Scheme of the split synthesis. Synthesis of 81 tetrapeptides from 3 amino acids (yellow, blue and red circles) in 4 coupling cycles. The divergent and vertical arrows show dividing of the solid support (green circles) into 3 portions and coupling a different amino acid to each portion. The convergent arrows indicate mixing. After the second coupling cycle 9 dipeptides are present in the mixture. In the third and fourth cycle 27 and 81 peptides are formed, respectively.