Prediction and improvement of protected peptide solubility in organic solvents

A predictive method of solubility for protected peptide fragments of globular proteins was described. The solubility prediction was performed on the basis of both the randomness of peptide structures in a solid state and the existence of tertiary peptide bonds such as X-Pro and X-(Z) Y bonds, in which X and Y are arbitrary amino acid residues and Z is a suitable protecting group for the X-Y peptide bond. In order to predict the randomness, the coil conformational parameter, Pc, was utilized. Solubility prediction by this method was successfully applied to the two classes of protected peptides composed solely of hydrophobic and of various amino acid residues. The solubility test results also indicate that the protection of peptide bonds at suitable intervals is effective in achieving a remarkable improvement in the solubility of extraordinarily insoluble peptides. Lastly, the strategy for the selection of the coupling routes in the protein syntheses was proposed.