1,N6-Etheno-7-deaza-2,8-diazaadenosine: syntheses, properties and conversion to 7-deaza-2,8-diazaadenosine

1,N6-Etheno-7-deaza-2,8-diazaadenosine (4) was synthesized from 8-aza-7-deazaadenosine (6) in 64% overall yield. The starting material 6 was obtained by the direct glycosylation of 8-aza-7-deazaadenine (7) with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-d-ribofuranose (8) (NO2 CH3, BF3 x Et2O; 77% yield). Compound 4 was transformed into 7-deaza-2,8-diazaadenosine (5). The fluorescence of compound 4 shows an emission maximum at 531 nm (phosphate buffer; pH 7.0), which is bathochromically shifted compared to 1,N(6)-etheno-2-azaadenosine (3a) (495 nm). A conformational analysis was performed in the solid state and in solution.     

Oligonucleotides incorporating 8-aza-7-deazapurines: synthesis and base pairing of nucleosides with nitrogen-8 as a glycosylation position

Oligonucleotides incorporating the unusually linked 8-aza-7-deazapurine N8-(2'-deoxyribonucleosides) 3a,b (purine and 6-amino-2-chloropurine analogues) were used as chemical probes to investigate the base pairing motifs of the universal nucleoside 8-aza-7-deazapurin-6-amine N8-(2'-deoxyribofuranoside) 2 (adenine analogue) and that of the 2,6-diamino compound 1. Owing to the absence of an amino group on the nucleoside 3a the low stability of oligonucleotide duplexes incorporating this compound opposite to the four canonical DNA-constituents indicate hydrogen bonding and base pairing for the universal nucleosides 1 and 2 which form much more stable duplexes. When the 6-amino-2-chloro-8-aza-7-deazapurine nucleoside 3b replaces 1 and is located at the same positions, two sets of duplexes are formed (i) high Tm duplexes with 3b located opposite to dA or dC and (ii) low Tm duplexes with 3b located opposite to dG or dT. These results are due to the steric clash of the 2-chloro substituent of 3b with the 2-oxo group of dT or the 6-oxo group of dG while the 2-halogeno substituents are well accommodated in the base pairs formed with dA or dC. For comparison duplexes incorporating the regularly linked nucleosides 4-6a,b containing the same nucleobases as those of 1-3a,b were studied.     

Halogenated 7-deazapurine nucleosides: stereoselective synthesis and conformation of 2'-deoxy-2'-fluoro-beta-D-arabinonucleosides

The stereoselective syntheses of 5-halogenated 7-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-7H-pyrrolo[2,3-d]pyrimidine nucleosides 3b-d, 4a-c as well as 7-deaza-2'-deoxyisoguanosine are described. Nucleobase anion glycosylation of 2-amino-4-chloro-7H-pyrrolo[2,3-d]pyrimidine (5) with 3,5-di-O-benzoyl-2-deoxy-2-fluoro-alpha-D-arabinofuranosyl bromide (6) exclusively gave the beta-D-anomer, which was deblocked (--> 8), aminated at C4 (--> 3a) and selectively deaminated at C2 to yield 2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl 7-deazaisoguanine (2). Condensation of the 5-halogenated 4-chloro-2-pivaloylamino-7H-pyrrolo[2,3-d]pyrimidines 9a-c with 6 furnished the N7-nucleosides 10a-c together with N2,N7-bisglycosylated compounds 11a-c. The former was converted to the corresponding 2,4-diamino-compounds 3b-d, and the latter was deblocked by NaOMe/MeOH to yield the 4-methoxy-nucleosides 4a-c. Conformational analysis of the sugar moiety of the nucleosides 2 and 3a-d was performed on the basis of vicinal [1H,1H] coupling constants. The fluorine atom in the sugar moiety shifts the sugar conformation from S towards N by about 10%, while the halogen substituents in the base moiety increase the hydrophobicity and polarizability of the nucleobases.     

Pyrazolo[3,4-d]pyrimidine ribonucleosides related to 2-aminoadenosine and isoguanosine: synthesis, deamination and tautomerism

The syntheses and properties of 8-aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) ribonucleosides related to 2-aminoadenosine and isoguanosine are described. Glycosylation of 8-aza-7-deazapurine-2,6-diamine 5 with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (12) in the presence of BF(3) x Et(2)O as a catalyst gave the N(8) isomer 14 (73%) with a trace amount of the N(9) isomer 13a (4.8%). Under the same reaction conditions, the 7-halogenated 8-aza-7-deazapurine-2,6-diamines 6-8 afforded the thermodynamically more stable N(9) nucleosides 13b-d as the only products (53-70%). Thus, a halogen in position 7 shifts the glycosylation from N(8) to N(9). The 8-aza-7-deazapurine-4,6-diamine ribonucleosides 1a-d were converted to the isoguanosine derivatives 3a-d by diazotization of the 2-amino group. Although compounds 1a,b do not contain a nitrogen at position 7 (the enzyme binding site), they were deaminated by adenosine deaminase; however, their deamination occurred with a much slower velocity than that of the related purines. The pK(a) values indicate that the 7-non-functionalized nucleosides 1a (pK(a) 5.8) and 15 (pK(a) 6.4) are possibly protonated in neutral conditions when incorporated into RNA. The nucleosides 3a-d exist predominantly in the keto (lactam) form with K(TAUT) (keto/enol) values of 400-1200 compared to 10(3)-10(4) for pyrrolo[2,3-d]pyrimidine isoguanosine derivatives 4a-c and 10 for isoguanosine itself, which will reduce RNA mispairing with U.     

3-Bromopyrazolo[3,4-d]pyrimidine 2'-deoxy-2'-fluoro-beta-D-arabinonucleosides: modified DNA constituents with an unusually rigid sugar N-conformation

The nucleobase anion glycosylation of 3-bromo-4-isopropoxy-1H-pyrazolo[3,4-d]pyrimidin-6-amine (6) with 3,5-di-O-benzoyl-2-deoxy-2-fluoro-alpha-d-arabinofuranosyl bromide (5) furnished the protected N(1)-beta-d-nucleosides 7 (60%) and 8 (ca. 2%) along with the N(2)-beta-d-regioisomer 9 (9%). Debenzoylation of compounds 7 and 9 yielded the nucleosides 10 (81%) and 11 (76%). Compound 10 was transformed to the 2'-deoxyguanosine derivative 1 [6-amino-3-bromo-1-(2-deoxy-2-fluoro-beta-d-arabinofuranosyl)-1H-pyrazolo[3,4-d]pyrimidin-4-one] (85% yield) and the purine-2,6-diamine analogue 2 [3-bromo-1-(2-deoxy-2-fluoro-beta-d-arabinofuranosyl)-1H-pyrazolo[3,4-d]pyrimidin-4, 6-diamine] (78%). Both nucleosides form more than 98% N-conformer population (P(N) ca. 358 degrees and psi(m) ca. 37 degrees ) in aqueous solution. Single-crystal X-ray analysis of 1 showed that the sugar moiety displays also the N-conformation [P = 347.3 degrees and psi(m) = 34.4 degrees ] in the solid state. The remarkable rigid N-conformation of the pyrazolo[3,4-d]pyrimidine 2'-deoxy-2'-fluoro-beta-d-arabinonucleosides 1 and 2 observed in solution is different from that of the parent purine 2'-deoxy-2'-fluoro-beta-d-arabinonucleosides 3 and 4, which are in equilibrium showing almost equal distribution of the N/S-conformers.     

Oligonucleotide duplexes containing N8-glycosylated 8-aza-7-deazaguanine and self-assembly of 8-aza-7-deazapurines on the nucleoside and the oligomeric level

The 8-aza-7-deazaguanine N8-(2'-deoxy-beta-D-ribofuranoside) (1) was synthesized, converted into the phosphoramidite 4 and incorporated into oligonucleotides. Nucleoside 1 forms stable base pairs with 2'-deoxy-5-methylisocytidine in DNA with antiparallel chain orientation (aps) and with 2'-deoxycytidine in duplexes with parallel chains (ps). According to the CD spectra self-complementary oligonucleotides d(1-m5isoC)3 and d(1-C), form autonomous DNA-structures. Neither the nucleoside 1 nor the regularly linked 8-aza-7-deaza-2'-deoxyguanosine form G-like tetrads while the regularly linked 8-aza-7-deaza-2'-deoxyisoguanosine gives higher molecular assemblies which are destroyed by bulky 7-bromo substituents. This was verified on monomeric nucleosides by ESI-MS spectrometry and on oligonucleotides by HPLC analysis.     

The C(8)-(2'-deoxy-beta-D-ribofuranoside) of 7-deazaguanine: synthesis and base pairing of oligonucleotides with unusually linked nucleobases

The 7-deazaguanine (2-aminopyrrolo[2,3-d]pyrimidin-4-one) C(8)-(2'-deoxy-beta-D-ribofuranoside) (6b), which possesses an unusual glycosylation site, was synthesized and incorporated in oligonucleotides. The oligonucleotides were prepared by solid-phase synthesis using phosphoramidite chemistry and were hybridized to form duplex DNA. Compound 6b is able to form base pairs with 2'-deoxy-5-methylisocytidine (m(5)isoC(d)) in oligonucleotide duplexes with antiparallel chain orientation and with dC in parallel duplex DNA. Thus, the C(8)-nucleoside 6b shows a similar base recognition as 2'-deoxyisoguanosine but not as 2'-deoxyguanosine. This indicates that the nucleic acid recognition not only depends on the donor-acceptor pattern of the nucleobase but is influenced by the glycosylation site. Base pairs of compound 6b formed with canonical and modified nucleosides are proposed.     

8-Aza-2′-deoxyguanosine and Related 1,2,3-Triazolo[4,5-d]pyrimidine 2′-Deoxyribofuranosides

The synthesis of 8-azaguanine N⁹-, N⁸-, and N⁷-(2′-deoxyribonucleosides) 1–3 , related to 2′-deoxyguanosine ( 4 ), is described. Glycosylation of the anion of 5-amino-7-methoxy-3H-1,2,3-triazolo[4,5-d]pyrimidine ( 5 ) with 2-deoxy-3,5-di-O-(4-toluoyl)-α-D -erythro-pentofuranosyl chloride ( 6 ) afforded the regioisomeric glycosylation products 7a/7b, 8a/8b , and 9 (Scheme 1) which were detoluoylated to give 10a, 10b, 11a, 11b , and 12a . The anomeric configuration as well as the position of glycosylation were determined by combination of UV, ¹³C-NMR, and ¹H-NMR NOE-difference spectroscopy. The 2-amino-8-aza-2′-deoxyadenosine ( 13 ), obtained from 7a , was deaminated by adenosine deaminase to yield 8-aza-2′-deoxyguanosine ( 1 ), whereas the N⁷- and N⁸-regioisomers were no substrates of the enzyme. The N-glycosylic bond of compound 1 (0.1 N HCl) is ca. 10 times more stable than that of 2′-deoxyguanosine ( 4 ).     

Oligonucleotides Containing Pyrazolo[3,4-d]pyrimidines: The Influence of 7-Substituted 8-Aza-7-deaza-2′-deoxyguanosines on the Duplex Structure and Stability

Oligonucleotides containing 7-substituted 8-aza-7-deazaguanines (=6-amino-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-ones) were prepared by automated solid-phase synthesis. A series of 7-alkynylated 8-aza-7-deaza-2′-deoxyguanosines (see 4a – d ) were synthesized with the 7-iodonucleoside 3c as starting material and by the Pd⁰/Cu^I-catalyzed cross-coupling reaction with various alkynes. Phosphoramidites were prepared from the 7-substituted 8-aza-7-deaza-2′-deoxyguanosine derivatives carrying halogeno, cyano, and hexynyl substituents. From the melting profiles of oligonucleotide duplexes, the T_m values as well as the thermodynamic data were determined. A significant duplex stabilization by the 7-substituents was observed for the DNA⋅DNA duplexes, but not in the case of DNA⋅RNA hybrids.     

Cyclocondensation reactions of heterocyclic carbonyl compounds.. Synthesis of 2,6-bis(6-azauracil-5-yl)aniline and its use for synthesis of some other polynuclear 1,2,4-triazines

5-(2-Aminophenyl)-6-azauracil 1 was converted to 7-(6-azauracil-5-yl)isatin 3 , semicarbazone 4 of which was recyclized to 2,6-bis(6-azauracil-5-yl)aniline 5 . This one served as a starting compound for preparation of other noncondensed two nuclear heterocycles 7, 9, 10 and condensed 1,2,4-triazines 11 and 12 as well.     

Oligonucleotides forming an i-motif: the pH-dependent assembly of individual strands and branched structures containing 2'-deoxy-5-propynylcytidine

Non-branched and branched oligonucleotides incorporating consecutive runs of 2'-deoxy-5-propynylcytidine residues () instead of 2'-deoxycytidine () were synthesized. For this, phosphoramidite building blocks of 2'-deoxy-5-propynylcytidine () were prepared using acetyl, benzoyl or N,N-di-n-butylaminomethylidene protecting groups. The formation of the i-motif assemblies incorporating 2'-deoxy-5-propynylcytidine residues was confirmed by temperature-dependent CD- and UV-spectra as well as by ion-exchange chromatography. The low pK(a)-value of nucleoside (pK(a) = 3.3) compared to dC (pK(a) = 4.5) required strong acidic conditions for i-motif formation. Branched oligonucleotide residues with strands in a parallel orientation lead to a strong stabilization of the i-motif allowing aggregation even at non-optimal pH conditions (pH = 5). The immobilization of oligonucleotides incorporating multiple residues of on 15 nm gold nanoparticles generated DNA-gold nanoparticle conjugates which are able to aggregate into i-motif structures at pH 5.     

Synthesis of Novel Nucleic Acid Mimics via the Stereoselective Intermolecular Radical Coupling of 3'-Iodo Nucleosides and Formaldoximes(1)

A highly convergent free radical coupling of alkyl iodides and oximes, mediated by bis(trimethylstannyl) benzopinacolate (8), has been utilized to prepare a series of dimeric nucleosides as mimics of natural nucleic acids. The systematic optimization of the reaction conditions allowed for the single-step conversion of the appropriate iodides and oximes into the 2'-deoxy dimers 9 in moderate to excellent yields. For example, the reaction of 3'-deoxy-3'-iodo-5'-(triphenylmethyl)thymidine (6a) with 3'-O-(tert-butyldiphenylsilyl)-5'-O-(methyleneimino)thymidine (7a) in the presence of 8 in degassed benzene gave an 81% yield of 3'-de(oxyphosphinico)-3'-(methyleneimino)-5'-O-(triphenylmethyl)thymidylyl-(3'-->5')-3'-O-(tert-butyldiphenylsilyl)thymidine (9a). Similarly prepared were dimers containing both pyrimidine (thymine, 5-methylcytosine) and purine (adenine, guanine) bases. The reaction was highly stereoselective, giving only a single dimeric species having the ribo-configuration of the newly introduced C-3'-branched methylene moiety. Also prepared were dimers 16, incorporating 2'-O-methyl ribonucleosides in both halves of the dimer. This required the synthesis of 3'-deoxy-3'-iodo-2'-O-methyl nucleosides 12 as well as 2'-O-methyl-5'-O-methyleneimino nucleosides 15. For example, 5'-O-(tert-butyldiphenylsilyl)-3'-deoxy-3'-iodo-2'-O-methyl-5-methyluridine (12e) was prepared in 80% yield by displacement of the corresponding triflate with Bu(4)NI. Also prepared were the suitably protected 3'-deoxy-3'-iodo adenosine and guanosine derivatives. Compounds 15 were prepared in high yield by a regioselective Mitsunobu reaction to give the corresponding 5'-O-phthalimido nucleosides 13, which were subsequently converted to the requisite oximes 15. In the 2'-O-methyl series, the pinacolate coupling reaction proceeded with efficiency equal to that observed for the 2'-deoxy series 9, but with slightly less stereoselectivity, giving predominantly the C-3'ribo products 16, contaminated with 5-25% of the epimeric material. Mixed base dimers containing both pyrimidine and purine bases at all possible positions, including purine-purine dimers were prepared. The hydroxylamine or methyleneimino (MI) backbone of several representative dimers so prepared was converted via methylation to give the corresponding methylenemethylimino (MMI)-linked compounds, which are novel phosphate surrogates for use in antisense oligonucleotides.     

3-deoxy-3,3-difluoro-D-arabinofuranose: first stereoselective synthesis and application in preparation of gem-difluorinated sugar nucleosides

The design and synthesis of gem-difluorinated sugar nucleosides were described. The key intermediate, 3-deoxy-3,3-difluoro-d-arabinofuranose 9, was first stereoselectively prepared from the chiral gem-difluorohomoallyl alcohol 12. The kinetic formation of single anti-14 in the benzylation of 12 could be accomplished by controlling the amount of sodium hydride used. The dihydroxylation of 14 (a mixture of anti and syn isomers) followed by deprotection and oxidation stereoselectively afforded furanose 9 with the arabino configuration at the C2 position. N(1)-(3-Deoxy-3,3-difluoro-beta-D-arabinofuranosyl)cytosine 6 was prepared from 9 by the glycosylation reaction. 4'-Thiofuranose 25 was easily synthesized from 9. The oxidation of 25 followed by the condensation with silylated N(4)-benzoylcytosine (Pummerer reaction) failed to give our desired protected nucleoside l-3'-deoxy-3',3'-difluoro- 4'-thiocytidine 27', but the regioisomer 27 was obtained. The regiochemistry of the Pummerer reaction was determined by the kinetic acidity of the alpha-proton of 4'-thiofuranose 25.     

Stereocontrolled syntheses of deoxyribonucleosides via photoinduced electron-transfer deoxygenation of benzoyl-protected ribo- and arabinonucleosides

The stereocontrolled, de novo syntheses of beta-2'-deoxy-, alpha-2'-deoxy-, beta-3'-deoxy-, and beta-2', 3'-dideoxyribonucleosides are described. Strategically protected ribose, arabinose, and xylose glycosylation precursors were synthesized bearing C2-esters capable of directing Vorbrüggen glycosylation. The key step is the regioselective deoxygenation of the desired hydroxyl group as either the benzoyl- or 3-(trifluoromethyl)benzoyl derivative. This deoxygenation is accomplished via a photoinduced electron-transfer (PET) mechanism using carbazole derivatives as the photosensitizer. The syntheses of the desired deoxynucleoside generally proceed in three steps from a common, readily available precursor.     

Synthesis of new 3'-, 5-, and N(4)-modified 2'-O-methylcytidine libraries on solid support

A versatile solid phase combinatorial approach was developed and utilized for the rapid synthesis of new 2'-O-methylcytidine nucleoside libraries 1-7 containing 672 compounds with 3'-deoxy-3'-C-methyl, 3'-deoxy-3'-C-hydroxymethyl, and 5-alkyl/alkynyl modifications. The modified uridine scaffolds 8-10, 23-25, and 31 were loaded onto the 4-methoxytrityl chloride (MMT-Cl) polystyrene resin through the hydroxyl groups at the 5'-position as well as on the substituents at the 3'- and 5-positions. The scaffolds loaded on the resin were orthogonally protected by MMT group on the resin itself and TBDMS or acetyl protecting groups. The 4-position of the uridine derivatives was activated by 2,4,6-triisopropyl benzene sulfonyl chloride for further derivatization. The resins 14-16, 28-30, and 32 loaded with the corresponding activated scaffolds were reacted with the selected and validated amino building blocks in the 96 well format on the semiautomated synthesizer. The high-quality 2'-O-methylcytidine libraries 1-7 were thus generated and characterized by liquid chromatography-mass spectrometry (LC-MS) analysis with 63-99% successful rates.     

Synthesis of Conformationally Locked 2＇-Deoxy-2＇-nucleobase-5＇- Isonucleosides: 2＇-Deoxy-2＇-nucleobase-5＇ deoxy-1＇, 4＇ :3＇, 6＇-dianhydro-D-mannitol

Novel bicyclic isonucleosides, 2＇ -deoxy- 2＇ -nucleobase- 5＇ -deoxy- 1＇ , 4＇ : 3＇ , 6＇ -dianhydro D-mannitol 10a-10c, were synthesized from D-glucose. The computer-assisted molecular simu lation indicated that the sugar conformations of compounds 10a-10c were restricted to N-con formation.     

Synthesis of nucleoside analogues bearing the five naturally occurring nucleic acid bases built on a 2-oxabicylo[3.1.0]hexane scaffold

Hitherto unknown nucleoside analogues incorporating the five naturally occurring nucleic acid bases built on a 2-oxabicyclo[3.1.0]hexane template were synthesized. The synthesis of these new conformationally restricted nucleoside analogues involved the preparation of a suitable sugar precursor bearing the 2-oxabicyclo[3.1.0]hexane scaffold. This sugar was readily obtained from [(3aS,6aS)-2,2-dimethyl-3a,6a-dihydrofuro[2,3-d][1,3]dioxol-5-yl]methyl benzyl ether (4) following a Simons-Smith-type cyclopropanation reaction. Finally, glycosylation reactions and deprotection provided the nucleoside analogues. Using nucleoside 14 bearing thymine base as a model, we found that the conformation of such nucleoside analogue was restricted toward a (0)T(1) conformation.     

2'-Deoxyimmunosine: stereoselective synthesis, base pairing and duplex stability of oligonucleotides containing 8-oxo-7-thiaguanine

Oligonucleotides containing 7-thia-8-oxoguanine represent a new class of molecules in which sulfur replaces the 7-nitrogen of a purine base. The monomeric 7-thia-8-oxoguanine 2'-deoxyribonucleoside (2'-deoxyimmunosine, 4) was prepared by nucleobase anion glycosylation in a regio- and stereoselective way employing 5-{[(di-n-butylamino)methylidene]amino}thiazolo[4,5-d]pyrimidine-2,7(3H,6H)-dione (18) and 1-chloro-2-deoxy-3,5-di-O-p-toluoyl-alpha-d-erythro-pentofuranose (6). The nucleoside was converted into the phosphoramidite and oligonucleotides were prepared by solid-phase synthesis. Oligonucleotide duplexes containing the 4-dC base pair show a similar stability as those containing the dG-dC motif. Thus the sterically demanding sulfur and the additional 8-oxo group are well accommodated in the major groove of DNA. As expected, compound 4 does not form a Hoogsteen pair, as reported for 8-oxo-2'-deoxyguanosine. Compared to 2'-deoxyguanosine, 2'-deoxyimmunosine shows a better mismatch discrimination in Watson-Crick base pairs.     

Stereoselective synthesis of 2-C-branched (acetylmethyl) oligosaccharides and glycoconjugates: Lewis acid-catalyzed glycosylation from 1,2-cyclopropaneacetylated sugars

1,2-Cyclopropaneacetylated sugars as glycosyl donors reacted with a series of glycosyl acceptors (monosaccharides, amino acids, and other alcohols) in the presence of Lewis acid to produce oligosaccharides and glycoconjugates containing 2-C-acetylmethylsugars. Galactosyl donor gave good to excellent α-selectivities with TMSOTf as a catalyst, whereas galactosyl donor offered moderate to good β-selectivities when BF(3)·Et(2)O was used as a catalyst. However, glucosyl donors produced β-exclusive selectivity under both conditions. The stereoselectivities of glycosylation depend on the reactivity of donor sugars and Lewis acid catalyst, which effectively dictated the glycosylation pathways. The evidence suggests that galactosyl donors (e.g., 7) can undergo S(N)1 pathway with a strong Lewis acid (TMSOTf) and S(N)2 pathway under BF(3)·Et(2)O, whereas the glucosyl donors (e.g., 8 and 10) followed S(N)2 pathway. The stereoselectivity was also consequential to the formation of a C2'-acetal intermediate formed via the 2-C-acetylmethyl group and the anomeric carbonium intermediate in glycosylation.     

2-aza-2'-deoxyadenosine: synthesis, base-pairing selectivity, and stacking properties of oligonucleotides

2-Aza-2'-deoxyadenosine (2, z2Ad) is synthesized via its 1,N6-etheno derivative 7 and enzymatically deaminated to 2-aza-2'-deoxyinosine (3). Compound 2 is converted into the phosphoramidite building block 10b. This is employed in solid-phase oligonucleotide synthesis. The 2-azapurine base forms a strong base pair with guanine, but a much weaker one with adenine, thymine, and cytosine. Oligonucleotide duplexes with dangling nucleotide residues, such as 2-aza-2'-deoxyadenosine and 7-deaza-2'-deoxyadenosine (4, c7Ad), either on one or both termini, are synthesized, and the thermal stability of the duplexes is correlated with the hydrophobic properties of the dangling nucleotide residues.