EVIDENCE FOR STACKING INTERACTIONS BETWEEN 5-MERCURATED POLYURIDYLIC ACID AND HIV-1 P7 NUCLEOCAPSID PROTEIN OBTAINED BY PHOSPHORESCENCE ANDOPTICALLY DETECTED MAGNETIC-RESONANCE (ODMR)

The photoexcited triplet state of Trp-37 in the C-terminal zinc finger of the HIV-1 p7 nucleocapsid protein was used as a probe of p7 intera ctions with the heavy atom-derivatized RNA homopolymer, poly-5-mercuri uridylic acid (5-HgU). Binding of p7 to 5-HgU (Hg blocked with 2-merca ptoethanol) produces an external heavy atom effect (HAE) on Trp-37 cha racterized by fluorescence quenching, reduction of the phosphorescence lifetime by three orders of magnitude, and the appearance of the D E phosphorescence-detected ODMR signal, absent in unperturbed Trp, but induced by a HAE. The details of the HAE are consistent with out-of-p lane van der Waals contact of Hg with the indole chromophore of Trp-37 . Steric requirements suggest further that the Trp-RNA contact occurs via an aromatic stacking interaction.