Photophysical
studies were carried out for simple amino acids like
alanine and valine with resorcinol-based aqueous acridinedione (ADDR)
dyes. ADDR dyes exhibit interesting excited-state characteristics
on altering the substituents at the 9th and 10th sites (Scheme 1).
The longest-wavelength absorption maxima remain the same on adding
the amino acids to the fluorophore, whereas the excited-state behavior
varies significantly mostly based on the nature of the substituent
at the 9th position. The absence of fluorescence enhancement was observed
with addition of β-alanine,
l
-alanine, and
l
-valine to ADDR1 dye (photoinduced electron transfer, PET), whereas
addition of glycine exhibits enhancement accompanied with a shift
toward a longer-wavelength region. Interestingly, the addition of
amino acids to non-PET dyes results in a fluorescence quenching accompanied
with a larger shift toward the shorter-wavelength region. The properties
of fluorophore and nonfluorophore dyes in the presence of alanine
or valine are found to be entirely different from those of glycine.
The interaction of alanine with ADDR dyes is predominantly through
H-bonding, but the structural aspects of H-bonding interactions of
alanine and water are completely different from those of glycine and
water. The time-correlated single-photon counting method portrays
the existence of fluorophore in two distinguishable microenvironments
in the presence of amino acids. The fluorescence spectral technique
used as a tool in elucidating the mode of interaction of dye with
neutral amino acids in aqueous solution is illustrated in the present
study.
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