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Selective dissociation of non-covalent bonds in biological molecules by laser spray.

The laser spray developed in our laboratory was applied to the analysis of bovine serum albumin (BSA), double-stranded DNA (dsDNA) and a protein-DNA complex. The tip of a stainless-steel capillary was irradiated with a 10.6 micro m infrared laser by increasing the laser power from 0 W (electrospray) to 1.4 W. The laser beam was focused to about 0.3 mm at the tip of the stainless-steel capillary. When BSA aqueous solution was irradiated by the laser, highly charged monomer ions were newly observed in addition to the multiply charged ions of non-denatured monomer, dimer and trimer moieties. This indicates that BSA suffers from denaturation on irradiation with an infrared laser in solution. A 1.4 W laser power is not sufficient to cause the complete denaturation of BSA under the present experimental conditions. Whereas dsDNA was found to dissociate almost completely to single-stranded DNA constituents on laser irradiation with a power of 1.2 W, no fragmentation of DNA molecules was observed. For a protein-DNA complex, i.e. a complex of c-Myb DNA binding domain and dsDNA, dissociation of the complex to the component moieties was observed. These findings indicate that the laser spray can selectively dissociate non-covalent complexes into subunits without causing dissociation of the covalent bonds of the subunits. The laser spray will be a versatile method for the investigation of the structures and stabilities of biomolecules including non-covalent complexes.