The Effect of Solvation on the Mean Excitation Energy of Glycine

Theoretical studies of energy deposition by fast ions on biological molecules are often carried out using isolated (gas phase) target molecules, while in fact the molecules are in an intracellular aqueous environment. The question then arises as to whether conclusions drawn from fast ion collisions with isolated biomolecules are applicable to the in vivo situation. In this contribution, we examine the prototypical case of the mean excitation energy of glycine for the isolated molecule and for the solvated molecule, using the polarizable quantum mechanics/molecular mechanics (QM/MM) approach employing both Hartree-Fock and density functional theory wave functions. The solvent shifts are approximately 2% of the isotropic mean excitation energy and are thus larger than the effect of electron correlation.