Table 2. Classification of amino acids by two independent schemes based on physicochemical properties.

Classification by charge (Hughes et al., 1990)
Positive	R, H, K
Negative	D, E
Neutral	A, N, C, Q, G, I, L, M, F, P, S, T, W, Y, V
Classification by volume and polarity (Miyata et al., 1979)
Special	C
Neutral and small	A, G, P, S, T
Polar and relative small	N, Q, Q, E
Polar and relative large	R, H, K
Nonpolar and relatively small	I, L, M, V
Nonpolar and relatively large	F, W, Y