From: Performance of rotation forest ensemble classifier and feature extractor in predicting protein interactions using amino acid sequences
Aliphatic amino acids
D1 = {A,V,L,I,M,C}
Aromatic amino acids
D2 = {F,W,Y,H}
Polar amino acids
D3 = {S,T,N,Q}
Positive amino acids
D4 = {K,R}
Negative amino acids
D5 = {D,E}
Special shape
D6 = {G,P}