Interactions of fluoroaniline with a heme-octapeptide of cytochrome c: a 19F NMR study

Abstract

Interactions of ortho-, meta- and para-fluoroaniline with a heme-octapeptide (H8PT) prepared from cytochrome c were studied by means of optical absorption and 19F NMR. The binding constant ( K b) of para-fluoroaniline to H8PT was estimated from the Soret absorption spectral change to be 4.5 x 10 4 M −1, which was larger than the K b values of ortho- (1.8 x 10 M −1) and meta-fluoroaniline (1.2 × 10 M −1) by three orders. On adding H8PT to the fluoroaniline solutions, the 19F NMR bands of ortho- and meta- fluoroaniline shifted to a lower magnetic field region by a chemical shift of nearly 100 Hz together with a small band-broadening, while that of para- fluoroaniline was shifted to a higher magnetic field by a chemical shift of more than 300 Hz together with heavy line-broadening up to nearly 300 Hz in half- band-width. The line-broadening and the low-field movement of the chemical shift of the 19F NMR signal of para-fluoroaniline on adding H8PT were ascribed to the binding of para-fluoroaniline to the H8PT on the basis of the K b values estimated from the optical absorption change. Assuming the fast chemical exchange regime, the exchange rate of para- fluoroaniline to H8PT and the residence time of para-fluoroaniline in the bound state were estimated to be 5.3 x 10 2 s −1 and 3.0 x 10 −4 s, respectively.