Strong coupling between adenine nucleobases in DNA single strands revealed by circular dichroism using synchrotron radiation.

Circular dichroism (CD) experiments on DNA single strands (dA(n)) at the ASTRID synchrotron radiation facility reveal that eight adenine (A) bases electronically couple upon 190 nm excitation. After n=8, the CD signal increases linearly with n with a slope equal to the sum of the coupling terms. Nearest neighbor interactions account for only 24% of the CD signal whereas electronic communication is limited to nearest neighbors for two other exciton bands observed at 218 and 251 nm (i.e., dimer excited states). Electronic coupling between bases in DNA is important for nonradiative deexcitation of electronically excited states since the hazardous energy is spread over a larger spatial region.