The polypeptide has a natural selection due to an optimized sequence. It folds properly. At the outside it has hydrophobic residues. Inside it has polar residues. Now the random sequence of the synthetic peptide is going to be unable to give a good direction to a folding process. This will cause aggregation of hydrophobic side chains on various molecules. Bringing about the polypeptides precipitation from solution. Polypeptides that occur naturally have have the same length when they are in aqueous solution because of amino acid side chains that are present and are large with reactive functional groups.
