TABLE 1.

Classification of amino acids.

No.	Dipolescalea	Volumescaleb	Class
1	−	−	Ala, Gly, Val
2	−	+	Ile, Leu, Phe, Pro
3	+	+	Tyr, Met, Thr, Ser
4	+ +	+	His, Asn, Gln, Tpr
5	+ + +	+	Arg, Lys
6	$+{}^{\prime }+{}^{\prime }+{}^{\prime }$	+	Asp, Glu
7	+ c	+	Cys

^aDipole   scale (Debye): −, Dipole < 1.0; +, 1.0 < Dipole <2.0; + +, 2.0 < Dipole < 3.0; + + +, Dipole > 3.0; $+{}^{\prime }+{}^{\prime }+{}^{\prime }$, Dipole > 3.0 with opposite orientation. ^bVolume   scale (ų): −, Volume < 50; +, Volume > 50. ^cCys is separated from class 3 because of its ability to form disulfide bonds.