[ Submit a Question, Answer or Critique]
From: Robin
Date: 04 Feb 2004
Time: 20:23:58 -0500
Remote Name: 67.128.224.54
"Figure 5-9 says that the nonionic form of amino acids does not occur in significant amounts in aqueous solutions, and that the zwitterion predominates at neutral pH. Why is this? I would have thought they would be in equilibrium." Rachel: Yes, the nonionic form and the zwitterion are in equilibrium at neutral pH. But at elelibrium, there is a much higher concentration of zwitterions than nonionic AA's. Remember that equilibrium means you have a constant exchange between the two states -- at one moment, 9 AA's may be zwitterionic and 1 nonionic, the next moment, that 1 may be zwitterionic, and one of the 9 might be nonionic. The reason zwitterions are favored is because zwitterionic AA's are lower in energy than nonionic AA's. The COOH group "prefers" to be COO- at pH 7 because of resonance. The resonance is energetically favorable enough to overcome the fact that the negative charge in energetically unfavorable. (under more acidic conditions, more of the amino acids will be COOH and less will be COO-, because of the high proton concentration -- the protons need somewhere to go, so it takes less energy to stick them onto COO-). As for the amine group, it "prefers" to be positively charged at pH 7 because of nitrogen's electronegativity. Or think of it like this: neutral nitrogen has a lone pair of electrons sticking out, and at pH 7 the proton concentration is high enough that most of the time that lone pair will grab an "extra" proton. (under more basic conditions, less of the amine groups would have the extra proton, because OH- would start competing with nitrogen for its protons.) Another thing that lowers the energy of the zwitterion is that the positive and negative charges help stabalize eachother just a little bit. I hope this answers your question and is not too confusing. Just remember, it's all about energy !!! :-)