[brett0123]

I‘m a bit confused on why L-Alanin is an non polar aminoacid but still soluble in water. Why is that? Does it have something to do with it‘s IEP?

[Babcock_Hall]

It is important to differentiate the side versus the entire amino acid.  The twenty common side chains have very different properties.  Drawing out the structure may help.

[Borek]

1. Polarity is not something that defines solubility on terms of soluble/nonsoluble, more like "easier to dissolve" vs "harder to dissolve".

2. I wouldn't call alanine non polar. Yes, -CH₃ is not something very polar, but it is just a small part of the molecule, -COOH and -NH₂ are much more important (and yes, they do ionize easily, making the "non polar" classification even more questionable).

[rolnor]

But the side-chain is non-polar compared to many of the other amino-acid side chains.

[Borek]

But the side-chain is non-polar compared to many of the other amino-acid side chains.

Compared to other amino-acids - yes. In general - not so.

And solubility is more in the "general" category.

[Babcock_Hall]

When one teaches the amino acids in introductory biochemistry classes, one often groups the side chains R into categories; everyone recognizes that the categories are somewhat arbitrary, but they are still useful.  A commonly used textbook provides five categories of R groups: nonpolar aliphatic (valine), non polar aromatic (phenylalanine), nonionizing but polar (asparagine), neutral acids (glutamate), and cationic acids (lysine).  I put an example of each one in parenthesis for convenience.  It is useful to classify the side chains, because in a polypeptide, the charges on the ammonium group and the carboxylate group of the free amino acids are all lost.

The OP's question sounds like a homework problem that is teaching by means of a paradox, which Borek solved.  The IEP (isoelectric point, pI) is another matter.  Amino acids are not uniformly soluble across a range of pH; however, I suspect that the question probably was not written with the isoelectric point in mind.

[rolnor]

Thanx Babcock. Enzymes are much like ordinary chemistry, I once had a prodrug with a phenol-group bound to a carboxylic acid, this acid was the prodrug tail to be cleaved off. In para-position to the phenol-group I had a fluorine. What we saw was that the prodrug tail was cleaved too rapidly, and we could not see the complete prodrug at all in plasma in the animal. So I changed the fluoro to a methoxy, and now we got a much better oral bioavailability, and we could also see the complete prodrug as a separate curve on HPLC in the animal's blood. The methoxy made the phenolic prodrug-ester more stable compared to the fluoro, you can compare this with the pentafluorophenol-group which is used as a good leaving group when you want to acylate something. The project then failed because of too rapid liver metabolism of the drug molecule itself, not poor oral bioavailability of the prodrug.