Binding of the nucleocapsid protein of type 1 human immunodeficiency virus to nucleic acids studied using phosphorescence and optically detected magnetic resonance.

The binding of p7 nucleocapsid protein of type 1 human immunodeficiency virus (HIV-1) to various oligonucleotides and polynucleotides has been investigated by phosphorescence and optically detected magnetic resonance (ODMR) spectroscopy. The intrinsic spectroscopic probe used in these studies is the photoexcited triplet state of Trp37, which is associated with the C-terminal zinc finger of p7 and is its only tryptophan residue. Complex formation produces a red-shift of the phosphorescence 0, 0-band (DeltaE0,0) of Trp37 as well as a reduction of the zero field splitting (zfs) D parameter. Increases of -DeltaE0,0 (A < C < U < G <I) rank with increasing binding affinity to nucleic acid homooligomers (A approximately C < U < G approximately I). It is proposed that the magnitude of the shift reflects the extent of aromatic stacking interactions. We propose also that -DeltaD increases not only with increased aromatic stacking but also with the extent of charge transfer (CT) character admixed into the triplet state. The quantity DeltaD/DeltaE0,0 correlates with the electron affinity of the bases (G < A < C < U approximately T), suggesting that this quantity reflects the extent of CT character admixed with the triplet state by the aromatic stacking interaction. Also affected by nucleic acid binding of p7 are the kinetic parameters of Trp37. We find a selective increase in the relative populating rate, and of the decay rate constant of the Tx sublevel. In binding of p7 to either d(IT)2 or d(IT)4, two distinct sets of triplet states of Trp37 are resolved, suggesting the existence of specific nucleic acid binding modes of these heterooligomers.