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Effect of Self-Association
and Biopolymer Binding on the Photonuclease/Photoprotease Activity of
Triarylmethane Photosensitizers
Indig, Guilherme1,
Bartlett, Jeremy1 and Lewis, Lavinia1
University of Wisconsin1
Abstract-
The fluorescence lifetime of cationic triarylmethane (TAM) dyes in low-viscosity
media is very short (typically 1-5 ps) due to fast nonradiative relaxation
processes that occur via rotational motions of their aromatic rings.
Consequently, TAM dyes show poor photoreactivity in aqueous solutions.
However, these dyes efficiently bind to natural and synthetic anionic
biopolymer polyelectrolytes in water, and macromolecular binding is
a phenomenon that has remarkable effects on their photoreactivity. In
biopolymer-TAM complexes the steric hindrance imposed by the macromolecule
on the rotational motions of the dye moiety typically leads to enhancements
of about 3 orders of magnitude in TAM fluorescence lifetime. Therefore,
in biopolymer-TAM complexes the excited dye singlet has a much better
chance to either cross to the respective triplet or engage in electron
transfer reactions with the host macromolecule. In complexes of Ethyl
Violet and Crystal Violet with BSA, hexokinase, and DNA, the excitation
of the dye moiety with 532 nm light leads both to dye bleaching and
macromolecular damage. Both dye bleaching and macromolecular damage
do not require the involvement of molecular oxygen to occur, and are
initiated by a photoinduced electron transfer from the host biopolymer
to the guest photosensitizer. BSA damage was characterized by a site
specific cleavage of the protein. DNA damage was characterized by extensive
strand-breaks in the host macromolecule. Hexokinase damage was characterized
primarily by a decrease in enzymatic activity upon photolysis of hexokinase-TAM
complexes. With the formation of dye aggregates on the host macromolecules
(conditions of relatively high biopolymer loading) the reaction mechanism
is altered, and the degree of macromolecular damage tends to decrease
concomitantly with an increase in the efficiency of dye photobleaching.
In these complexes the initial electron transfer event presumably does
not involve the host macromolecule. Supported in part by PharmaLux,
LLC.
Keywords: triarylmethanes,
photonucleases, photoproteases, molecular exciton
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