Nonlinear protein - nucleic acid crosslinking induced by femtosecond UV laser pulses in living cells

We investigate the interactions between proteins and DNA in solutions of living cells, triggered by UV, 200-fs laser pulses. DNA-protein crosslinking is obtained with two different techniques: conventional chemical or laser-induced. Our results evidence a nonlinear response of the cells to the laser irradiation in the investigated intensity range. The laser induced crosslink efficiency can reach a value approximately twice higher than that obtained with ordinary chemical methods. The experimental results are very well reproduced by a simple phenomenological model based on the interplay between two-photon absorption by DNA bases and cell damage induced by high intensity laser pulse.