We use a mixed quantum-classical (MQC) approach to study energy transfer to a peptide's torsional modes through collision with an atomic ion. In this study, the torsional motion of the peptide is treated quantum mechanically while the translational motion of the attacking atomic ion is treated classically. The MQC approach is applied to study the excitations of torsional modes of GLY-ALA dipeptide by colliding the peptide with a sodium ion Na+. In particular, we study the energy transfer to torsional motions around the C-alpha-C and C-alpha-N bond and rotation of the CH3 side chain. Calculation shows that the efficiency of energy transfer is strongly dependent on initial orientation of the colliding partners. For the majority of approaching angles, the energy transfer to torsional mode is rather small. However, for certain attacking angles, the torsional mode is strongly excited and the Na+ is found to be trapped near the peptide. The average ratios of energy transfer to torsional motions along C-alpha-C, C-alpha-N and the side chain CH3 are about 58%, 27%, and 17%, respectively. (C) 2003 American Institute of Physics.