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Unique Excited
State Properties of a Bacteriochlorophyll-serine Conjugate - the Key
to its Action as a Sensitizer in PDT?
Eichwurzel, Ines1,
Stiel, Holger1, Teuchner, Klaus1,
Scheer, Hugo 2, Scherz, Avigdor3,
Salomon, Yoram3 and Leupold, Dieter1
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie,
Max-Born-Str. 2a, D-12489 Berlin1
Botanisches Institut der LMU, D-80638 München2
Weizmann Institute of Science, Rehovot 76100, Israel3
Abstract-
Bacteriochlorophyll a (BChl) is not only an essential pigment in bacterial
photosynthesis, it also has been invoked to be a potential sensitizer
for photodynamic therapy of cancer (PDT). The interest in the latter
application has been renewed since BChl has been rendered water-soluble
by trans-esterification with the amino acid serine (BChl-ser) [1]. Absorption-
and fluorescence spectra together with fluorescence decay- and quantum
yield measurements, enable a differentiation between the influence of
covalent coupling to serine and influences of the aqueous environment
on ground-state absorption and first excited-state deactivation channels
of BChl-ser (as compared to BChl in organic solvents) [2]. Here we report
on excited state absorption and primary photochemistry of BChl-ser,
obtained by ps transient as well as nonlinear absorption measurements.
The results are again compared to BChl in polar solvents. Immediately
following ps-pulsed excitation in the Qy-absorption region,
BChl in polar solvents shows a structureless S1 absorption
as described earlier [3]. However, S1 decay of BChl doesnt
simply result in a rise of triplet absorption. Instead, a well-structured
transient absorption (slightly red-shifted with respect to the S0-S1
absorption band) is observed. Under the same excitation conditions,
the excited state behavior of BChl-ser in an aqueous environment is
completely different. Immediately after excitation, two new absorptions
in the red spectral region with very short lifetimes are apparent. Their
assignment will be discussed in the frame of the proposed mode of action
of BChl-ser as a sensitizer in PDT. [1] Rosenbach-Belkin, V., Chen,
L., Fiedor, L, Tregrub, I., Pavlotsky, I., Brumfeld, V., Salomon, Y.,
Scherz, A., Photochem. Photobiol.,1996, 64, 174; [2] Eichwurzel, I.,
Stiel, H., Teuchner. K., Leupold, D., Sheer, H., Salomon, Y., Scherz,
A., Photochem. Photobiol.,2000, 72, 204-209 [3] Becker, M., Nagarijan,
V., Parson, W., Biochemistry, 1991, 113, 6840 Supported by the German-Israeli-Foundation
(GIF project #. I-0497-140.02/96)
Keywords: PDT,
Bacteriochlorophyll
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