THE STRUCTURE OF STREPTONIGRIN SEMIQUINONE IN SOLUTION

Streptonigrin semiquinone (SQ(.)), a free radical intermediate implica ted in the biological functioning of the antitumor antibiotic strepton igrin has been prepared and its structural properties in solution have been characterized. Through the use of electron paramagnetic resonanc e spectroscopy, the spin densities of the unpaired electron have been determined, indicating that the unpaired electron is largely confined to the quinolinesemiquinone moiety of the antibiotic. Unambiguous assi gnment of the hyperfine coupling constants was achieved employing isot opically labeled semiquinone radical, INDO molecular orbital calculati ons, and the study of unsubstituted 5,8-quinolinesemiquinone as a refe rence system. The assignments point to a negative spin density at the carbon para to the pyridine nitrogen in the radicals derived from both streptonigrin and the unsubstituted 5,8-quinolinequinone. Characteriz ation of the properties of the streptonigrin semiquinone in solution i ndicate that the radical is stable in solution: it can be conveniently studied in 0.1 M methanolic lithium hydroxide or in aqueous organic s olvent mixtures buffered with 0.06 M K3PO4 at pH 12.0. Under these con ditions, the semiquinone shows completely reversible spectral changes between -10 to 60 degrees C, Lowering the pH from 12.0 to 7.0 in aqueo us DMSO decreases the lifetime of the radical from two weeks to a few minutes. Changes in structural properties of streptonigrin semiquinone in solution have been found to occur mainly due to variation in solva tion and freedom of rotation of the amino group. Decreasing the temper ature of SQ(.) solution in methanol from 60 to - 10 degrees C leads to an increase in the hyperfine coupling constant to the amino nitrogen from 1.28 to 1.40 G, and those of the two amino protons from 0.73 and 0.73 to 1.02 and 1.11 G respectively, while the other coupling constan ts change less than 3%. Greater electron spin delocalization onto the -NH2 group has been found throughout the solvent systems examined, yet the temperature at which the two amino protons become equivalent chan ges with the nature of the solvent (e.g., from 25 degrees C in MeOH to 40 degrees C in aq. DMSO). The effective rotational diffusion constan t as measured from EPR spectra in a series of solvents with fixed pola rity (E-T) follows the Stokes-Einstein equation only in solvent mixtur es of low to moderate viscosities (10-12.4 cP) suggesting that in addi tion to the viscosity of the medium a more specific mechanism (e.g., h ydrogen bonding to the solvent) restricts the motion of the amino grou p. This hydrogen bonding mechanism is further supported by the fact th at the degree of inequivalence in the amino proton hyperfine coupling constants varies monotonically with the spin density at the amino nitr ogen. Studies of SQ(.) in aqueous micellar dispersions using neutral, cationic, and anionic sufractants indicate that SQ(.) is located at th e surface of these aggregates. The structure of SQ(.) changes little a t neutral and anionic micellar surfaces; however, more severe structur al changes occur in cationic micelles which appear to be consistent wi th a conformational change induced by the positively charged surface.