Something that could perhaps be of use here is what I like to call the
"Anchovie Pizza Equivalent" (APE), which is about 1 microgram of
mercury. According to the Food and Drug Administration website here in
the USA the average mercury content of anchovies is 0.34 ppm, which is
about 1 microgram per ounce of fish. Tuna can be higher, but varies a
lot from fish to fish. My point here is that most institutions regard
the amount of mercury you bring onsite for purposes of eating for lunch,
be it sushi or pizza, as small enough to be negligible. I tend to
agree. So, one could argue that 1 microgram of Hg per day is a "safe
amount". Especially if you don't eat it.
In terms of protein crystals, a 100 micron wide cube has a volume of 1
nanoliter, and if it were soaked to a final concentration of 50 mM Hg
that is 1e-9 L * 50e-3 mol/L *200 g/mol = 10 ng. So, 100 protein
crystals soaked with Hg add up to roughly 1 APE. Please note that I am
in no way encouraging you to eat your protein crystals, and especially
not the solutions you soak them in. You should do your own APE
calculations for those. But I do think it important to note just how
tiny the amount of metal in our crystals really is.
Now, mercury is purportedly the second-most-toxic metal after
Plutonium. But Pu derivatives are uncommon. In fact, until recently
(4zhd) Pu derivatives were unheard of. The authors I'm sure will tell
you 4zhd involved no small amount of paperwork. But as long as you are
not working with Pu, you can regard every other metal as less toxic than Hg.
Another good example is selenium; by far the most common metal
derivative. Although toxic, Se is also a dietary requirement. I
suppose this is an excellent demonstration of what "moderation" really
means. The Recommended Daily Allowance (RDA) of selenium in the USA for
adult men and pregnant women is 55-60 micrograms per day. In crystals,
one Se atom per 100 amino acids at 50% solvent comes to an overall
concentration of 50 mM. So, a 100 micron crystal contains about 4 ng of
Se. It would take 15,000 such crystals to add up to the US RDA. The
synchrotrons I work at don't go thought that many crystals every day.
But even if they did, I'd stick to my commercially available
multivitamin to get my dietary selenium.
So, although it is never a good idea to be sloppy with chemicals in the
lab, I think it is also important to do the math and think about not
just the toxicity of the things we work with on the bench, but the
everyday items all around us. It is never a good idea to be
antagonistic with regulators about such things. They are only trying to
do their job, and all they are trained to know about are LD50s and how
to stay as far below them as possible. A little gently-pointed-out
insight into non-lethal applications of heavy metals can be helpful all
around. The over-the-counter drug Pepto Bismol (bismuth subsalicylate)
is almost 50% bismuth by weight, a metal that is right next to mercury
on the periodic table. Brominated vegetable oil contains no bromine, by
the way. And dandruff shampoos such as Selsun Blue make an excellent
and surprisingly radiation-hard reference for the selenium edge.
-James Holton
MAD Scientist
On 9/4/2017 3:13 AM, Dr Stephen Graham wrote:
Hi all,
(This email is aimed primarily at my UK colleagues, but feel free to
read on and gloat that you don't have to write safety forms in your
lab/country!).
I need to sort out written risk assessments for heavy atom soaking of
crystals in my lab. I wondered whether anyone would be willing to
share the risk assessments they have in their institute/company so
that I can seek inspiration and make sure I'm keeping up with best
practice.
Many thanks,
Stephen
