Collisions with biomolecules embedded in small water clusters

We have studied fragmentation of water embedded adenosine 5'-monophosphate (AMP) anions after collisions with neutral sodium atoms. At a collision energy of 50 keV, loss of water molecules from the collisionally excited cluster ions is the dominant process and fragmentation of the AMP itself is almost completely prohibited if the number of attached water molecules is larger than 13. However, regardless of the initial number of water molecules attached to the ion, capture of an electron, i.e. formation of a dianion, always leads to loss of a single hydrogen atom accompanied by evaporation of water molecules. This damaging effect becomes more important as the size of the water cluster increases, which is just the opposite to the protective behavior observed for collision induced dissociation (CID) without electron transfer. For both cases, the loss of water molecules within the experimental time frame is qualitatively well described by means of a common model of an evaporative ensemble. These simulations, however, indicate that characteristically different distributions of internal energy are involved in CID and electron capture induced dissociation.