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 immunodeficienc y virus (HIV-I) to various oligonucleotides and polynucleotides has be en investigated by phosphorescence and optically detected magnetic res onance (ODMR) spectroscopy, The intrinsic spectroscopic probe used in these studies is the photoexcited triplet state of Trp37, which is ass ociated with the C-terminal zinc finger of p7 and is its only tryptoph an residue. Complex formation produces a red-shift of the phosphoresce nce 0,0-band (Delta E-0,E-0) of Trp37 as well as a reduction of the ze ro field splitting (zfs) D parameter. Increases of -Delta E-0,E-0 (A < C < U < G < I) rank with increasing binding affinity to nucleic acid homooligomers (A similar to C < U < G similar to I). It is proposed th at the magnitude of the shift reflects the extent of aromatic stacking interactions. We propose also that -Delta D increases not only with i ncreased aromatic stacking but also with the extent of charge transfer (CT) character admired into the triplet state. The quantity Delta D/D elta E-0,E-0 correlates with the electron affinity of the bases (G < A < C < U approximate to T), suggesting that this quantity reflects the extent of CT character admired 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 T-x su blevel. 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.