Chemical and structural characterization of interstrand cross-links formed between abasic sites and adenine residues in duplex DNA

A new type of interstrand DNA–DNA cross-link between abasic (Ap) sites and 2′-deoxyadenosine (dA) residues was recently reported, but the chemical structure and properties of this lesion were not rigorously established. Here we characterized the nucleoside cross-link remnant released by enzymatic digestion of duplex DNA containing the dA-Ap cross-link. A synthetic standard was prepared for the putative nucleoside cross-link remnant 6 in which the anomeric carbon of the 2-deoxyribose residue was connected to the exocyclic N⁶-amino group of dA. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that the synthetic material 6 matched the authentic cross-link remnant released by enzymatic digestion of cross-linked DNA. These findings establish the chemical structure of the dA-Ap cross-link released from duplex DNA and may provide methods for the detection of this lesion in cellular DNA. Both the nucleoside cross-link remnant 6 and the cross-link in duplex DNA were quite stable at pH 7 and 37°C, suggesting that the dA-Ap cross-link could be a persistent lesion with the potential to block the action of various DNA processing enzymes.     

Site specific labelling of sugar residues in oligoribonucleotides: reactions of aliphatic isocyanates with 2' amino groups.

Considerable effort has been directed towards studying the structure and function of nucleic acids and several approaches rely on the attachment of reporter groups or reactive functional groups to nucleic acids. We report here the selective modification of 2-amino groups in oligoribonucleotides, through their reaction with aliphatic isocyanates, to give the corresponding 2'-urea derivatives in >95% yield. Furthermore, such modification with (2-isocyanato)ethyl 2-pyridyl disulfide enables subsequent coupling to other thiols (such as those contained in peptides and proteins) or to thiol-reactive electrophiles. A modified decamer was not significantly destabilized by the 2'-urea group, compared with a 2'-amino group, as demonstrated by a mere 1.3 degrees C drop in the melting temperature of the duplex.     

Thermal stability and energetics of 15-mer DNA duplex interstrand crosslinked by trans-diamminedichloroplatinum(II)

The effect of the location of the interstrand cross-link formed by trans-diamminedichloroplatinum(II) (transplatin) on the thermal stability and energetics of 15-mer DNA duplex has been investigated. The duplex containing single, site-specific cross-link, thermodynamically equivalent model structures (hairpins) and nonmodified duplexes were characterized by differential scanning calorimetry, temperature-dependent uv absorption, and circular dichroism. The results demonstrate that the formation of the interstrand cross-link of transplatin does not affect pronouncedly thermodynamic stability of DNA: the cross-link induces no marked changes not only in enthalpy, but also in "reduced" (concentration independent) monomolecular transition entropy. These results are consistent with the previous observations that interstrand cross-links of transplatin structurally perturb DNA only to a relatively small extent. On the other hand, constraining the duplex with the interstrand cross-link of transplatin results in a significant increase in thermal stability that is primarily due to entropic effects: the cross-link reduces the molecularity of the oligomer system from bimolecular to monomolecular. Importantly, the position of the interstrand cross-link within the duplex modulates cooperativity of the melting transition of the duplex and consequently its thermal stability.     

N(4)C-alkyl-N(4)C cross-linked DNA: bending deformations in duplexes that contain a -CNG- interstrand cross-link

Short DNA duplexes containing a 1,3-N(4)C-alkyl-N(4)C interstrand cross-link that joins the two C residues of a -CNG- sequence were prepared using either a phosphoramidite or convertible nucleoside approach. The alkyl cross-link consists of 2, 4, or 7 methylene groups. The duplexes, which contain a seven-base-pair core and A(3)/T(3) complementary 3'-overhanging ends, were characterized by enzymatic digestion and MALDI-TOF mass spectrometry. Ultraviolet thermal denaturation studies showed that the duplexes denature in a cooperative manner and that the length of the cross-link affects the thermal stability. Thus, the transition temperature of the ethyl cross-linked duplex, 42 degrees C, is 16 degrees C higher than the melting temperature of the corresponding non-cross-linked control, whereas the transition temperatures of the butyl and heptyl cross-linked duplexes, 73 and 72 degrees C, respectively, are 46-47 degrees C higher. The reduced molecularity of denaturation of the cross-linked duplexes versus melting of the non-cross-linked duplex most likely accounts for these differences. Examination of molecular models suggests that the ethyl cross-link is too short to span the distance between the two C residues at the site of the cross-link in B-form DNA without causing distortion of the helix, whereas less and no distortion would be expected for the butyl and heptyl cross-links, respectively. The circular dichroism spectra, which show greatest deviation in the ethyl cross-linked duplex from B-form DNA, are consistent with this expectation. Anomalous mobilities on native polyacrylamide gels of multimers produced by self-ligation of each of the cross-linked duplexes suggest that the ethyl and butyl cross-linked duplexes undergo bending deformations, whereas multimers derived from the heptyl cross-linked duplex migrated normally. The bending angle was estimated to be 20 degrees, 13 degrees, and 0 degrees for the ethyl, butyl, and heptyl cross-linked duplexes, respectively. Thus, it appears that the degree of bending in these N(4)C-alkyl-N(4)C cross-linked duplexes is controlled by the length of the cross-link.     

Synthesis of a new disulfide Fmoc monomer for creating biologically susceptible linkages in peptide nucleic acid oligomers

Peptide nucleic acids (PNA) are one of many synthetic mimics of DNA and RNA that have found applications as biological probes, as nano-scaffold components, and in diagnostics. In an effort to use PNA as constructs for cellular delivery we investigated the possibility of installing a biologically susceptible disulfide bond in the backbone of a PNA oligomer. Here we report the synthesis of a new abasic Fmoc monomer containing a disulfide bond that can be incorporated into a PNA oligomer (DS-PNA) using standard solid phase peptide synthesis. The disulfide bond survives cleavage from the resin and DS-PNA forms duplexes with complementary PNA oligomers. Initial studies aimed at determining if the disulfide bond is cleavable to reducing agents while in a duplex are explored using UV thermal analysis and HPLC.     

N(4)C-ethyl-N(4)C cross-linked DNA: synthesis and characterization of duplexes with interstrand cross-links of different orientations.

The preparation and physical properties of short DNA duplexes that contain a N(4)C-ethyl-N(4)C interstrand cross-link are described. Duplexes that contain an interstrand cross-link between mismatched C-C residues and duplexes in which the C residues of a -CG- or -GC- step are linked to give "staggered" interstrand cross-links were prepared using a novel N(4)C-ethyl-N(4)C phosphoramidite reagent. Duplexes with the C-C mismatch cross-link have UV thermal transition temperatures that are 25 degrees C higher than the melting temperatures of control duplexes in which the cross-link is replaced with a G-C base pair. It appears that this cross-link stabilizes adjacent base pairs and does not perturb the structure of the helix, a conclusion that is supported by the CD spectrum of this duplex and by molecular models. An even higher level of stabilization, 49 degrees C, is seen with the duplex that contains a -CG- staggered cross-link. Molecular models suggest that this cross-link may induce propeller twisting in the cross-linked base pairs, and the CD spectrum of this duplex exhibits an unusual negative band at 298 nm, although the remainder of the spectrum is similar to that of B-form DNA. Mismatched C-C or -CG- staggered cross-linked duplexes that have complementary overhanging ends can undergo self-ligation catalyzed by T4 DNA ligase. Analysis of the ligated oligomers by nondenaturing polyacrylamide gel electrophoresis shows that the resulting oligomers migrate in a manner similar to that of a mixture of non-cross-linked control oligomers and suggests that these cross-links do not result in significant bending of the helix. However, the orientation of the staggered cross-link can have a significant effect on the structure and stability of the cross-linked duplex. Thus, the thermal stability of the duplex that contains a -GC- staggered cross-link is 10 degrees C lower than the melting temperature of the control, non-cross-linked duplex. Unlike the -CG- staggered cross-link, in which the cross-linked base pairs can still maintain hydrogen bond contacts, molecular models suggest that formation of the -GC- staggered cross-link disrupts hydrogen bonding and may also perturb adjacent base pairs leading to an overall reduction in helix stability. Duplexes with specifically positioned and oriented cross-links can be used as substrates to study DNA repair mechanisms.     

Thermal and thermodynamic properties of duplex DNA containing site-specific interstrand cross-link of antitumor cisplatin or its clinically ineffective trans isomer

The effect of the single, site-specific interstrand cross-link formed by cisplatin or transplatin on the thermal stability and energetics of a 20-base pair DNA duplex is reported. The cross-linked or unplatinated 20-base pair duplexes were investigated with the aid of differential scanning calorimetry, temperature-dependent UV absorption, and circular dichroism. The cross-link of both platinum isomers increases the thermal stability of the modified duplexes by changing the molecularity of denaturation. The structural perturbation resulting from the interstrand cross-link of cisplatin increases entropy of the duplex and in this way entropically stabilizes the duplex. This entropic cross-link-induced stabilization of the duplex is partially but not completely compensated by the enthalpic destabilization of the duplex. The net result of these enthalpic and entropic effects is that the structural perturbation resulting from the formation of the interstrand cross-link by cisplatin induces a decrease in duplex thermodynamic stability, with this destabilization being enthalpic in origin. By contrast, the interstrand cross-link of transplatin is enthalpically almost neutral with the cross-link-induced destabilization entirely entropic in origin. These differences are consistent with distinct conformational distortions induced by the interstrand cross-links of the two isomers. Importantly, for the duplex cross-linked by cisplatin relative to that cross-linked by transplatin, the compensating enthalpic and entropic effects almost completely offset the difference in cross-link-induced energetic destabilization. It has been proposed that the results of the present work further support the view that the impact of the interstrand cross-links of cisplatin and transplatin on DNA is different for each and might also be associated with the distinctly different antitumor effects of these platinum compounds.     

NMR studies on metal complexes of DNA oligomers

A short overview of NMR spectroscopic applications for the study of metal ion complexes of DNA oligomers is presented. One typical example is given to illustrate the scope of the methods: the NMR structure of a trans-DDP interstrand cross-linked duplex, d(CTCCTG*TGTCTC) x d(GAGATA*AGGAG). The solution structure of this double-stranded DNA oligonucleotide, containing a trans-diammineplatinum(II) interstrand cross-link, was determined using two-dimensional nuclear magnetic resonance (2D NMR) and NOE-restrained molecular dynamics and energy minimization refinement. The duplex is a non-palindromic 12/11-mer with a missing central residue in the lower strand and in addition it contains a GT mismatched base pair. The analysis indicated that an interstrand cross-link is established between G6-N7 of the upper strand and A18-N1 of the lower strand.     

Sequence preferences of DNA interstrand cross-linking agents: dG-to-dG cross-linking at 5'-CG by structurally simplified analogues of mitomycin C

The nucleotide sequence preferences of the DNA interstrand cross-linking agents dehydroretronecine diacetate (DHRA), 2,3-bis(acetoxymethyl)-1-methylpyrrole (BAMP), dehydromonocrotaline, and dehydroretrorsine were studied by using synthetic DNA duplex fragments and polyacrylamide gel electrophoresis (PAGE). These agents have structural features in common with the reductively activated aziridinomitosene of mitomycin C (MC). Like MC, they preferentially cross-linked DNA duplexes containing the duplex sequence 5'-CG. For DHRA and BAMP interstrand cross-linked DNA duplexes, PAGE analysis of iron(II)-EDTA fragmentation reactions revealed the interstrand cross-links to be deoxyguanosine to deoxyguanosine (dG-to-dG), again analogous to DNA cross-links caused by MC. Unlike MC, DHRA could be shown to dG-to-dG cross-link a 5'-GC sequence. Furthermore, the impact of flanking sequence on the efficiency of interstrand cross-linking at 5'-CG was reduced for BAMP, with 5'-TCGA and 5'-GCGC being equally efficiently cross-linked. Possible origins of the 5'-CG sequence recognition common to all of the agents are discussed. A model is presented in which the transition state for the conversion of monoadducts to cross-links more closely resembles ground-state DNA at 5'-CG sequences.     

4-vinyl-substituted pyrimidine nucleosides exhibit the efficient and selective formation of interstrand cross-links with RNA and duplex DNA

The formation of interstrand cross-links in nucleic acids can have a strong impact on biological function of nucleic acids; therefore, many cross-linking agents have been developed for biological applications. Despite numerous studies, there remains a need for cross-linking agents that exhibit both efficiency and selectivity. In this study, a 4-vinyl-substituted analog of thymidine (T-vinyl derivative) was designed as a new cross-linking agent, in which the vinyl group is oriented towards the Watson–Crick face to react with the amino group of an adenine base. The interstrand cross-link formed rapidly and selectively with a uridine on the RNA substrate at the site opposite to the T-vinyl derivative. A detailed analysis of cross-link formation while varying the flanking bases of the RNA substrates indicated that interstrand cross-link formation is preferential for the adenine base on the 5′-side of the opposing uridine. In the absence of a 5′-adenine, a uridine at the opposite position underwent cross-linking. The oligodeoxynucleotides probe incorporating the T-vinyl derivative efficiently formed interstrand cross-links in parallel-type triplex DNA with high selectivity for dA in the homopurine strand. The efficiency and selectivity of the T-vinyl derivative illustrate its potential use as a unique tool in biological and materials research.     

NMR solution structure of a DNA dodecamer containing a transplatin interstrand GN7-CN3 cross-link.

The DNA duplex d(CTCTCG*AGTCTC).d(GAGAC-TC*GAGAG) containing a single trans- diammine-dichloroplatinum(II) interstrand cross-link (where G* and C* represent the platinated bases) has been studied by two-dimensional NMR. All the exchangeable and non-exchangeable proton resonance lines were assigned (except H5'/H5") and the NOE intensities were transformed into distances via the RELAZ program. By combining the NOESY and COSY data (330 constraints) and NMR-constrained molecular mechanics using JUMNA, a solution structure of the cross-linked duplex has been determined. The duplex is distorted over two base pairs on each side of the interstrand cross-link and exhibits a slight bending of its axis ( approximately 20 degrees ) towards the minor groove. The platinated guanine G* adopts a syn conformation. The rotation results in a Hoogsteen-type pairing between the complementary G(6)* and C(19)* residues which is mediated by the platinum moiety and is stabilized by a hydrogen bond between O6(G(6)*) and N4H(C(19)*). The rise between the cross-linked residues and the adjacent residues is increased owing to the interaction between these adjacent residues and the ammine groups of the platinum moiety. These results are discussed in relation to the slow rate of closure of the monofunctional adducts into interstrand cross-links.     

Interaction of psoralen-derivatized oligodeoxyribonucleoside methylphosphonates with single-stranded DNA

Oligodeoxyribonucleoside methylphosphonates derivatized at the 5' end with 4'-(amino-alkyl)-4,5',8-trimethylpsoralen were prepared. The interaction of these psoralen-derivatized methylphosphonate oligomers with synthetic single-stranded DNAs 35 nucleotides in length was studied. Irradiation of a solution containing the 35-mer and its complementary methylphosphonate oligomer at 365 nm gave a cross-linked duplex produced by cycloaddition between the psoralen pyrone ring of the derivatized methylphosphonate oligomer and a thymine base of the DNA. Photoadduct formation could be reversed by irradiation at 254 nm. The rate and extent of cross-linking were dependent upon the length of the aminoalkyl linker between the trimethylpsoralen group and the 5' end of the methylphosphonate oligomer. Methylphosphonate oligomers derivatized with 4'-[[N-(2-aminoethyl)amino]methyl]- 4,5',8-trimethylpsoralen gave between 70% and 85% cross-linked product when irradiated for 20 min at 4 degrees C. Further irradiation did not increase cross-linking, and preirradiation of the psoralen-derivatized methylphosphonate oligomer at 365 nm reduced or prevented cross-linking. These results suggest that the methylphosphonate oligomers undergo both cross-linking and deactivation reactions when irradiated at 365 nm. The extent of cross-linking increased up to 10 microM oligomer concentration and dramatically decreased at temperatures above the estimated Tm of the methylphosphonate oligomer-DNA duplex. The cross-linking reaction was dependent upon the fidelity of base-pairing interactions between the methylphosphonate oligomers and the single-stranded DNA. Noncomplementary oligomers did not cross-link, and the extent of cross-linking of oligomers containing varying numbers of noncomplementary bases was greatly diminished or eliminated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Solution structure of a DNA duplex containing mispair-aligned N4C-ethyl-N4C interstrand cross-linked cytosines

The solution structure of an interstrand cross-linked self-complementary oligodeoxynucleotide containing directly opposed alkylated N(4)C-ethyl-N(4)C cytosine bases was determined by molecular dynamics calculations guided by NMR-derived restraints. The undecamer d(CGAAACTTTCG)(2), where C represents directly opposed alkylated N(4)C-ethyl-N(4)C cytosine bases, serves as model for the cytotoxic cross-links formed by bifunctional alkylating agents used in cancer therapy. The structure of the duplex shows the cross-link protruding into the major groove. An increase in the diameter of the DNA at the pseudoplatform formed by the cross-linked residues creates an A-DNA characteristic hole in the central portion of the DNA. This results in a centrally underwound base step and a number of subsequent overwinding steps leading to an overall axis bend toward the major groove. The structure shows narrowing of both minor and major grooves in the proximity of the cross-link. The perturbation leads to preferential intrastrand base stacking, disruption of adjacent canonical (A.T) base pairing, and buckling of base pairs, the extent of which diminishes with progression away from the lesion site. Overall, the distortion induced by the cross-link spreads over three base pairs on the 5'- and 3'-sides of the cross-link.     

Sequence-specific DNA interstrand cross-linking by an aziridinomitosene in the absence of exogenous reductant

The aziridinomitosene derivative (1S,2S)-6-desmethyl(methylaziridino)mitosene (4) was shown to alkylate plasmid DNA at pH 7.4 in the absence of a reducing agent [Vedejs, E., Naidu, B. N., Klapars, A., Warner, D. L., Li, V. -s., Na, Y., and Kohn, H. (2003) J. Am. Chem. Soc. 125, 15796-15806], an activity not found in the parent mitomycins. We sought to evaluate aziridinomitosene 4 for the presence of DNA interstrand cross-linking activity using nonreductive reaction conditions. Radiolabeled DNA treated with 4 was analyzed by denaturing polyacrylamide gel electrophoresis (DPAGE), a technique that readily separates the less mobile cross-linked ds DNA from the more mobile ss DNA products. Nonreduced 4 produced an interstrand cross-link (ICL) in duplex DNA containing 5'-d(CG) sites, and the yield of ICL was comparable to that obtained from reduced MC under similar conditions. A ds DNA having the central tetranucleotide 5'-d(ACGT) provided the greatest ICL yield from both nonreduced 4 and reduced MC. Substitution of 5'-d(CG) with the inverted sequence 5'-d(GC) completely abolished interstrand cross-linking by 4, revealing 5'-d(CG) as its specific site of ICL formation. Replacement of dG at 5'-d(CG) with 2'-deoxyinosine (dI), which lacks the exocyclic C2 amino group present in dG, also prevented DNA ICL formation by 4, revealing an essential role for the dG C2 amino group in the interstrand cross-linking reaction between 4 and duplex DNA. This report directly demonstrates the presence of bifunctional alkylating activity in a nonreduced aziridinomitosene and clearly shows that unreduced 4 alkylates residues in the minor groove of ds DNA, cross-linking with the same 5'-d(CG) sequence specificity displayed by reduced MC.     

NMR and computational characterization of mitomycin cross-linked to adjacent deoxyguanosines in the minor groove of the d(T-A-C-G-T-A).d(T-A-C-G-T-A) duplex

Two-dimensional homonuclear and heteronuclear NMR and minimized potential energy calculations have been combined to define the structure of the antitumor agent mitomycin C (MC) cross-linked to deoxyguanosines on adjacent base pairs in the d(T1-A2-C3-G4-T5-A6).d(T7-A8-C9-G10-T11-A12) duplex. The majority of the mitomycin and nucleic acid protons in the MC-X 6-mer complex have been assigned from through-bond and through-space two-dimensional proton NMR studies in aqueous solution at 5 and 20 degrees C. The C3.G10 and G4.C9 base pairs are intact at the cross-link site and stack on each other in the complex. The amino protons of G4 and G10 resonate at 9.36 and 8.87 ppm and exhibit slow exchange with solvent H2O. The NMR experimental data establish that the mitomycin is cross-linked to the DNA through the amino groups of G4 and G10 and is positioned in the minor groove. The conformation of the cross-link site is defined by a set of NOEs between the mitomycin H1" and H2" protons and the nucleic acid imino and amino protons of G4 and the H2 proton of A8 and another set of NOEs between the mitomycin geminal H10" protons and the nucleic acid imino and amino protons of G10 and the H2 proton of A2. Several phosphorus resonances of the d(T-A-C-G-T-A) duplex shift dramatically on mitomycin cross-link formation and have been assigned from proton-detected phosphorus-proton two-dimensional correlation experiments. The proton chemical shifts and NOEs establish fraying at the ends of the d(T-A-C-G-T-A) duplex, and this feature is retained on mitomycin cross-link formation. The base-base and base-sugar NOEs exhibit similar patterns for symmetry-related steps on the two nucleic acid strands in the MC-X 6-mer complex, while the proton and phosphorus chemical shifts are dramatically perturbed at the G10-T11 step on cross-link formation. The NMR distance constraints have been included in minimized potential energy computations on the MC-X 6-mer complex. These computations were undertaken with the nonplanar five-membered ring of mitomycin in each of two pucker orientations. The resulting low-energy structures MX1 and MX2 have the mitomycin cross-linked in a widened minor groove with the chromophore ring system in the vicinity of the G10-T11 step on one of the two strands in the duplex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Structure of the DNA interstrand cross-link of 4,5',8-trimethylpsoralen.

4,5',8-Trimethylpsoralen (TMP) cross-links a 5' TpA or a 5' ApT site by photoreacting with one thymine moiety in each DNA strand. We are interested in whether psoralen interstrand cross-links all share one structure or whether there are significant differences. In this paper, we employed a rapid method for probing the structure of the cross-link by making a series of TMP cross-linked duplexes containing specific base-pair mismatches. The relative stability provided by a base pair can be correlated with neighboring base pairs by comparing the extents of gel retardation when base-pair mismatches happen in each position. From our studies, we infer that with respect to the furan-side strand, the 5'T.A base pair of the two T.A base pairs in the TpA site is not hydrogen bonded. Immediately on each side of the cross-linked TpA site is a highly stabilized base pair. Next, a region of decreased stability occurs in each arm of a cross-linked duplex and these base pairs of least stability are located farther away from the cross-linked thymines as the lengths of the arms of the cross-linked helix increase. Finally, even in 7 M urea at 49 degrees C the cross-linked helix is hydrogen bonded at both ends of a duplex of 22 base pairs. We propose that the structures of interstrand cross-links in DNA vary appreciably with the DNA sequence, the length of the DNA duplex, and the structures of the DNA cross-linking agents.     

Delivery of photoreactive psoralen derivatives to specific biological targets

Psoralens are bifunctional molecules which photoreact with the pyrimidine bases of nucleic acids to form monoadducts and diadducts, or interstrand cross-links. We have prepared psoralen derivatives with additional functional groups which can be specifically directed to chosen biological targets. A sulfhydryl-containing psoralen which can form site-specific cross-links in plasmid DNA has been used to study psoralen repair and mutagenesis. Cloned DNA containing psoralen monoadducts has been cross-linked to specific regions of viral RNA and used to probe virus assembly. A biotinylated psoralen derivative which binds specifically to avidin has been used to detect small amounts of DNA. Finally, a psoralen derivative of insulin has been used to deliver psoralen specifically to activated lymphocytes.     

Triplex formation by a psoralen-conjugated oligodeoxyribonucleotide containing the base analog 8-oxo-adenine.

Oligodeoxyribonucleotides containing thymidine and 8-oxo-2'-deoxyadenosine can form pyr.pur.pyr type triplexes with double-stranded DNA. Unlike triplexes whose third strands contain thymidine and deoxycytidine, the stability of these triplexes is independent of pH. We have prepared d-ps-TAAATAAATTTTTAT-L [I(A)], where A is 8-oxo-2'-deoxyadenosine, ps is 4'-hydroxymethyl-4,5',8- trimethylpsoralen and L is a 6-amino-2-(hydroxymethyl)hexyl linker. The oligomer is designed to interact with a homopurine sequence in the promoter region of the human gene coding for the 92 kDa form of collagenase type IV. Oligomer I(A) and oligomer I(C), which contains 2'-deoxycytidine in place of 8-oxo-2'-deoxycytidine, both form stable triplexes at pH 6.2, but only I(A) forms a stable triplex with a model duplex DNA target at pH 7.5, as determined by UV melting experiments. Triplex formation is stabilized by the presence of the psoralen group. Upon irradiation both I(A) and I(C) form photoadducts with the DNA target at pH 6.2, but only I(A) forms a photoadduct at pH 7.5. In these photoreactions oligomer I(A) appears to selectively form a photoadduct with a C in the purine-rich strand of the duplex target. Although a T residue is present in the pyrimidine-rich strand of the target at the duplex/triplex junction, essentially no adduct formation takes place with this strand, nor is interstrand cross-linking observed. The extent of photoadduct formation decreases with increasing temperature, behavior which is consistent with the UV melting curve of the triplex. A tetramethylrhodamine derivative of I(A) was prepared and found to cross-link less extensively than I(A) itself. Oligomer I(A) is completely resistant to hydrolysis when incubated for 24h in the presence of 10% fetal bovine serum at 37 degree C, although it is hydrolyzed by S1 nuclease. The properties of oligomer I(A) suggest that 8-oxo- containing oligomers may find utility as antigene oligonucleotide reagents.