[Kcv]

Hello
So I have 2 questions that I hope you guys might help me out with.

1) So first one is about Amino Acids in their Zwitterion or (0) form, near isoionic point. According to my book at this point, amino acids tend to take a solid form because '' hydrofobic interactions increase'' but I don't seem to grasp that.


I mean if the amine group remains NH3 + and the carbocylic one COO- ,maybe the interactions between different amino acids would increase and the positive amine groups would attract the negative COO- groups. But are these interactions the cause of the low solubility near isoionic point? And if they were, why would these interactions be stronger than the ones between these groups and water ?? Or maybe there are other factors.

2) Question Number 2 is about Choline.
 

Apparently, the charge on the N atom is stable, but why though ? I know CH3 groups are electron rich, there's what we call the ''electron repulsion effect''. But N is quite electronegative why would a positive charge be stable on it ? Or does the fact that N+ is sitting in the middle of some pretty electron rich groups make it more stable ? Electrostatic attractions ?

[Vidya]

For Q1
Solubility in pure water or water mixed with alcohol or in organic non polar solvent...
Q2
You are right that N is very electronegative  with a positive charge but still it is considered a more stable due to its octet ...like carbocation is not stable because its octet is incomplete.

[Babcock_Hall]

It might be easier to start with one of the amino acids that has an non-ionizing side chain, such as glycine, alanine, methionine, etc.  Your choice of lysine complicates matters a bit, in that the side chain can protonate, as well as the amino and carboxylic acid groups common to all of the amino acids.