MULTIPLE FLUORESCENCE LIFETIMES FOR OLIGONUCLEOTIDES CONTAINING SINGLE, SITE-SPECIFIC MODIFICATIONS AT GUANINE AND ADENINE CORRESPONDING TOTRANS ADDITION OF EXOCYCLIC AMINO-GROUPS TO 8-DIHYDROXY-9,10-EPOXY-7,8,9,10-TETRAHYDROBENZO[A] PYRENE AND 8-DIHYDROXY-9,10-EPOXY-7,8,9,10-TETRAHYDROBENZO[A] PYRENE())

Fluorescence decay profiles of four oligonucleotide duplexes, [GRAPHIC S] ((+)- and (-)- trans-1) and [GRAPHICS] ((+)- and (-)-trans-2), in w hich an exocyclic amino group of deoxyadenosine (A) or deoxyguanosine (G) has been alkylated by trans opening at C-10 of the epoxide group of either the (+)-(R,S,S,R)- or (-)-(S,R,R,S)-enantiomer of (+)-7 bet a,8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a ]pyrene (BPDE in which the benzylic 7-hydroxy group and the epoxide ox ygen are trans), exhibit more than one fluorescence lifetime. Decay pr ofiles of the oligomers, measured at 15 degrees C with excitation and emission wavelengths of 335 and 400 nm, respectively, have been analyz ed using a triple-exponential decay law. Results for (+)- and (-)-tran s-1 and -2 have been compared with results for the modified, single-st randed oligonucleotides ((+)- and (-)-trans-SS-1, and (+)- and (-)-tra ns-SS-2) and for the cis and trans opened products formed on alkylatio n at the g-amino group of 2'-deoxyadenosine 5'-phosphate by (+)-(R,S,S ,R)-BPDE ((+)-trans- and(+)-cis-A). The profiles of(+)-trans- and (+)- cis-A are well represented by single-exponential decay laws with lifet imes of 86 and 110 +/- 3 ns, respectively. For the single- and double- stranded oligomer adducts, which exhibit at least three fluorescence l ifetimes, two of the lifetimes are short (0.5-14 +/- 1 ns) and one is long (35-59 +/- 3 ns). The fluorescence lifetimes and the amplitudes o f the long-lived components in the decay profiles of the double-strand ed oligomer adducts are generally smaller than those for the correspon ding single-stranded adducts. The data provide evidence that the doubl e-stranded oligomer adducts exist as multiple conformations. Previousl y reported NMR results suggest that the short-lifetime fluorescence co mponents are due to major adduct conformations in which the pyrenyl gr oup is intercalated ((+)- and (-)-trans-1) or lies in the minor groove ((+)- and (-)-trans-2). The observation of long lifetime fluorescence species for the double-stranded oligomers is consistent with the pres ence of minor conformations (similar to 1-5%) in which the double-stra nded oligomer either is locally denatured or is a mixture of locally d enatured double-stranded conformations and equilibrium concentrations of single-stranded oligomers.