Laser Selective Photobiology: Dye-Biomolecule Complexes

Abstract

After the successful use of laser beams to perform selective photochemistry [1], the possibility of laser selective photobiology has become of current interest at a few laboratories. For this purpose, a first proposal, based on two-step dissociation of a given biomolecule was made by Letokhov [2]. In this proposal, however, level thermalization must be prevented in order to retain selectivity. In the liquid phase, thermalization of the vibrational modes of a molecule usually occurs in a time of the order of a few picoseconds, if not shorter [3]. Accordingly, this scheme requires picosecond or even sub-picosecond pulses for both steps. Despite this difficulty, selective action on nucleic acid components have recently been reported by Kryukov et al. [4]. The experiment was performed using high-intensity UV pulses. A different proposal for selective laser photobiology was advanced by our group in a previous work [5]. This proposal is based on two-step photoion-ization (or dissociation) of a dye molecule bound to a given biological site. This process induces irreversible chemical damage to the biomolecule [6]. The proposal makes use of the fact that the dye fluorescence lifetime is a very sensitive function of the biological environment (i.e. of the binding site or of the biomolecule). Selectivity is achieved through the use of two laser pulses with suitable delay.