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Looking at the structure of an amino acid (as a zwitterion), it seems that the part of the molecule with the amino group and the carboxylic acid group are quite polar (the amino group has a positive hydrogen and the carboxylic acid group has a negative oxygen). However, my textbook says that the character of the R group completely determines the polarity of the molecule (i.e. if R is non-polar, the molecule is non-polar). Why is this?

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    $\begingroup$ Compared to other amino acids, that is. $\endgroup$ – Ivan Neretin Mar 13 '17 at 4:10
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    $\begingroup$ What @IvanNeretin said, but taking it a little further, when amino acids are called "polar" or "nonpolar," I think it usually means the amino acid residues within a protein, i.e. whether the side chain makes that part of the protein hydrophilic (or so-called "polar") or hydrophobic (or "non-polar"). $\endgroup$ – SendersReagent Mar 13 '17 at 7:29
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All amino acids have the capability to form the zwitter ion...but that doesnt necessarily make all of them polar....consider the example of a saponification reaction...we have a polar end and a non polar alkyl group attacked to it making that whole part of the chain hydrophobic...and non polar Indeed since the general structure of an amino acid being NH2-CH(R)-COOH The R group has a significant part to play in determining if an amino acid is polar or not...which makes amino acids like alanine, cysteine, glycine, isoleucine, leucine, methionine, phenylalanine, proline, tryptophan, tyrosine and valine show more of a non polar character than polar

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