  In this work it is shown that three pairs of the factors appear to be the
key, i.e. main factors of a natural classification of protein (canonical) amino
acids within the amino acid (genetic) code. First pair: the factors of the
habit of an amino acid molecule (size and polarity). Second pair: the factors
of the association (type of the amino acid/enzyme reactivity and degree of the
hydrophobicity/hydrophilicity of an amino acid molecule). Third pair: the
factors of the dissociation (degree of the acidity-basicity, over acidic group,
COOH and degree of the basicity/acidity over the basic group, NH2). As a result
of the influence and interdependence of all six factors (measured through
correspondent valid parameters) it appears still one natural classification
into polar and non-polar amino acids, where polar amino acids possess negative
and non-polar, the positive values of hydropathy index.
