The vacuum UV CD Spectra of G.G.C. triplexes

Vacuum UV circular dichroism (CD) spectra were measured down to 175 nm for d(C) 10 , d(G) 10 , the d(G) 10 -d(C) 10 duplex, and the d(G) 10 -d(G) 10 -d(C) 10 triplex. A CD difference spectrum was calculated for d(G) 10 -d(C) 10 giving the change in CD induced by forming the duplex from d(G) 10 and d(C) 10 . The d(G) 10 -d(G) 10 -d(C) 10 CD difference spectrum gave the CD Induced by triplex formation from binding of d(G) 10 to the d(G) 10 -d(C) 10 duplex. In the near-UV, the d(G) 10 -d(C) 10 and d(G) 10 -d(G) 10 -d(C) 10 difference spectra resembled the difference spectrum for poly[r(G)-r(C)] (Blopolymers 29, 325-333). This similarity may be an indication of similar purine base stacking. The d(G) 10 -d(G) 10 -d(C) 10 vacuum UV difference spectrum had a negative band at 195 nm and a positive band at 180 nm, making it similar to difference spectra for homopolymer triplexes containing T.A.T and U.A.U triplets (Nucl. Acids Res.19, 2275-2280). The appearance of these bands in difference spectra should be good Indicators of triplex formation. The complementary oligonucleotides c-myc/d(CCCCACCCTCCC) and c-mycll d(GGGAGGGTGGGG) are part of the regulatory sequences of the human c-myc gene. G-G-C rich triplexes formed by binding c-mycll or c-mycll I d(GGGGTGGGTGGG) to the c–myclc–mycll duplex had CD difference spectra similar to that of d(G) 10 -d(G) 10 -d(C) 10 in both the vacuum UV and near UV regions, indicating similar triplet structures.