Optical Detection Of Triplet-state Magnetic Resonance Research Articles

Fluorescence, phosphorescence, and optically detected magnetic resonance studies of the nucleic acid association of the nucleocapsid protein of the murine leukemia virus.

Fluorescence, phosphorescence, and optical detection of triplet state magnetic resonance (ODMR) are employed to investigate the interaction of p10, the nucleocapsid protein of the Moloney murine leukemia virus, with nucleic acids. p10 is a 55-amino acid protein containing a single zinc finger motif, C26C29H34C39, that includes Y at position 28 and W at position 35. In addition, the interactions of a zinc finger peptide, p10-ZF, comprising residues 24-41 of p10, and a doubly mutated 24-41 peptide, p10-ZF' in which the positions of Y and W are interchanged, also are reported. The measurements focus on the direct involvement of the sole W residue in the nucleic acid interaction. Fluorescence quenching and salt-back titrations indicate complex formation of p10 with several octanucleotides--(dT)8, (dI)8, (dU)7dT, and (5-BrdU)7dT--and with the polynucleotides poly(dT) and poly(dI). Poly(dI) binds with the highest affinity. Apparent binding constants and salt-back midpoints are reported. Neither p10-ZF nor p10-ZF' exhibits significant fluorescence quenching by these DNA substrates. Binding of p10-ZF to fluorescent poly(ethenoadenylic acid) was detected with greatly reduced affinity relative to p10, but binding of p10-ZF' was undetectable. These results are in general agreement with phosphorescence and ODMR measurements monitoring W. Addition of poly(I) to p10 leads to a phosphorescence red shift, reduction in the zero-field splitting (ZFS) parameters D and E, and a significantly reduced phosphorescence lifetime, each consistent with aromatic stacking interactions between W and the nucleobases. These effects are smaller with p10-ZF and undetectable with p10-ZF'. Poly(U) produces no significant changes in the triplet state parameters of W; no stacking interactions are observed even for p10. (5-BrdU)7dT yields large phosphorescence red shifts in p10 and p10-ZF, and reductions of D, but no significant heavy atom effects. These effects probably are due to enhanced local polarizability caused by Br, but any stacking interactions in these complexes would exclude van der Waals contacts between W and the Br atoms.

Optically detected triplet-state magnetic resonance studies of the DNA complexes of the bisquinoline analog of echinomycin

The polymeric DNA and model duplex oligonucleotide complexes of the bisquinoline analogue of echinomycin (2QN) have been studied by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy, with the quinoline chromophores of the drug used as intrinsic probes. Plots of ODMR transition frequencies versus monitored wavelength revealed heterogeneity in the phosphorescence emission of 2QN which was ascribed to the presence of a major and minor conformation of the drug in aqueous solutions (referred to as the red and blue forms of 2QN, respectively, in this report). ODMR results, in conjunction with findings from low-temperature phosphorescence investigations, indicate that the quinoline chromophores of the major (red) form of 2QN are involved in aromatic stacking interactions in complexes with the natural DNAs from Escherichia coli, Micrococcus lysodeikticus, Clostridium perfringens, and calf thymus as evidenced by red shifts in the phosphorescence 0,0-band of the drug, reductions in the phosphorescence lifetime and zero-field splitting (zfs) D and E parameters, and polarity reversals of the ODMR slow passage signals upon complex formation between the analogue and DNA. The polarity reversals, which reflect shifts in the triplet-state sublevel populations induced by complex formation, apparently result from changes in the triplet sublevel decay constants upon binding to the natural DNAs. The 2QN complexes of the double-stranded alternating copolymers poly(dG-dC).poly(dG-dC) [abbreviated as poly[d(G-C)2]] and poly(dA-dT).poly(dA-dT) [abbreviated as poly(dA-dT).poly(dA-dT) [abbreviated as poly[d(A-T)2], the homopolymer duplexes poly(dG).poly(dC) [abbreviated as poly(dG.dC)] and poly(dA).poly(dT) [abbreviated as poly(dA.dT)], and the self-complementary oligonucleotides d(ACGT)2, d(TCGA)2, and d(ACGTACGT)2 were also investigated. The extent of reduction of the zfs D parameter (delta D) for the major form of 2QN upon complex formation with the polymeric DNAs was found to scale linearly with the standard free energy of the drug-DNA interaction (delta G degrees) calculated from previously reported binding studies for these targets [Fox, K. R., et al. (1980) Biochem. J. 191, 729-740]. This relationship between spectroscopic and thermodynamic properties of the 2QN-polynucleotide complexes is a consequence of the effects of base stacking interactions on the electronic states of the intercalator, which were postulated to arise from second-order shifts of the ground-state and the triplet-state energies of the complex on the basis of a modification of the solvent effect theory of van Egmond et al. [(1975) Chem. Phys. Lett. 34, 423-426].

Application of ODMR to the study of DNA complexes of bisintercalating antibiotics and their biosynthesized derivatives

The DNA complexes of triostin A, echinomycin, and the monoquinoline (1QN) and bisquinoline (2QN) biosynthesized derivatives of echinomycin were investigated by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy, with the quinoxaline and quinoline moieties of the DNA-binding peptides used as intrinsic probes. Plots of zero-field splitting (zfs)D parameter versus monitored wavelength revealed heterogeneity in the phosphorescence emission of echinomycin, triostin A, and 2QN ascribed to the occurrence of major and minor forms of the peptides in aqueous solution. ODMR results, in conjunction with findings from phosphorescence studies, indicate that the quinoxaline and quinoline chromophores of the major forms of the peptides are involved in aromatic stacking interactions in complexes with the natural DNAs fromMicrococcus lysodeikticus, Escherichia coli, and calf thymus as evidenced by red shifts in the phosphorescence 0,0 bands of the drugs, reductions in the phosphorescence lifetimes and zfsD andE parameters, and polarity reversal of the ODMR slow passage signals upon drug complexation. The reversal in ODMR signal polarity of echinomycin and 2QN is a consequence of changes in the triplet-state sublevel decay constants upon peptide binding to the natural DNAs. The extent of reduction of theD parameter for the major form of echinomycin, 2QN, and the quinoline moiety of 1QN upon complexation with polymeric DNAs was found to correlate with the binding affinities measured for these targets [1], but no correlation was found for the quinoxaline moiety of 1QN. Preliminary studies of triostin A-DNA complexes also revealed no correlation between the reduction in zfsD-value upon complexation and binding affinity, although the largest reductions inD-value among the peptides investigated in this report were exhibited by the poly(dG-dC)·poly(dG-dC) and natural DNA complexes of triostin A.

Optically detected magnetic resonance study of the interaction of an arsenic(III) derivative of cacodylic acid with EcoRI methyltransferase

The interaction of the enzyme Escherichia coli RI methyl transferase (methylase) with an arsenic(III) derivative of cacodylic acid has been investigated by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy in zero applied magnetic field. The reactive derivative (CH3)2AsSR is formed by the reduction of cacodylate by a thiol. The As(III) derivative binds to the enzyme by mercaptide exchange with a cysteine (Cys) residue located close to a tryptophan (Trp) site. The arsenical binding selectively induces an external heavy-atom effect, perturbing the nearby Trp residue in the enzyme. Zero-field splittings (ZFS) and total decay rate constants of the individual triplet-state sublevels of the Trp residue in the presence and absence of perturbation by As(III) have been determined. The perturbed Trp shows a large reduction in the overall decay lifetime compared with unperturbed Trp residue, exhibiting a high selectively for the Tx sublevel. This selectivity suggests that the As atom lies in the xz plane of the principal magnetic axis system of Trp, but not directly along the z (out-of-plane) axis. The accessibility of this enzyme binding site to the arsenical is decreased upon forming a ternary complex of methylase with sinefungin and a DNA oligomer, d[GCGAA(BrU)(BrU)CGC], containing two 5-bromouracil (BrU) bases in place of thymine within the hexadeoxynucleotide recognition sequence. This result indicates that the arsenical binding site in methylase which produces the Trp heavy-atom effect is protected from this ligand by ternary complex formation or the enzyme undergoes a conformation change, removing the Cys from the Trp site. This protection is also observed in fluorescence quenching experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorescence and optically detected magnetic resonance studies of echinomycin-DNA complexes

Echinomycin complexes with polymeric DNAs and model duplex oligonucleotides have been studied by low-temperature phosphorescence and optical detection of triplet-state magnetic resonance (ODMR) spectroscopy, with the quinoxaline chromophores of the drug used as intrinsic probes. Although not optically resolved, plots of ODMR transition frequencies versus monitored wavelength revealed heterogeneity in the phosphorescence emission of echinomycin, which was ascribed to the presence of two distinct quinoxaline triplet-state environments (referred to as the blue and red triplet states of echinomycin in this report). We think that a likely origin of the two triplet states of echinomycin is the occurrence of two or more distinct conformations of the drug in aqueous solutions. Spectroscopically observed perturbations of the triplet-state properties of echinomycin such as the phosphorescence emission spectrum, phosphorescence lifetime, ODMR spectrum, and zero-field splitting (zfs) energies were investigated upon drug binding to the double-stranded alternating copolymers poly(dG-dC).poly(dG-dC) [abbreviated as poly[d(G-C)2]] and poly(dA-dT).poly(dA-dT) [abbreviated as poly[d(A-T)2]], the homopolymer duplexes poly(dG).poly(dC) [abbreviated as poly(dG.dC)] and poly(dA).poly(dT) [abbreviated as poly(dA.dT)], and the natural DNAs from Escherichia coli, Micrococcus lysodeikticus, and calf thymus. Echinomycin bisintercalation complexes with the self-complementary oligonucleotides d(ACGT), d(CGTACG), and d(ACGTACGT), which are thought to model drug binding sites, were also investigated. Phosphorescence and ODMR spectroscopic results indicate that the quinoxaline chromophores of the drug are involved in aromatic stacking interactions in complexes with the natural DNAs as evidenced by red shifts in the phosphorescence 0,0 band of the drug, a small but significant reduction in the phosphorescence lifetime of the red triplet state, and reduction of the zfs D-value of both the blue and red triplet states upon drug complexation. These changes in the triplet-state properties of echinomycin are consistent with stacking interactions that increase the polarizability of the quinoxaline environment. The extent of the reduction of the D parameter for the red triplet state upon complexation with the polymeric DNAs was found to correlate with the binding affinities measured for these targets [Wakelin, L. P. G., & Waring, M. J. (1976) Biochem. J. 157, 721-740], with the single exception of the drug-poly[d(G-C)2] complex, for which an increase in the D-value was noted. In addition, upon drug binding to the natural DNAs, there is a reversal of signal polarity in the ODMR spectra of the red triplet state. Among the synthetic DNA polymers investigated, a reversal of ODMR signal polarity was found only with the echinomycin-poly[d(A-T)2] complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Perturbation of tryptophan residues by point mutations in bacteriophage T4 lysozyme studied by optical detection of triplet-state magnetic resonance spectroscopy.

We have investigated perturbations of the triplet-state properties of Trp residues in bacteriophage T4 lysozyme caused by point mutations using low-temperature phosphorescence and optical detection of triplet-state magnetic resonance (ODMR) spectroscopy. Five temperature-sensitive mutants have been studied in detail. These include lysozymes with the point mutations Gln-105----Ala, Gln-105----Gly, Gln-105----Glu, Ala-146----Thr, and Trp-126----Gln. Changes in phosphorescence 0,0 band wavelength, intensity, the triplet-state zero-field splitting (ZFS), and the wavelength dependence of the ZFS were detected only from Trp-138 in each mutant. In the case of the Q105A mutation, the perturbations on Trp-138 have been ascribed to the combination of an increase in the polarizability of the environment and to the loss of hydrogen bonding of the enamine nitrogen of indole. For the Q105G mutation, we believe that Q is replaced by a solvent molecule in H bonding, leading to relatively small changes. In the Q105E mutation, the perturbation results largely from the introduction of a charged residue. In the case of the mutation A146T, the perturbation is associated with a local conformational change in which Trp-138 is shifted to a more solvent-exposed location. On the other hand, no significant spectroscopic changes in Trp-126 and Trp-158 were found in any of the mutants, suggesting that the perturbations are probably localized near Trp-138 for the mutations of positions 105 and 146. However, in the mutation W126Q, which occurs approximately 16 A away from Trp-138, significant changes of Trp-138 are detected, suggesting that the effects of this mutation are propagated over large distances.

Optically detected magnetic resonance study of tyrosine residues in point-mutated bacteriophage T4 lysozyme.

Two spectroscopically distinct types of tyrosine (Tyr) residues in triply point mutated bacteriophage T4 lysozyme, which contains no tryptophan (Trp), have been detected by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy. Their triplet states are characterized by similar E but different D values. The Tyr site which exhibits the lower D value and has the red-shifted phosphorescence origin is quenched by energy transfer to Trp and has D and E values comparable to previously studied Tyr residues. The blue-shifted Tyr site, which is not quenched by Trp, exhibits a larger D value that has been found previously. Calculation of energy-transfer efficiencies of Tyr-Trp pairs based on the crystal structure of the native enzyme provides a possible assignment of Tyr sites to the two different spectral types.

Investigation of the role of individual tryptophan residues in the binding of Escherichia coli single-stranded DNA binding protein to single-stranded polynucleotides. A study by optical detection of magnetic resonance and site-selected mutagenesis.

Fluorescence and optical detection of triplet state magnetic resonance (ODMR) spectroscopy have been employed to study the complexes formed between single-stranded polynucleotides and Escherichia coli ssb gene products (SSB) in which tryptophans 40, 54, and 88 are selectively, one residue at a time, replaced by phenylalanine using site-specific oligonucleotide mutagenesis. Fluorescence titrations and ODMR results indicate that tryptophans 40 and 54 are the only tryptophan residues in E. coli single-stranded DNA binding protein that are involved in stabilizing the protein-nucleic acid complexes via stacking interactions. Wavelength-selected ODMR measurements on E. coli SSB reveal the presence of two spectrally distinct tryptophan sites (Khamis, M. I., Casas-Finet, J. R., and Maki, A. H. (1987) J. Biol. Chem. 262, 1725-1733). Our present results indicate that tryptophan 54 belongs to the blue-shifted site, while tryptophan 40 belongs to the red-shifted site of the protein.

Investigation of complexes formed between gene 32 protein from bacteriophage T4 and heavy-atom-modified single-stranded polynucleotides using optical detection of magnetic resonance.

Optical detection of triplet-state magnetic resonance (ODMR) is employed to study the complexes formed between gene 32 protein (GP32), a single-stranded DNA-binding protein from bacteriophage T4, and the heavy-atom-derivatized polynucleotides poly(5-HgU) and poly(5-BrU). The triplet-state properties of some of the tryptophan (Trp) residues in the complexes are dramatically different from those in the free protein, in that they are subject to an external heavy-atom effect. Direct evidence for the presence of a heavy-atom effect, and hence a close-range interaction between mercurated or brominated nucleotide bases and Trp residues in the complex, is provided by the observation of the zero-field (D) + (E) ODMR transition of Trp, which is not normally observed in the absence of a heavy-atom perturbation. The amplitude-modulated phosphorescence-microwave double-resonance (AM-PMDR) technique is employed to selectively capture the phosphorescence spectrum originating from the heavy-atom-perturbed Trp residue(s) in the GP32-poly(5-HgU) complex. Arguments based on our experimental results lead to the conclusion that the heavy-atom perturbation arises from aromatic stacking interactions between Trp and mercurated bases. Wavelength-selected ODMR measurements reveal the existence of two environmentally distinct and spectrally different types of Trp in GP32. One of these types is perturbed selectively by the heavy atom and hence undergoes stacking interactions with the heavy-atom-derivatized bases of the polynucleotide while the second type of Trp residue is unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of phospholipase-lipid interactions by optical detection of triplet state magnetic resonance spectroscopy

We have investigated the binding of porcine pancreatic phospholipase A 2 (PA 2) to n-hexadecylphosphocholine (C 16PN) micelles using optical detection of triplet state magnetic resonance (ODMR) spectroscopy. The zero field splittings (zfs) of the single Trp 3 residue undergo significant changes upon binding of PA 2 to C 16PN micelles. Zfs titrations of PA 2 vs C 16PN indicate that the binding stoichiometry is C 16PN:PA 2 ~ 25. A reduction of the ¦ E¦parameter from 1.227 to 1.135 GHz is postulated to result from Stark effects caused by the binding of a polar group (possibly phosphocholine) near Trp 3 in the PA 2-C 16PN micelle complex.

Optically detected magnetic resonance of the phosphorescent bases of Escherichia coli valine-specific transfer ribonucleic acid

Phosphorescence spectroscopy and optical detection of triplet state magnetic resonance (ODMR) spectroscopy have been used to characterize bases that contribute to the phosphorescence emission of Escherichia coli valine-specific transfer ribonucleic acid. When it is excited with 335-nm light, a short-lived phosphorescence with an origin near 435 nm is observed and is assigned to 4-thiouridine (s4U) at position 8 of the tRNA sequence. With excitation at 290-300 nm, a structured, long-lived phosphorescence is observed with an origin near 380 nm, in addition to the s4U phosphorescence. Comparison was made of the phosphorescence and ODMR spectra between Mg2+-containing and Mg2+-free tRNA samples. The s4U phosphorescence of the Mg2+-containing sample is more structured, and the peak is blue shifted relative to the Mg2+-free sample. Both samples give a single low-frequency (ca. 2.9 GHz) ODMR signal, but the high-frequency signal region (ca. 19-20 GHz) is structured. The Mg2+-containing sample has a partially resolved group of lines centered at 19.3 GHz, whereas the Mg2+-free sample has two broad bands centered at 19.2 and 20.0 gHz. The differences are attributed to effects of Mg2+ on the tRNA conformation. The ODMR signals observed by monitoring the long-lived phosphorescence are assigned to a pyrimidine nucleoside, possibly 5-(carboxy-methoxy)uridine in the anticodon.