Anomalous signal of phosphorus used for phasing DNA oligomer: importance of data redundancy.

The crystal structure of a left-handed Z-DNA hexamer duplex d(CGCGCG)(2) has been solved based on the anomalous diffraction signal of inherent P atoms using data collected at the single wavelength of 1.54 A. The anomalous signal of about 2% of the total diffracted intensity, constant for all nucleic acids, may be generally useful for solving crystal structures of DNA and RNA oligomers. The multiplicity of intensity measurements is shown to crucially affect the data quality and the ability to solve the phase problem. The anisotropic model refined to an R factor of 8.9% at 0.95 A resolution.