Phosphorus SAD Phasing for Nucleic Acid Structures: Limitations and Potential

Abstract

Phasing of nucleic acid crystal diffraction data using the anomalous signal of phosphorus, P-SAD, at Cukα wavelength has been previously demonstrated using Z-DNA. Since the original work on P-SAD with Z-DNA there has been, with a notable exception, a conspicuous absence of applications of the technique to additional nucleic acid crystal structures. We have reproduced the P-SAD phasing of Z-DNA using a rotating-anode source and have attempted to phase a variety of nucleic acid crystals using P-SAD without success. A comparison of P-SAD using Z-DNA and a representative nucleic acid, the Dickerson-Drew dodecamer, is presented along with a S-SAD using only two sulfurs to phase a 2’-thio modified DNA decamer. A theoretical explanation for the limitation of P-SAD applied to nucleic acids is presented to show that the relatively high atomic displacement parameter of phosphorus in the nucleic acid backbone is responsible for the lack of success in applying P-SAD to nucleic acid diffraction data.

Keywords

• SAD phasing
• nucleic acid
• phosphorus