The synthesis of protected peptide fragments of a staphylococcal nuclease.

We have recent,ly determined the t,otal amino acid sequence of an extracellular nuclease of Staphylococcus aureus consisting of a single chain of 149 residues devoid of half-cystinelf 2 (see Fig. 1). This globular protein contains some regions of helical structure,3 can undergo completely reversible unfolding in a variety of denaturing solvents,4 and readily yields cryst’als suitable for X-ray analysis.5 Because of these features, organic synthesis of the polypeptide chain of the nuclease would be of particular value in the direct study of catalytic and binding properties, and of factors determining tertiary structure. The proposed plan for synthesis involves an assembly of the chain through successive couplings of protected peptide fragments to the free a-amino group of a protected C-terminal sequence. Each peptide fragment must be fully protected except for a free terminal carboxyl group. Short of total stepwise assembly of the nuclease chain in one operation, these considerations preclude the use of the solid phase method6 of Merrifield, as usually employed, since in this method most blocking groups are lost upon detachment of the completed peptide from the supporting resin. At the present time, the uninterrupted solid phase synthesis of a chain of