concept of weighing things by just looking at them, won’t feel like such an odd concept anymore in a decade’s time...


Add a comment