We developed a dual basis approach within the fragment molecular orbital formalism enabling efficient and accurate use of large basis sets. The method was tested on water clusters and polypeptides and applied to perform geometry optimization of chignolin (PDB: ) in solution at the level of DFT/6-31++G鈭椻垪, obtaining a structure in agreement with experiment (RMSD of 0.4526 脜). The polarization in polypeptides is discussed with a comparison of the 伪-helix and 尾-strand.