According to Levinthal a protein chain of ordinary size would require enormous time to sort its conformational states before the final fold is reached. Experimentally observed time of folding suggests an estimate of the chain length for which the time would be sufficient. This estimate by order of magnitude fits to experimentally observed universal closed loop elements of globular proteins -25-30 residues.

Key words: Levinthal?s paradox, protein folding, chain conformation, closed loops, loop fold structure.