Nanocomposites (NC) were formed using cationic poly(L-lysine) (PLL), a semicrystalline polypeptide, that was reinforced by sodium montmorillonite (MMT) clay via solution intercalation technique. By varying solution conditions such as pH, temperature, and polypeptide concentration in the presence of clay platelets, the secondary structure of PLL was controllably altered into alpha-helical, beta-sheet, and random coil. The high molecular weight polypeptide shows a strong propensity to fold into the beta-sheet structure when cast as films, irrespective of the initial secondary structure in solution. Nanocomposite local morphology confirms intercalated MMT platelets with PLL over a wide range of compositions. (c) 2006 Wiley Periodicals, Inc.