Workshop on Theoretical Developments for Radiation Damage
Working Group Four
The meeting in Fréjus was dedicated to develop extensive discussions on current studies and new projects of theoretical research on radiation damage in biomolecular systems. In that sense, relatively short communications were given with full time for discussion. With participants from all the research groups relied to the WG4, a wide range of theoretical methods, from static to dynamic, were covered during the workshop. Moreover, new research groups as the “Laboratoire de Physique Théorique”, IRSAMC, University of Toulouse; the “Chimie, Biologie, Radicaux libres” group, University of Marseille; the Department of Chemistry, University of Gdansk were for the first time involved in the theoretical group. In addition to discussions on existing calculation methods, the development of new methodologies including hybrid approaches was considered. The two main directions handled by the working group, photochemistry and photodissociation processes and molecular reaction dynamics in electron attachment or ion/biomolecule interactions were developed.
The basics of the hybrid quantum mechanics/molecular mechanics (QM/MM) methodology were widely discussed around applications to photochemistry in large molecular systems. The specific requirements for studies in biological macromolecular systems and solvent were developed. New developments in the ab-initio calculation of ground and excited potential energy hypersurfaces were reviewed. The correct location of the important features of the energy landscapes such as minima, transition states or conical intersections was pointed out. In that sense, non-adiabatic dynamics around conical intersections appear to be a general mechanism of de-excitation of excited states and a correct choice of both reaction path and active coordinates for defining the nuclear motions are necessary to describe the dynamical processes. A wide variety of problems in photochemistry and photophysics has been presented as the degradation of the 5-bromo uracil or the photophysics of Ru(II) polypyridyl complexes, in particular intercalated in DNA. The electron transfer in stacked clusters of DNA bases as guanine was also addressed. Such a problem is the basic mechanism of electron migration in DNA induced by ionizing radiations.
Concerning the groups involved into dynamical approaches, an opening to time-dependent microscopic description of the irradiation of clusters and molecules, with extension to the problem faced for clusters in contact with a substrate by means of hierarchical approaches has been proposed. The laboratories interested in dynamics are using a large variety of methods: from classical trajectories, semi-classical approaches and full quantum treatments. A great number of processes have been discussed: photodissociation, charge transfer mechanisms, ionization or fragmentation processes induced by collision of ions on biomolecules. Direct and indirect processes occurring in the action of radiation on the biological medium have been considered.
As a conclusion, it is worth noting that the biological systems considered are not only the DNA and RNA bases or peptides, but also chromophores of proteins or visual pigments. The use of hybrid methods or hierarchical treatments opens the possibility to extend theoretical approaches to realistic models of the biological medium including environment effects.