Back in September, talking about the insides of cells, I said:
There's not a lot of bulk water sloshing around in there. It's all stuck to and sliding around with enzymes, structural proteins, carbohydrates, and the like. . ."
But is that right? I was reading this new paper in JACS, where a group at UNC is looking at the NMR of fluorine-labeled proteins inside E. coli bacteria. (It's pretty interesting, not least because they found that they can't reproduce some earlier work in the field, for reasons that seem to have them throwing their hands up in the air). But one reference caught my eye - this paper from PNAS last year, from researchers in Sweden.
That wasn't one that I'd read when it came out - the title may have caught my eye, but the text rapidly gets too physics-laden for me to follow very well. The UNC folks appear to have waded through it, though, and picked up some key insights which otherwise I'd have missed. The PNAS paper is a painstaking NMR analysis of the states of water molecules inside bacterial cells. They looked at both good ol' E. coli and at an extreme halophile species, figuring that that one might handle its water differently.
But in both cases, they found that about 85% of the water molecules had rotational states similar to bulk water. That surprises me (as you'd figure, given the views I expressed above). I guess my question is "how similar?", but the answer seems to be "as similar as we can detect, and that's pretty good". It looks like all the water molecules past the first layer on the proteins are more or less indistinguishable from plain water by their method. (No difference between the two types of bacteria, by the way). And given that the concentration of proteins, carbohydrates, salts, etc. inside a cell is rather different than bulk water, I have to say I'm at a loss. I wonder how different the rotational states of water are (as measured by NMR relaxation times) for samples that are, say, 1M in sodium chloride, guanidine, or phosphate?
The other thing that struck me was the Swedish group's estimate of protein dynamics. They found that roughly half of the proteins in these cells were rotationally immobile, presumably bound up in membranes or in multi-protein assemblies. It's been clear for a long time that there has to be a lot of structural order in the way proteins are arranged inside a living cell, but that might be even more orderly than I'd been picturing. At any rate, I may have to adjust my thinking about what those environments look like. . .