Tetsuro Majima, Kiyohiko Kawai and colleagues from Osaka University made photosensitizer modified and charge acceptor modified DNA with different lengths of 'sticky end'. 

Sticky ends are single strand sections of DNA which 'work as glue' for nanostructure assembly. Majima and colleagues investigated the effects of changing the length of sticky ends on the assembly of nanostructures, by direct observation of the charge transfer kinetics. 

Charge transfer has been shown previously to occur between strands through adenine to its paired guanine, and along strands by guanine hopping. The charge transfer should therefore not occur between unattached segments. After exciting the photosensitizer modified DNA via laser, they watched for the charge acceptor to form a radical cation to indicate assembly. Formation of the radical cation occurred at 590 nm. 

Majima used charge transfer to determine whether assembly occurred using 2, 4, 6 and 8 nucleotide sticky ends. They saw charge transfer for all except 2 nucleotides, although this was weak for 4 nucleotides. Results for the same lengths of sticky end using gel mobility shift assays showed only assemblies with 8 nucleotides. The discrepencies were thought to be due to differences in conditions between gel electrophoresis and the solution structures used for laser experiments. Majima confirmed by monitoring the charge acceptor that no transfer occurred when non-complimentary sticky ends were used, i.e. when no assembly would be expected. 

The major challenge in this area for the future is to examine the conductivity of DNA nanostructures according to Majima. The next step in their lab will be to investigate the charge transfer in DNA segments constructed by cross-over DNA. 

Rachel Davies