The assembly of helical and b-sheet peptide blocks contg. reactive chain ends results in highly branched chain architectures (\"locked-in folds\") mimicking native tertiary structures. This mol. kit strategy allows to bypass the protein folding problem in protein de novo design and gives access to protein mimetics of high thermo-dynamic stability. The validity of this concept is exemplified for the design and synthesis of locked-in folds mimicking the zinc finger and MHC folding motifs. [on SciFinder (R)]