THE PHOTOBIOLOGICAL DIFFERENCES OF GILVOCARCIN-V AND GILVOCARCIN-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 investiga ted in various solvents. The spectra consist of a triplet-triplet abso rption in the visible region and a residual absorption observed betwee n 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 att ributed to the solvated electron is also seen in solutions containing a low DMSO/water volume ratio and at 266 nm irradiation of a 50% metha nol/water solvent mixture. The molar absorption coefficient of the tri plet 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 f or both drugs are between 1 and 5 x 10(4)/s. A similar initial yield a nd 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 st ate by thymine is estimated to be lower than 10(6)/Ms. The triplet qua ntum yields of both antibiotics determined by using the comparative me thod are higher in dimethylsulfoxide (DMSO) (0.18) than are those corr esponding to 25% DMSO/water (0,06), The decrease in phi(T) in the pres ence of water could be attributed to an enhanced internal conversion r ate constant from the S-1 state or to an increase in the photoionizati on yield. The similarity of the transient intermediates and their yiel ds for GV and GM suggest that their photobiological differences are du e to other factors such as DNA binding constants, preferential localiz ation of the drugs in the cell or the enhanced reactivity of the vinyl group toward cellular components.