We present mixed quantum-classical simulation of the internal conversion between the lowest energy ππ^* (S_La) and nπ^* (S_n) excited electronic states in adenine in the gas phase, adopting a quadratic vibronic model (QVC), parametrized with the help of PBE0 density functional calculations. Our approach is based on a hierarchical representation of the QVC Hamiltonian and a subsequent treatment of the most relevant coordinates at accurate time-dependent quantum level and of the other ‘bath’ modes at classical level. We predict an ultrafast transfer (∼30 fs) of ≈75% of the initial population excited on S_La to S_n. Within an adiabatic picture, on the same timescale the wave packet concentrates almost completely on the lowest S₁ state, where however it shows a very broad distribution with different characteristics (due to the different ‘diabatic’ character). It is shown that the proposed methodology offers a practicable route to describe the quantum dynamics of internal conversion processes in large semi-rigid systems.