The photobiological differences of gilvocarcins V and M are not related to their transient intermediates and triplet yields.

The transient absorption spectra of the intermediates produced by the 355 nm laser excitation of gilvocarcin derivatives have been investigated in various solvents. The spectra consist of a triplet-triplet absorption in the visible region and a residual absorption observed between 340 and 700 nm due to a long-lived species, assigned to the radical cation. A broad-fast decaying band with a maximum at around 700 nm attributed to the solvated electron is also seen in solutions containing a low DMSO/water volume ratio and at 266 nm irradiation of a 50% methanol/water solvent mixture. The molar absorption coefficient of the triplet state of gilvocarcin V (GV) and gilvocarcin M (GM), determined by the energy transfer method, is independent of the solvent properties and has a value of 3.0 x 10(4)/Mcm. The triplet decay rate constants for both drugs are between 1 and 5 x 10(4)/s. A similar initial yield and triplet decay rate constant of GV were observed in the presence of 3.4 mM thymine. Thus, a quenching rate constant of the GV's triplet state by thymine is estimated to be lower than 10(6)/Ms. The triplet quantum yields of both antibiotics determined by using the comparative method are higher in dimethylsulfoxide (DMSO) (0.18) than are those corresponding to 25% DMSO/water (0.06). The decrease in phi T in the presence of water could be attributed to an enhanced internal conversion rate constant from the S1 state or to an increase in the photoionization yield. The similarity of the transient intermediates and their yields for GV and GM suggest that their photobiological differences are due to other factors such as DNA binding constants, preferential localization of the drugs in the cell or the enhanced reactivity of the vinyl group toward cellular components.

Photophysical properties of gilvocarcins V and M and their binding constant to calf thymus DNA.

Absorption and emission techniques were used to characterize the ground (S0), singlet (S1) and triplet states (T1) of gilvocarcin V (GV) and gilvocarcin M (GM) in different solvents. Aggregation of GV with dimerization constant equal to 7800 M-1 is observed in 10% dimethyl-sulfoxide (DMSO)/water. The photophysical properties of the S1 state of these molecules are more sensitive to changes in solvent characteristics than the corresponding ground states. The absorption of visible light by GV and GM results in a higher dipole moment of the excited state causing a red shift in the fluorescence spectra with increasing solvent polarity. The fluorescence quantum yield remains practically unchanged with changes in solvent properties unless water is present as a co-solvent. Both phi f and tau f values corresponding to GV in DMSO are larger than those of GM, whereas in 10% DMSO/H2O the opposite is observed. Thus, GV is more susceptible to other deactivation pathways besides emission in the presence of water than GM. The relative phosphorescence quantum yield (phi p = 0.03) and the triplet energy (ET = 52 kcal/mol) of GV and GM are similar. The S0-S1 energy difference is 63 kcal/mol for GV, whereas for GM it is 67. Thus, the singlet-triplet energy difference is 11 and 15 kcal/mol, respectively. The PM3/CI calculated electronic structures of these compounds are consistent with the observed photophysical properties. The dark binding constants of GV to calf thymus DNA ([1.1-0.08] x 10(6) M-1) are about an order of magnitude larger than those of GM ([0.24-0.018] x 10(6) M-1) at different ionic strengths (0-2.00 M NaCl). Also, the number of gilvocarcin molecules bound per base pair is smaller for GM than for GV. These differences in dark DNA binding parameters between GV and GM could have implications in the large photocytotoxic ability of GV as compared to GM.