How amino acids react
Amino acids contain an amino group, mostly NH2, and a carboxylic group, -COOH.
Thats why an amino acid can react with bases ánd with acids. The amino acid is an ampholyt.
When the amino acid has, per molecule, 1 amino group and 1 carboxylic group, then the pH of the solution of this amino acid will be about neutral (6 - 8). Any amino acid with extra carboxylic groups will cause a lower pH and an amino acid with extra amino groups will cause a higher pH value.
In a solution, one amino acid molecule will donate the H+ of its carboxylic group to the amino group of a similar, but not the same molecule.
There is a real option that, in a solution, one amino acid molecule will donate the H+ of its carboxylic group to the amino group of a similiar molecule.
And this amino group can belong to another amino acid molecule, but could also belong to the same molecule. In that last case, we talk about internal proton transfer.
On the one side, this molecule became --COO- and on the other side ---NH3+.
A same molecule has so a positive side as well as e negative side; we call that particle a 'double ion'.
In short: an amino acid can - dependent on the environment - be neutral in two ways, be positive and be negative.
Condensation and Hydrolysis
Apart from participation in acid base reactions, amino acids do very well in another reaction type: condensation.
There the amino group of one molecule is linked to the OH-group of another molecule, losing a water molecule.
In this way two amino acids are linked to each other through a so called peptide bond.