Do zwitterions chase their own tails?

Question

Zwitterions are positive at one end and negative at the other, right?
So why don't the two ends curl round and join up?
When I make a model of glycine it's easy to get a proton in the NH3+ end to sit right next to an oxygen on the COO-
What stops that happening in real life?
If it does happen, how can the species still have two opposite charges? 
When they're right up next to one another, that proton is hydrogen bonded between the N and the O

Smeghead · Accepted Answer

Your model is inaccurately representing the stress of bending the bonds and curling the molecule.  The energy required to bend the molecule around like that is higher than the energy of the bond holding the positive and negative ends together.  In real life, bond angles are quite tightly constrained.  Apart from any other considerations, when you hold the molecule in one position as you propose (curled up), you are imposing a HUGE reduction in entropy, because instead of many thousands of slightly different configurations, the molecule now only has one single configuration.  That loss of entropy means a lot of energy is required to hold it.

novangelis · Answer

They can "chase", but the bonds angles required for cyclization require far too much energy to form.  The four membered beta-lactam ring of penicillins is unstable with accompanying bonds relieving the strain, and a spontaneous formation of a three-membered alpha-lactam   If there were sufficient bonds, spontaneous formation of a lactam (for example a six-membered delta-lactam from , the molecule would tend to be unionized at neutral pH.  

Glycine has pka1 and pka2 of 2.35 and 9.78, while for 5-Aminopentanoic acid they are 4.27 and 10.766.  (For comparison, acetic acid has a pka of 4.76 and pentaoic acid, 4.84.) There is decreased synergistic ionization with more intermediate bonds.

Do zwitterions chase their own tails?

2 Answers