The Factors of the Classification of Protein Amino Acids
  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. 
Miloje M. Rakocevic