The amino acids glutamine and asparagine have an amino group substituted in place of the OH of the carboxylic acid groups in the side chains of respectively glutamatic acid and aspartic acid. This means that the amino group neighbours a C=O and means that it cannot be protonated at low pH. I was just wondering why this is the case? Can it simply be explained with electronegativity or do we need to use MO theory? Any thoughts?