A closer look at the parallel RNA double helix poly(rA)

As early as 1961, Rich, Davies, Crick and Watson proposed a parallel double helix RNA structure for poly(rA) on the basis of diffuse fibre diffraction photographs [1], using similar arguments to those used to deduce the antiparallel double helix structure for DNA.It took over 50 years before this could be confirmed by a crystal structure determination [2].However the solution of this structure by ab initio direct methods required one week on an eight-CPU workstation.We recently obtained synchrotron data from better quality crystals of significantly higher resolution and completeness.With the new data the structure can be solved in a much shorter time and the data quality appears to be sufficient for a charge density analysis using the Invariom approach [3].