Question

How does an r group determine if an amino acid is acidic basic or nonpolar?

Answer 1

R group (or side chain) is an organic component of amino acids that is unique to each amino acid.  Three of the amino acids have basic side chains at neutral pH: arginine (Arg), lysine (Lys), and histidine (His). Their side chains contain nitrogen and resemble ammonia, which is a base and their pKa's are high enough that they tend to bind protons, (positive charge). Aspartic acid or aspartate (Asp) and glutamic acid or glutamate (Glu) are acidic amino acids because their side chains have carboxylic acid groups whose pKa's are low enough to lose protons, (negatively charged). The non-polar amino acids contain uncharged functional groups at physiological pH and are incapable of participating in hydrogen bonding.