About her work on DNA, there is much to say – but she also did work on coal – that is really very cool (and which you don’t often hear about in school). It’s hard enough to try to get Rosalind Franklin credit for her crucial contributions to solving the double helical structure of DNA – let alone getting her other work acknowledged. Turns out that DNA stuff was only a brief stint in an incredibly productive, though tragically short, career. Before it, she performed research on coal and carbon that made possible carbon fiber technologies and after it, she made key insights into the structure of viruses. I’m going to start with a bio I wrote for CSHL WiSE (Women in Science and Engineering) and then I’ll tell you more details about some of the her cool coal chemistry contributions!
Bio part: Rosalind Elsie Franklin was born in London, England July 25, 1920 and despite a tragically short life (she died in April 1958 from ovarian cancer) she made tremendous scientific achievements in figuring out what molecules look like and how that relates to what they do. Best known for “Photo 51,” the “blurry X” of an x-ray diffraction pattern showing that DNA forms a specific double helix, Franklin also was a pioneer of carbon technology and she discovered innerworkings of viruses including polio and tobacco mosaic virus (TMV).
Rosalind’s father wanted to become a scientist but WWI got in the way – and he almost got in the way of Franklin becoming a scientist. He discouraged Rosalind from pursuing her goal of becoming a scientist (which she had her hearts set on by the age of 15) – not out of malice, but because it was a very difficult career path for a woman. But, while Rosalind couldn’t be dissuaded, her father was able to be convinced. So, after attending one of the only schools at the time to teach girls physics and chemistry, St. Paul’s Girls’ School, she enrolled at Cambridge University’s Newnham College in 1938 as a chemistry student.
After graduating in 1941, she received a scholarship to carry out graduate research. She used the scholarship to spend a year in photochemist R.G.W Norrish’s lab, then took a position as an assistant research officer for the British Coal Utilization Research Association (CURA). She worked there until 1947, studying the physical structure of coal – and publishing numerous papers on the topic that are still widely-cited. Among other things, she found out that different forms of carbon can form different “meshes” at the molecular level that can filter out and trap various other molecules. By determining that different types of coal have different microstructures – and that their porosity is temperature-dependent – she was able to classify different types of coal and predict their usefulness for different tasks. Her work also helped make possible carbon fiber technology and earned her a PhD in physical chemistry from Cambridge in 1945.
After CURA, she then moved to Paris, where an old friend helped her get in touch with Marcel Mathieu, a big-wig in French research at the time. Mathieu saw the potential for Franklin to be great too and offered her a position as a “chercheur” in the Laboratoire Central des Services Chimiques de l’Etat. It was here where, through the tutelage of Jacque Mering, she learned X-ray diffraction techniques (which would later make her not-famous-enough). X-ray diffraction techniques beam x-rays at molecular samples – the x-rays scatter when they hit the sample and scientists can then use the scattered x-rays to figure out where they scattered from.
She took this newfound technical prowess back to England where, in 1951, a Turner and Newall Fellowship in hand, she went to work as a research associate at King’s College London in John Randall’s Biophysics Unit. Randall assigned Franklin a project to work on DNA structural studies while another researcher, Maurice Wilkins was away. When Wilkins came back, he acted as if Franklin were merely his technical assistant instead of being her own research group leader. After getting off on the wrong foot, things didn’t improve in their relationship (not helped by the fact that women at the time couldn’t even eat lunch in the same room as the men did).
Despite the difficult environment for a female researcher, Franklin was able to made incredible progress in her work. Together with graduate student Raymond Gossling, she figured out how to get informative x-ray diffraction information from DNA. Scientists had been stuck in part because they were only trying to look at a semi-crystal form of DNA, in which the DNA took on an “unnatural” shape and gave a confusing signal. But, by keeping the DNA “wet” and looking at its structure in the soluble fiber form (fiber diffraction instead of crystallography), she was able to capture a “picture” of DNA in its natural form.
This picture held the key information to calculating the geometry of the double helix DNA forms – not just the fact that it’s a double helix with the phosphate groups on the outside and the bases sticking in – but everything from the number of bases per turn, to the vertical spread and the radius. More on how here: http://bit.ly/fiberdiffractiondna
Franklin was on the cusp of unlocking that information when, without her knowledge, let alone approval, Maurice Wilkins showed some of Franklin’s work, including the iconic Photo 51, to Watson and Crick, who were independently carrying out theoretical modeling to figure out DNA structure at Cambridge. The picture provided the missing clues to getting their model to fit, and they were able to speedily publish their classic paper in Nature in 1953 (in a journal issue where Franklin’s Photo 51 paper also appeared but was greatly overshadowed).
Looking for a research environment where she had more autonomy and faced less hostility, Franklin left in 1953 to start her own research group at Birkbeck College in London. As a condition of leaving, Franklin had promised Kings College not to work on DNA, so she switched to the field of virology, studying the structures of viruses including the plant virus tobacco mosaic virus (TMV) and polio, and publishing 17 papers over the next five years (as well as creating giant 3D models for the 1958 Brussels World’s Fair).
We will never know what other great accomplishments Franklin would have undoubtedly made were her life not cut tragically short by ovarian cancer. She passed away in 1959 at the age of 37 and Watson, Crick, & Wilkins got the Nobel Prize in 1962 for the DNA structure discovery she played a key role in.
Usually when you (hopefully) hear about Franklin, it’s for this. So now I want to tell you more about some of her other work – specifically her work studying different forms of carbon – and why it continues to be important.
First – what is coal? As we looked at in more detail in a Christmas-themed coal-centric post, the same pure element can hook together into different 3D solid forms, called allotropes – coal, graphite, and diamond, for example are all made up of “all” carbon, so they’re called carbon allotropes. Graphite & diamond are both crystalline forms of coal but they have different arrangements of their carbon atoms – in graphite, carbon is organized into sheets, and in diamond it’s really locked in through connections to other carbons in all directions.
In coal, however, the carbons don’t have a strict orderly structure (it’s “amorphous”) – at least at the overarching structural level. But that doesn’t mean that it can’t have “microstructures” and Franklin found that different types of coal have different microstructures that influence how those coals react, what they can absorb, etc.
Another difference between coal and the others is that, while graphite and diamond are pure carbon, coal usually has impurities – it comes from decayed organic matter (plants, etc.) getting heated & pressurized over time and, since that organic stuff had other elements too, coal can incorporate some of those into its structure. Analogously to how inorganic rocks are said to contain “minerals,” coals are said to contain “macerals” which are the carbon-containing parts. These macerals can be further grouped and then ranked by by degree of “coalification” – the more carbon, the higher the “rank” and the more heat it can provide – from the low-carbon lignite (aka brown coal) to subbituminnous coal, bituminous coal, and then the “five-star coal” called anthracite.
People were (and still are) interested in being able to take lower-quality carbon products and making them “better” – through things like “carbonification” where they heat coal up to make it purer. But what was different at the atomic level? To get at this, Franklin started with some simple yet beautiful experiments…
When we looked at concentrations the other day, that was measuring how much of one thing there is compared to all the things, or all the space. So, for example, a frequent way to report concentration is molarity (M), which tells you the number of copies of something (such as sugar molecules) there is in 1 L. Since the things we’re looking at are really small, there are usually huge numbers of them in a liter, so instead of dealing with 20+ digit numbers, molarity counts in terms of moles – like a dozen except that instead of 12, a mole means 6.02×10^23 – of anything. So a 1M glucose solution would mean 6.02×10^23 glucose molecules per liter of whatever it’s dissolved in (usually water). 6.02×10^23 is called Avogadro’s number, and it’s often a bit too big for what we need, so we can talk of millimolar (mM) (thousandth of 1M), micro molar (μM) for millionth, etc.
But another way to talk about how stuffed something is is density. Unlike concentration, which just looked at amount of things, density takes into account how heavy those things are. At the definition level, density is mass/volume. We often use the terms “mass” and “weight” interchangeably, but weight takes into account gravity – gravity kinda pushes down on things, making them seem heavier the stronger the gravitational force – so when you’re on the moon, where gravity’s way weaker you’ll weigh less (~16.5% what you weigh on Earth) – but you’ll have the same mass because mass doesn’t depend on gravity – instead it just depends on how “massive” the things in it are.
And by “massive” I mean how much physical stuff there is in it – and the physical stuff of matter is typically atoms – some atoms are heavier than other atoms – it has to do with how many protons & neutrons they have – the more of these subatomic particles they have, the more massive the atom – though not necessarily the more space they take up – because how much space something takes up, while at some level limited by how much stuff is in each molecule, is also limited by how that stuff is spread out – both within individual molecules and between groups of molecules.
As a result, if you take the same number of copies of the same atom, you can arrange them so that they take up more or less space (occupy different volumes) and thus have different densities. The closer together you pack them, the higher the density, and vice versa. So, even if she couldn’t (yet) “see” how the carbon molecules were arranged in coal, Franklin could learn a lot about their packing just by measuring their densities.
You can easily measure weight on a scale, but if we want to calculate density we also need to know the volume. Say you have a rectangular brick of coal. To find the volume of a brick it’s just length times width times depth, right? Well, if you do that you get the volume of the thing as if it were completely full – it ignores any holes inside. So for example, that brick could be hollow or full and this “lump volume” would still be the same.
There’s no easy tape measure way to get the volume of something that’s lumpy, so instead, something called the Archimedes’ liquid displacement method is used – drop something in a liquid and see how much the liquid level rises. So, for example, if you had 50mL of water in a beaker and you plopped in a rock and the water level rose to 55mL, you can say the volume of the rock is 5mL.
This works if the liquid molecules don’t interact with the object. But things get complicated if the molecule you’re displacing (e.g. water) is actually messing with and/or getting into the thing you’re trying to measure.
The “apparent density” you measure can get skewed from the “lump density” in a couple of ways – I say “skewed” but it’s actually giving you the “true density” that takes into account gaps inside the object.
- the thing can chemically interact with the material, potentially even breaking it down (ok, this one does actually mess things up, but number 2’s good!)
- the thing can sneak into pores in the material
Franklin used this second “problem” as an asset – because the thing can only sneak in if the holes are big enough, she could use different size liquid molecules as “probes” to see how big the holes were in different coals.
Even a fine mesh can’t keep out really really small stuff – so she used Helium (He), the second smallest atom (hydrogen’s smaller, but it’s also more reactive) to get at the “true” density to compare to when she repeated the measurement with increasingly larger probes: methanol, water, hexane, & benzene. If the probing liquid molecules were small enough, they’d sneak in and the true volume as determined using helium would be the same as the apparent densities they measured. But if the probes couldn’t get in, the apparent density would appear lower. And this would imply that the pores (or at least their openings) were too small to allow them in. And since the size of those probing molecules were known, she could estimate the size of the pores
So, what happened? When she used methanol, she found that the apparent densities were higher than the true density – she had run into that issue #1 – the methanol was reacting chemically with the coal, making it seem more dense. not cool… But the other molecules didn’t react with the coal so they were usable. But could they get in? Sometimes…
She found that the “low-quality” coal – which had low amounts of carbon (lots of other elements in there) – had big pores that had no problem taking in the big probes like hexane & benzene. But the top-of-the-line coal (which had lots of carbon) didn’t just have lots of carbon atoms, it had those carbon atoms arranged into a finer mesh that kept out even small stuff
This ability to selectively let in certain molecules but not others is often called “molecular sieving” – it’s frequently used in carbon-based filters that do things like separate nitrogen from oxygen in the air. And it’s one of the reasons Britain’s government was interested in coal research – such sieving could be used in gas masks to protect soldiers in the war (they knew carbon could do it, but they didn’t know *why*/how – and Franklin provided the answers)
That was with “untreated coal” – but what about heat-treated coal? When she heated the coals, it became more porous, but some of the pores became closed off so that even helium couldn’t get in. And this led to the lower reactivity of such “carbonized” coals that people were wondering why was.
She also found that certain types of carbon which she named “graphitizing carbon” would change its structure so that the carbons arranged themselves into orderly sheets. While graphitization can be good if you want synthetic graphite, it can also be bad if you don’t want it. Graphitization can be a problem for certain metal alloys (mixtures of elements) that contain carbon – like steel, which has iron and carbon. If you heat it and the carbons come out of that nice amorphous (shapeless) mixture into crystalline sheets that don’t let iron in, the steel can crack. So it was important to figure out what was happening and how you could know what types of coal make good starting material for different tasks
To characterize what was going on, she turned to X-ray diffraction. You know how that Photo 51 is a fiber diffraction image that’s often mistaken for a crystal diffraction image? Well, Franklin used a 3rd type of x-ray diffraction here, powder diffraction – it uses tiny crystals that have all different orientations, so the resultant pattern is a series of rings, with distances between the rings providing information about the spacing between planes in the crystals. And if you rotate the sample you can get diffraction from different angles – so it’s commonly reported by a pilot of intensity of x-rays scattered at different angles
When she upped the temperature, the interplanar spaces fell, which she interpreted as a change from planes randomly rotated (disordered layers) to planes adopting a more orderly layering. I think of it kinda like how in “simulated annealing” in computer modeling you allow for more freedom to explore options so you don’t get trapped in local minima – if molecules don’t have enough energy to explore, the can get stuck without finding the best (lowest-energy-requiring) orientation – but if you let them explore more they can reach that happy place.
And graphite is the most thermodynamically stable form of solid carbon (its happy place) – so you’d think all carbon would want to get there – and would get there if you gave them enough exploration energy (high temp heat treatment). Franklin found if you did this with cokes (products of coal processing), that’s indeed what happened. But with chars (which typically come from things like wood I think), you only got a porous, isotropic material with little graphite-like regions, but not that overall layering throughout – this was the first time this key distinction between cokes & chars was shown.
Franklin thought this difference was because, in all coals there were little graphite-like minisheets – and in graphitizing carbons, these minisheets were already nearly parallel, but in non-graphitizing carbons, there were already different interatomic interactions that were harder to break up and get to reorganize. It’s still not clear exactly what’s going on, but it might involve big ball-like things called fullerenes.
Speaking of full-erenes, I’ve had a full day in the lab (with promising-looking preliminary results) and this post is pretty full too – so that’s all for now.
But you can learn more about her coal work in this article: http://www.personal.rdg.ac.uk/~scsharip/REF_paper.pdf