In an article from EurekAlert that doesn’t seem to appear on the Penn Medicine news site, the above image appears as an illustration of how blood clots exhibit stretchiness. The caption tries to explain, but… “Fibrinogen molecule pulled by probe of the atomic force microscope (yellow disk) stretched 23 nanometers by the uncoiling of three, tightly coiled coils within the molecule.” You just know the poor writer was, like, “Is there some word I can use besides ‘coil’?” To which the researcher evidently balked.
What’s evidently going on here is that the uppermost segment of the fibrinogen molecule uncoils (allowing it to stretch to more than twice its rest length) when tugged on. I actually had trouble seeing it right off the bat because the three lines that connect the top portion of the righthand molecule didn’t read as the same structure as on the left. Instead, the three nearly-straight lines on the right looked cartoonish, and it took me a moment to identify them as anything more than diagrammatic elements. Perhaps they could be illustrated as something a little more geometrically complex, or maybe one could have a third step in the series, showing an intermediate, partially-coiled state. An animation could be spiffy, too.
The other thing that gave me pause was wondering how they know that it’s only the uppermost segment that strecthes (um, sorry, uncoils). Of course, maybe they don’t. One of the researchers is quoted as saying, “But, how is the stretching happening at a molecular level? We think part of it has to be the unfolding of certain parts of the fibrin molecule, otherwise how can it stretch so much?” So the cartoon may in fact be showing something that differs considerably from reality. Tsk, tsk.