Tuesday, January 18, 2011
[I was discussing vitalism back here and this sort of a continuation]
There is one non obvious way in which synthetically produced molecules may differ from naturally occurring ones: they may differ ever so slightly in how and how many neutrons are sprinkled amongst their constituent atoms.
Consider carbon first because carbon is the central atom of life. The majority of carbon exists as carbon-12, but there are also carbon-13 as well a carbon-14 isotopes to consider. Let's forget carbon-14 for the moment and focus just on carbon-12 and carbon-13. The heavier isotope makes up only about one percent of total carbon. But that one percent translates to thousands of trillions of carbon-13 atoms when we're talking about something like a spoonful of sugar where one mole has 12 x 1023 number of atoms. In other words, there is a measurable quantity of sugar in that spoonful having one (or more) extra neutrons (a carbon-13 atom) than an "identical" neighbor.
The useful part of this neutron (isotopic) labeling is that, depending on the complexity of the molecule, the label can occur at distinct carbons. To illustrate, consider the lowly propane molecule. Most will be CH3CH2CH3 but there will be a smaller fraction of propanes having an extra neutron at one end, viz., *CH3CH2CH3 and also a fraction of propanes having an extra neutron in the middle, viz., CH3*CH2CH3, where the asterisk stands for a carbon-13 which differs from "normal" carbon by having one more neutron.
OK, so what? The upshot is that for most molecules, especially for ones more complex than propane, the relative amounts of label "at the end vs. in the middle" will depend on how that particular batch of molecules was made (and of course whether the method of making them is isotope sensitive). The reason for the latter is beyond the scope I can cover here and deals with kinetic isotope effects. The non-statistical distribution of isotopes is smaller than one might at first believe because kinetic isotope effects are small themselves. But suffice it to say that commercial companies have sprung up in recent years to analyze such batches of molecules. The technique has proven useful for distinguishing the source and origin of otherwise identical chemical entities. Wine producers, for example, have discovered that their appellation d’origine contrôlée products can be distinguished from fake products in this way. Also, drug manufactures can detect counterfeit (infringed) products and methods of making them in this way. Though I haven't seen it yet, drug manufacturers could deliberately "mark" their products by including a small but detectable amount of isotopic enrichment.
Isotopic labeling is an old trick in the chemical arts and so is the metaphorical term isotopic signature.