Abstract: The Trp⁃cage mini⁃protein has been taken as an example to study the influence of orientation properties of two serine side chains on its folding dynamics using a residue⁃specific force field（RSFF1）. It was obtained by improving the current OPLSAA/L force field through adding some special non⁃bonded interactions based on statistical analysis of intrinsic conformational features of coil regions in protein structures（PDB coil library）. The folding process of wild⁃type Trp⁃cage and its two mutations（S13G and S14G）were reproduced. It was found that this modified force field（RSFF1）could fold wild⁃type Trp⁃cage to its natively folded structure（N）with a low RMSD of only c.a. 0.67 Å，which resembled the experimental structure. Its folding pathway could be decomposed into three steps：D⇌I1⇌I2⇌N，where D，Iand N represented the denatured state，intermediate and natively folded state，respectively. The folding mechanism was in accordance with the diffusion collision model. Instead，only slight influences on secondary and tertiary structures were induced by mutation S13G，and the structure could arrive at the native⁃like state（N’）with a RMSD of c.a. 1.9 Å. The S14G mutation exerted observable influences on the folding dynamics. The N⁃terminal α⁃helices were locally loose，the destroy of 310⁃helices and hydrophobic collapses were concomitant in the S14G mutation，resulting a mis⁃folded state with a RMSD of c.a. 3.8 Å. Because S13 and S14 with the same side⁃chain exhibited the opposite orientation，their contributions to its structural stability and folding dynamics showed obvious discrepancy. It implied that both chemical（such as charge，hydrophobicity and polarity etc.）and orientation properties of side chains themselves should be fully considered during stabilizing active conformation of polypeptide/protein and improving the drugg ability of peptide inhibitor.

Key words: Trp?cage mini?protein, side?chain orientation, residue mutation, folding mechanism, inter?residue interaction