The binding of 4',6-diamidino-2-phenylindole to bovine serum albumin.

The binding of 4',6-diamidino-2-phenylindole (DAPI) to bovine serum albumin (BSA) has been investigated between pH 6 and 8, in 0.05 M phosphate buffer at 20 degrees C, by fluorescence titrations and the results analyzed according to a procedure previously reported (R. Favilla and A. Mazzini, Biochim. Biophys. Acta 788 (1984) 48). The dye binds to the protein with a blue shift of about 4 nm in its fluorescence emission maximum, but with an enhancement factor of 10 of its fluorescence quantum yield. The dissociation constant decreases from 100 microM to 54 microM as the pH is increased from 6 to 8, with a constant number of nearly three equivalent binding sites. The complete displacement of DAPI bound to BSA by Ca2+ suggests a possible specificity of this substantially electrostatic interaction. The fluorescence decay of DAPI bound to the protein shows a double exponential kinetics, with a tau 1 = 0.97 ns and tau 2 = 2.78 ns. These results, compared with those obtained for DAPI alone, tau 1 = 0.16 ns and tau 2 = 2.8 ns, are rationalized in terms of two different rotamers of DAPI. Both rotamers are able to bind to the protein, but only one of them undergoes an intramolecular proton transfer, from the 6-amidinium group to the indole aromatic ring, in the excited singlet state of DAPI alone. When DAPI interacts with BSA this transfer does not occur and consequently a large increase of fluorescence is observed.(ABSTRACT TRUNCATED AT 250 WORDS)