3.5. Cysteine residues
The fluorescence emission of mBrB was used to determine the
number of cysteines residues present in PV2. The emission of mBrB
displayed a clear kinetic behavior with t50=35±1 min. A similar
experiment using BSA, produced a similar kinetic with t50=37±1. A
comparison of the fluorescence saturation value between BSA (with
ca. 35 cysteine residues) and PV2 renders a value of 32 cysteine
residues for the latter.
3.6. Fluorescence quenching experiments
To gain insight about the environment of tryptophan residues in
PV2, fluorescence quenching experiments were performed by
monitoring the acrylamide-induced fluorescence quenching of tryptophan
emission. Increasing amounts of acrylamide scaled down the
emission spectra without any noticeable blue shift as would be
expected for the presence of tryptophan residues with different
solvent exposure [26] (Fig. 3A). To quantify the degree of solvent
exposure of the indole rings, the “sphere of action” quenching model
[26,27] and Eq. (1), a modified form of the Stern-Volmer model, were
Fig. 5. Proteinase K digestion of PV2. The PV2 protein solution (0.25 mM) was incubated
with the indicated concentrations of proteinase K. Products were analyzed on a SDS
polyacrylamide gel (4–15%) and visualized with Coomassie blue.