Study of adrenocorticotropic hormone conformation by evaluation of intramolecular resonance energy transfer in N -dansyllysine 21 -ACTH-(1-24)-tetrakosipeptide.

The solution conformation of ACTH was studied by intramolecular resonance energy transfer and fluorescence depolarization with the synthetic, biologically active, derivative N(epsilon)-dansyllysine(21)-ACTH-(1-24)-tetrakosipeptide. The Förster parameters involved in energy transfer from the donor Trp(9) to the dansyl acceptor attached to the side chain of Lys(21) were determined from measurements with ACTH fragments containing either the donor or the acceptor alone. From determinations of the fluorescence quantum yields of the donor in the derivative and in the native ACTH-(1-24)-tetrakosipeptide, it was shown that besides energy transfer no additional quenching of the donor fluorescence is introduced by the presence of the acceptor. The short measured rotational relaxation time of the dansyl chromophore reflects the flexibility of the lysine side chain and justifies the use of an average value for the orientation factor in the distance calculations. The calculated intramolecular distance is remarkably independent of solvent, indicating a "random coil" situation with regard to residues 9-21.