What are amino acids?
Most simply, amino acids are the building blocks of proteins. One of the main functions of DNA is as a "recipe," or code for a particular list of amino acids (this is a gross oversimplification, but is sufficient for our discussion). The ribosome reads the "recipe" from mRNA (which in turn is copied from the DNA) and links the required amino acids together in a polypeptide chain. These polypeptides then form the building blocks of our bodies. The proteins in muscle tissue, hair, fingernails - even enzymes and antibodies are built from these polypeptide chains.
Why are aminos helpful for studying fossils?
One of the peculiar aspects of amino acids produced by living organisms is the fact that while amino acids are chiral molecules - that is they have both "left-handed" (sinistral) and "right-handed" (dextral) versions, living organisms only produce the left-handed versions of amino acids. It's this sinistral quality (referred to as "laevo" in organic chemistry) of biologically produced amino acids that makes them useful for study of fossils. An important aside is how we can use this property to search for signs of life on other planets. If we find amino acids (which do exist in comets, and interstellar debris) consisting of only one chiral form, that's a pretty good hint that we're looking at biologically produced amino acids. This of course assumes that alien life forms use amino acids as their building blocks.
Image from the NAU geochronology laboratory.
Over time, these biologically produced amino acids will change chirality: switch from left-handed to right-handed. This is matched by the right-handed aminos switching to left-handed. Eventually, all this switching evens out and a 50:50 ratio of left- to right-handed amino acids (this is referred to as a "racemic" mixture).
This alteration of aminos from left to right and vise versa is called "racemization." The rate of racemization is time and temperature dependant. For a pile of snail shells that were buried a long time ago, we can use this rate of racemization to estimate age. If we know the rate of racemization and the temperature history for the particular shells in question, we are left with age as the unknown vvariable. It should be apparent, then, that the older shells will have amino acid ratios closer to a 50:50 mix of left- and right- handed forms. We can use these amino acid racemization measurements to extimate the age of particular shells. Conversely, if we know the age and the rate of racemization, we can estimate temperature. For those of us studying the last Ice Age, this can be really important. We will go into this in more detail in future posts.
The amino acid data that I've gotten was processed and analyzed by the team at the Northern Arizona Geochronology lab. I want to thank Darrell Kaufman and Jordan Bright for all their help with my snails so far. Thanks!
Where are they in snail shells?
Snail shell is mostly calcium carbonate in the form of Aragonite. Binding these Aragonite crystals together is a proteinaceous material. The snail shell is also covered by the proteinaceous periostracum. These aminos can then be separated from the shell and analyzed with a spectrometer to measure the relative amounts of left- and right-handed amino acids. And that is where my work comes in, which will be addressed in the next episode.