The problem with molecules with a nitrogen backbone is that they tend to be highly explosive. Generally more explosive the larger the molecules. I suppose if organisms had evolved in really low temperatures, life might be able to use such molecules, but under any conditions familiar to us humans such life would be walking talking piles of plastic explosive.
fasquardon
To put into perspective how explosive this is, aziroazide azide, a fairly small 2 Carbon 14 Nitrogen compound, is so explosive that the force of
light hitting it while it's at near absolute zero temperatures will cause the chemical to explode. Testosterone, for reference, is C
19H
28O
2, substantial larger and something like a million times more chemically stable. If you look at the list of Most Powerful Non-Nuclear Explosives, it's basically a resume for Nitrogen solids. There are no known primary nitrogen compounds bigger than aziroazide azide; any hypothetical example would explode too quickly to be studied.
Nitrogen is really, really bad at bonding to things because it has 5 valence electrons. Two of those electrons 'pair up' and are happy while the other three are open to forming covalent bonds (carbon can form 4 bonds). The fact that Nitrogen has a lone pair (two paired up electrons) means that its covalent bond angle will be smaller than carbon's (lone pairs take up more 'space' than bonding electrons) and thus have to overcome a higher degree of electrostatic repulsion as bonding electrons are forced closer together.
Nitrogen also has issues because it's strongly electronegative (4th highest on the periodic table). That means it tends to snatch electrons from other atoms and hold onto them tightly. This tends to lead to 'lopsided' chemicals that are unevenly electrically charged as every nitrogen is trying to snatch electrons for itself. This electrochemical stress tends to make things very high energy and very unstable. Carbon doesn't tend to have this problem nearly as bad because its electronegativity is very similar to Hydrogen (a staple of many biomolecules) so the bonds tend to have very little electrochemical charge.
So not only does nitrogen not cooperate as well as carbon, but it's a lot greedier.
Now, Nitrogen can be useful biologically, but mostly as a solvent. NH
3, ammonia, can serve as a Lewis acid (creating NH
2-) or Lewis base (NH
4+). That's basically the role water serves in our cells, alternating between OH
- and OH
3+ to donate or store electrons as needed. Being able to act as both a Lewis base (donates a lone pair of electrons) and Lewis acid (receive of a lone pair of electrons) is chemically unusual and necessary for life. It's necessary to temporarily store electrochemical charge for biochemistry. Ammonia wouldn't be as good at this as water, but it does work.
