As to explaining this note :
A general definition of acids and bases comes from Lewis. Here, acids are defined as compounds which accept an electron pair from a base, and bases are defined as compounds which can donate an electron pair to an acid. Two other terms that are used extensively in organic chemistry are electrophile (electron-seeking species) and nucleophile (nucleus-seeking, or electron-donating species). These terms have essentially the same meaning as Lewis acid and base respectively, but in organic reactions they are used to refer specifically to reactions occurring at carbon.
Amino acids are highly polar compounds, with very low solubility in organic solvents. Their solubility in water is somewhat greater than in alcohols, and the compounds are very soluble in aqueous acid or base. When a simple amino acid such as glycine, NH2CH2CO2H, dissolves in aqueous acid, it is present in the form of the ammonium cation +NH3CH2CO2H which has two acidic groups, +NH3 and CO2H. Upon adding a base OH - to the solution the species now will be present as NH2CH2CO2-. And it is that glycine (and the other alpha-amino acids in neutral solution [alpha-amino acids have their COOH and NH2 groups attached at the same C-atom] ) may exist as a so-called zwitterion or dipolar ion in which the amino and carboxyl groups are mutually neutralized to give an "inner salt" :
cation, below pH = 2.5 zwitterion, at pH = 2.5-9.0 anion, above pH = 9.0
The dipolar structure of amino acids accounts for the salt-like properties of the compounds [such as Na+ Cl -]. Although the formulas of amino acids may be written as if they were neutral compounds (e.g. NH2CH2CO2H ), it must be kept in mind that they are in fact dipolar ions. The pH [= the degree of acidity : pH lower than 7, acid, pH = 7, neutral, pH above 7, basic] of the solution at which the positive and negative charges are exactly balanced to give an electricallly neutral molecule [the zwitterion] is called the iso-electric point. For most amino acids this value is in the range of pH from 5.5 to 6.5. If an additional NH2 group is present, as in lysine, the isoelectric point will be higher (9.5), and if an additional CO2H group is present, the isoelectric point will be lower.
Picking up Müller's text (present note) now :
Features do appear that closely follow protein constitution. Many of them are expressed already at the bonding of building blocks, when, for instance, against the nucleophilous nitrogen of one amino acid there is a carboxylic carbon of the other amino acid, which [carboxylic carbon] has largely lost its electrophilous nature as a result of the zwitter-ion mesomery [i.e. an ionic state mediating between two ionic states].