methylcoumarin is reported. Itwas found that the coumarin-maslinic derivative (MaCo) forms an excellent fluorescence
resonance energy transfer (FRET) pair with the tryptophan (Trp) residue of human serum albumin
(HSA). This feature allowed for monitoring HSA conformational alterations by measuring the distance between
donor (Trp) and acceptor (MaCo) through Förster energy transfer mechanism. Displacement experiments confirmed
that MaCo binds to subdomain IIA of HSA with independence of temperature. It was observed that, in
the temperature range 35–45 °C, the fluorescence emission maximumofHSA-MaCo complex decreased,whereas
in the range 45 °C–65 °C, an incrementwas detected. The concomitant change in the polarity of environment surrounding
Trp was confirmed by red edge excitation shift experiments. Thermal denaturation of HSA was
followed by time-resolved fluorescence spectroscopy. Average lifetime of Trp residue decreased with temperature
due to the increment of solvent collisions and changes in the solvent exposure of Trp. To discriminate the
importance of each effect, lifetime of N-Acetyl-L-tryptophanamide (NATA) at different temperatures was measured.
Circular dichroism(CD) studies confirmed the loss of secondary structure of HSA with increasing temperature
and showed a different trend in the conformational transformation below and above 45 °C, in agreement
with steady-state and time-resolved fluorescence experiments.
