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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)
The Terpenes
1. The Terpenes:
- we seen a number of reactions for alkenes and alkynes. Which of these occur in real life? Many of them do, but not as we have seen them in class. In biological systems reactions take place in aqueous solutions, or on membrane surfaces and they are very specifically catalyzed by enzymes. Enzymes are proteins which catalyze reactions in very specific ways. Of equal importance is the property that enzymes can be regulated, their ability to catalyze a reaction can be increased or decreased very quickly. We'll talk about all of that in biochemistry next semester, one just has to remember that many of the basic reactions that we have learned can apply to real biological molecules, but they might be a little less obvious to see.
- one of the very interesting classes of alkenes in nature are the terpenes. These are very important molecules, so we'll spend a few minutes discussing their structures and properties.
- the base unit for all of the terpenes is isoprene, the proper IUPAC name for which is 2-methyl-1,3-butadiene. If we number the carbons correctly, we call the number one carbon the tail and the number 4 carbon the head
- the terpenes then are made of stringing together isoprene units in a head to tail fashion. We won't go into the chemistry of how they are put together, that requires more chemistry than you have, but you should be able to identify the carbon skeleton units.
- if you put two isoprenes together, you get the base 10 carbon unit or group called a geranyl group. There are a number of examples in the text book of 10 carbon terpenes.
- the base 15 carbon unit made up of 3 isoprene units is called a farnesyl group. Again a number of examples are given in the text of 15 carbon terpenes.
- the last base unit that has a proper name is the 20 carbon group, called a geranyl-geranyl group.
- what do these things do in nature? They have tons of different roles. The base compound isoprene may be involved in heat stress (read the box on page 88 of the text). The medium sized terpenes are often highly scented. They make up the scents of many flowers and plants. They often act as pheromones, given their volatility this makes sense. The larger terpenes make up all kinds of molcules like vitamin A, used in the vision system, cholsterol, all of the steroid hormones...
- an important aspect of biological chemistry emerges here. Organisms string together these 5 carbon units is a fairly common mechanism to generate "generic" larger structures which are then modified very specifically by enzymes to carry out their purpose. What possible advantage can this have? The base mechanisms for stringing carbon skeletons together for these molecules only evolved once, but the diversity of function only required a much smaller number of evolutionary events.
