Self-assembly of Td.
All single strands(S1, S2, S3,S4) in equimolar quantities were mixed in
TM buffer (12.5 mM Tris, 5 mM MgCl2, pH=7.8-8.0), heated
to 95 °C for 5 min with an MJ MiniTM 48-well personal thermal cycler,
and then cooled rapidly on ice for at least 1 h.