2.4. Isothermal titration calorimetry (ITC)
Isothermal Titration Calorimetry (ITC) experiments were carried
out at 25 _C on the Nano-ITC II Instruments (TA Instrument).
Prior to analysis, GA, MEGA and PCA were dissolved in 30 mM of
Hepes buffer (containing 100 mM citrate) at pH 7.40, respectively.
The Fe(III)–citrate solution was also in the same buffer. Before each
titration, all solutions were degassed thoroughly under vacuum
with stirring. The solution in the sample cell was stirred at
300 rpm to ensure rapid mixing of the titrant upon injection. Titrations
were performed by an automated sequence of 32 injections,
each of 1.5 ll Fe(III)–citrate titrate into the sample cell containing
190 ll GA/MEGA/PCA, spaced at 360 s intervals to ensure complete
equilibration. A background titration, consisting of the same titrate
solution but only the buffer solution in the sample cell, was subtracted
from each experimental titration to account for heat of
dilution. Three titrations were carried out for each measurement,
and the reported experimental values are the average of individual
best-fit values, fitting with independent binding model in the Nano
Analyze software provided by TA Instruments. Raw data were processed
using the Origin 8.0 software.