Exocyclic Oxygen Atom Research Articles

Outer-sphere coordination, N-coordination and O-coordination of the deprotonated saccharin in copper(II) saccharinato complexes. Implications for the saccharinato carbonyl stretching frequency

Tetraaquabis( o-sulfobenzimidato- N)copper(II) reacts with neutral N-heterocycles to form complexes in which the o-sulfobenzimide (saccharin) entity interacts directly with the copper atom (through either the endocyclic nitrogen atom or the exocyclic oxygen atom) or indirectly, through coordinated water molecules. With 4-aminopyridine, it yields diaquatetrakis(4-aminopyridine)copper(II)di( o-sulfobenzimidate)dihydrate, whose metal atom shows trans-N 4O 2 octahedral coordination. The o-sulfobenzimidate anions interact with the copper atom through the coordinated water molecules, and they link with the lattice water molecules to furnish three-dimensional network architecture. The reagent when treated with pyrazole affords tetrakis(pyrazole)bis[1,2-benzisothiazolyl-3-olato 1,1-dioxide]copper(II); in this neutral compound, the metal atom and the o-sulfobenzimidate moieties are linked by covalent copper–oxygen bonds. The aqua complex with di-2-pyridylamine has the copper atom in a square-pyramidal configuration: one of the o-sulfobenzimidate ligands binds through its nitrogen atom whereas the other binds through the exocyclic oxygen atom in aqua(di-2-pyridylamine)[1,2-benzisothiazolyl-3-olato 1,1-dioxide]( o-sulfobenzimidato- N)copper(II), which adopts a linear hydrogen-bonded chain motif. When treated with nicotinamide, tetraaquabis( o-sulfobenzimidato- N)copper(II) affords a monohydrated di(nicotinic acid) adduct, the amide group being oxidized to a carboxylic group. In this square-pyramidal complex, the molecules are linked by hydrogen bonds involving the two carboxylic acid ends into a linear chain that propagates along the a– c diagonal of the unit cell. The coordination mode of the o-sulfobenzimidate entities in the complexes is reflected in the stretching frequencies of the carbonyl groups, the respective band(s) being blue-shifted for N-coordination and red-shifted for O-coordination relatively to the o-sulfobenzimidate ions, leading to the frequency order N-coordinated>uncoordinated> O-coordinated. These shifts should be considered in vibrational frequency versus bond order correlations.

Silver complexes with triazolopyrimidine ligands containing an exocyclic oxygen atom: X-ray evidence for an unusual tautomeric form

The crystal structures of the silver complexes [Ag2(5HtpO)2][NO3]21, [Ag2(5HtpO)2][ClO4]2·2H2O 2 and {[Ag(7HtpO)]ClO4}n3, where the ligands are the triazolopyrimidine derivatives 4,5-dihydro-5-oxo[1,2,4]triazolo[1,5-a]pyrimidine (5HtpO) and 4,7-dihydro-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine (7HtpO), are described. 1 and 2 are binuclear compounds, with two ligands bridging two silver atoms through their nitrogen atoms at positions 3 and 4. The ligand in these compounds is in an unusual tautomeric form with its acidic proton attached to the exocyclic oxygen atom, which influences its geometric and spectroscopic (infrared) properties. On the other hand, the ligand 7HtpO in compound 3 is in its usual tautomeric form and binds two different silver atoms through its nitrogen atoms in positions 1 and 3, building infinite –Ag–7HtpO–Ag–7HtpO– chains.

Molecular structure and activity toward DNA of baicalein, a flavone constituent of the Asian herbal medicine "Sho-saiko-to".

Baicalein (5,6,7-trihydroxyflavone, 1) is of interest because of its broad spectrum of biological activity. It is a constituent of the east Asian herbal remedy, "Sho-saiko-to". The 3D structure of 1 was determined using X-ray diffraction. The compound exists in an almost planar conformation with a C-2-C-1' bond distance of 1.476(5) A. Hydrogen-bonding interactions predominate in the crystal structure. The position of the three hydroxyl groups maximizes intramolecular hydrogen bonding, and each of the hydroxyl hydrogen atoms is a donor in a three-center hydrogen bond. The carbonyl oxygen, O-4, is an acceptor in an intramolecular hydrogen bond (with OH-5). Two molecules of 1 exist as hydrogen-bonded dimers related by inversion center (-x + 1, -y, -z + 1). O-4 is also an acceptor in an intermolecular hydrogen bond with OH-6. The planarity of the flavone framework is dependent on structural and/or electronic forces that stabilize the negative charge on the exocyclic oxygen atom, O-4. Compound 1, therefore, is planar in any situation where forces can stabilize the negative charge on O-4. Consistent with this, UV absorbance studies performed on 1-DNA complexes with varying concentrations of 1 strongly suggest intercalation of 1 within the double helix, followed by possible interstrand cross-links.

Solvent effects on the nitrogen NMR shieldings of 3-methylsydnone andab initio calculations of the shieldings including related oxazole and oxadiazole systems

Solvent effects on the nitrogen shieldings of 3-methylsydnone (1), a zwitterionic isomer of the hypothetical 5-methoxy-1,2,3-oxadiazole, are found to be significantly different from those observed for oxazoles and oxadiazoles. A detailed analysis of the solvent-induced variations reveals contributions from three interactions, those due to solvent polarity, hydrogen bonding from solvent to solute, involving the lone pairs of the exocyclic oxygen atom in 1, and those involving basic centers in the solvent and the positively charged heteroaromatic ring in the vicinity of N-3 which bears a formal positive net charge. Ab initio DFT-GIAO magnetic shielding calculations employing B3PW91 functionals, a 6–311++G** basis set and geometries optimized using the same set are reported for 1 and some related molecules which include all of the existing oxazoles and oxadiazoles. The experimental range of nitrogen shielding considered is about 177 ppm and shows an excellent linear correlation with the calculated results. The least-squares standard deviation is only 1.4% of the observed shielding range. The effects on the nitrogen shielding of 1 caused by hydrogen bonding and the calculations indicate that the exocyclic oxygen atom in 1 is the primary acceptor of hydrogen bonds from solvent molecules. The calculations fully corroborate recent assignment for the 17O NMR shieldings of 1. Copyright © 2000 John Wiley & Sons, Ltd.

Exocyclic oxygen atoms of platinated nucleobases as binding sites for alkali metal ions †

Three complexes of model nucleobases with exocyclic oxygen atoms (1-methyluracilate, mura; 1-methylcytosine, mcyt; 9-methylguanine, Hmgua) which contain PtII bonded to a ring N atom and an alkali metal ion (Cs+, K+, Na+) bonded to a keto oxygen of the bases, trans-Cs[Pt(NH3)(mura)I2]·4H2O 1, trans-K[Pt(NH3)2(mcyt)2][PF6]3·H2O 2, and trans-[Pt(NH3)(Hmgua)2(mcyt)Na(H2O)2][ClO4]3·0.5H2O 3, have been prepared and their crystal structures determined. The compounds have been studied, among others, with regard to the role of alkali metal ions for the rotation of nucleobases when bound to PtII. While in the case of 1 the alkali metal ion is necessary for charge compensation and for this reason its binding to the platinated mura is not fully unexpected, it is surprising to see that alkali metal ions even bind to cationic complexes of PtII containing neutral nucleobases (2, 3).

(Ethylenediamine)Pt(uracilate)]4 – A Metal Analogue of Calix[4]arene: Coordination Chemistry of Its 1,3-Alternate Conformer towards First-Row Transition-Metal Ions

The open “molecular box” [(en)Pt(U-N,N)]4 (1) (where en = structurally characterized heteronuclear compounds show a 1,3-alternate arrangement of the uracil nucleobases. Pt–M ethylenediamine, U = uracil dianion) is able to act as an efficient ligand for additional metal ions. Reactions of the 1,3contacts are in the range 2.743(2)–2.804(2) A; the Pt atoms act as weak σ donors to the heterometals. In spite of the high alternate conformer of 1 with divalent metal ions result in the formation of octanuclear compounds of the general formula degree of positive charge associated with compounds 2a–5, they do not behave as hosts for anions. This is a consequence [(en)Pt(U-N,N,O,O)M]4X8 [M = Cu2+, X = Cl (2a), X = (SO4)0.5 (2b), and M = Co2+ (3), Ni2+ (4), Zn2+ (5) with X = of the orientation of the four nucleobases which, in contrast to a cone arrangement, does not produce a sufficiently large (SO4)0.5], in which the heterometals are bound at the exocyclic oxygen atoms of the uracil nucleobases. The cavity for the incorporation of anions.

In-Situ Infrared Spectroscopic and Scanning Tunneling Microscopy Investigations of the Chemisorption Phases of Uracil, Thymine, and 3-Methyl Uracil on Au(111) Electrodes

The complementary techniques of in-situ infrared spectroscopy and scanning tunneling microscopy (STM) have been used in this study to build detailed structural models for the chemisorbed forms of uracil, thymine, and 3-methyl uracil on Au(111) electrodes. The infrared spectra, in water and D2O electrolytes, show that both uracil and thymine adopt similar coordination forms with the surface with both exocyclic oxygen atoms and a deprotonated N3 facing in toward the surface in a vertically oriented chemisorbate. 3-Methyl uracil cannot exhibit such a surface coordination and its IR signature in the carbonyl stretching region is quite different. This is interpreted as the chemisorbate interacting through its deprotonated N1 and C2O. STM has been used to characterize and compare the molecular ordering of the three respective adsorbates. Uracil exhibits the highest coverage structure c(3 × ), while thymine exhibits smaller ordered domains which are expanded in one direction to allow for the spatial requirements of the methyl group on thymine. The domain size for the thymine chemisorbate could be improved by temperature annealing the electrode in-situ and a “pseudo c( × 4)” structure was observed. Both the uracil and thymine chemisorbate structures feature chains of molecules, stacked like “rolls of coins”, close enough for π-stacking to occur. The structure of thymine overlayers differs from uracil, since there are a number of different possible orientations of adjacent molecular rows, which results in a high frequency of stacking faults. These differences are discussed. 3-Methyl uracil is quite different, exhibiting a rather low coverage, albeit a highly ordered structure (5 × 2 ) which cannot allow π-stacking. On the basis of these observations, the factors governing the formation of the respective chemisorbed phases are discussed.

Molecular structure of the novel 3-coordinated Hg(II) complex, aqua-bis(3,3-Dimethylglutarimidato)mercury (II)

The novel complex of biologically active 3,3-dimethylglutarimide, aqua-bis(3,3-dimethylglutarimidato)mercury(II) has been obtained and its structure determined by single crystal X-ray diffraction and vibrational spectroscopic studies. The crystals of C14H20N2O4Hg·H2O are monoclinic, space group P21/c, a=6.085(2), b=19.686(4), c=14.433(3)Å, β=93.32(3)° and Z=4. The structure was solved by heavy atom method and refined to R=0.035, Rw=0.093 for 2075 unique reflections. The mercury atom is almost linearly bonded to two deprotonated nitrogen atoms of the glutarimidate rings [Hg–N=2.035(8) and 2.028(8)Å, N–Hg–N′=169.2(3)°] and is also coordinated to one water molecule in the axial position [Hg–O=2.643(8)Å, N–Hg–O=98.5(3)°]. The water molecule forms strong hydrogen bonds to the carbonyl groups of the neighbouring molecules in the unit cell.The infrared and Raman spectra have provided further evidence for bonding in this complex. It follows that binding of the mercury(II) ion to the deprotonated nitrogen atom of the cyclic 6-membered imides leads to a smaller accommodation of the negative charge on the exocyclic oxygen atoms than analogous binding of Pd(II) with these imides.

Crystal structures of a protonated form of trans-[Pt(NH3)2(mura)2] and of a derivative containing three different metal ions, Pt2+, Ag+, and Na+ (mura = 1-methyluracilate). Major difference in packing between heteronuclear pyrimidine nucleobase complexes of cis- and trans-(NH3)2PtII

The complex trans-[Pt(NH3)2(mura)2] 1 (mura = 1-methyluracilate), a compound of very low water solubility, is markedly solubilised in the presence of acid or suitable metal salts due to protonation and metal binding to the exocyclic oxygen atoms, respectively. The perchlorate salt trans-[Pt(NH3)2(Hmura)2][ClO4]2·2H2O 2 has been characterised by X-ray analysis. With Ag+, 1 formed heteronuclear species of varying stoichiometries, e.g. Pt2Ag3 3, the composition of which can be further varied by the presence of alkali metal salts. The complex trans-[{Pt(NH3)2(mura)2}2AgNa(H2O)4][ClO4]2·6.5H2O 4 appears to be the first structurally characterised example of a nucleobase complex containing three different metal ions. Tetranuclear cations of 4 are arranged in the crystal in such a way as to permit both intermolecular hydrogen bonding between NH3 ligands and O2 sites of mura nucleobases and π stacking between adjacent trans-[Pt(NH3)2(mura)2] entities. This feature is radically different from that observed in related di- and tri-nuclear complexes derived from cis-(am)2PtII. With mercury(II) salts initially binding to exocyclic oxygen atoms of the mura ligand takes place, followed by metal binding to the C5 atoms of both uracil ligands of 1.

Exo-anomeric effects on energies and geometries of different conformations of glucose and related systems in the gas phase and aqueous solution

Ab initio calculations predict that in 2-hydroxy- and 2-methoxytetrahydropyran, hyperconjugative delocalization of lone-pair density on the exocyclic oxygen atom at C-1 into the σ ∗ orbital of the C-1-O-5 bond is maximized when the OR group at C-1 is oriented gauche to C-1-O-5. This exo-anomeric effect lengthens the C-1-O-5 bond, shortens the exocyclic C-1-O bond, and stabilizes the gauche conformers by about 4 kcal/mol over the anti. In the anti orientation, hyperconjugative interaction of the OR group at C-1 with other appropriately oriented σ ∗ orbitals increases, but the geometric and energetic consequences are less marked. Solvation effects reduce the energetic stabilization associated with the exo-anomeric effect in the tetrahydropyrans. This derives from a combination of changes in the overall electrostatics and also from decreased accessibility of the hydrophilic groups in the gauche conformers. For glucose or glucosides, instead of the simple tetrahydropyran model systems, the interactions of the exocyclic OR group at C-1 with the hydroxy group at C-2 can significantly affect these hyperconjugative delocalizations. In the glucose and glucoside systems, solvation effects oppose the formation of intramolecular hydrogen bonds. MM3(94) force field calculations show systematic deviations in the relative energies and structures of the various model systems with respect to the more reliable HF/cc-pVDZ predictions.

REACTIONS OF THE RHODIUM TRIFLUOROACETATE DIMER WITH NUCLEOSIDES AND NUCLEOTIDES

Abstract Complexes of rhodium trifluoroacetate with adenosine (ado), adenosine-5′-momophosphate (AMP), cytidine (cyd), guanosine (guo) and inosine (ino) have been prepared from aqueous or methanolic solutions. They were characterized by elemental analysis, magnetic susceptibility measurements, mass spectra, infrared, visible and nuclear magnetic resonance spectroscopies. In some cases their formation was followed by NMR prior to isolation. They all proved to be diamagnetic and in every compound contain two ligand molecules per rhodium dimer except for the ado complex where the ratio is 1:1. In the solid state it was found that adenosine interacts with the metal via the N(1) and N(7) atoms, its nucleotide through N(7), whereas in the case of cyd, guo and ino their exocyclic oxygen atoms take part in coordination. Solution study seem highly interesting since in some cases more than one species appear.

A heterodinuclear anti oximate-bridged copper(II)-nickel(II) complex: crystal structure and magnetic properties

The crystal structure of [Cu(ClO 4)(μ-L)Ni(CTH)](ClO 4)·H 2O (where H 2L is α,ω-bis(1,3-dimethyl-5-nitrosouracil-6-yl-amine)propane and CTH is d, l-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) has been determined by X-ray methods. The complex crystallizes in the monoclinic space group C2/ c with a=41.848(16), b=12.738(3), c=20.005(7) A ̊ , β=114.92(2)° . The structure consists of discrete dimetallic cations [Cu(ClO 4)(μ-L)Ni(CTH)] +, one perchlorate anion and one non-bonded disordered molecule of water. Within the dinuclear cation, copper(II) and nickel(II) are bibridged by an exocyclic oxygen atom of the uracil ring and an anti nitroso-oximato group, resulting in a Cu···Ni distance of 4.972(2) Å. The copper(II) ion occupying the inside site of the double deprotonated ligand L 2− exhibits a N 4O distorted square-pyramidal coordination environment, whereas the nickel(II) ion is in a distorted octahedral geometry. Magnetic susceptibility measurements in the temperature range 300-2 K revealed the occurrence of a strong antiferromagnetic exchange interaction between copper(II) and nickel(II) ions with a J value of −136.4 cm −1. Magnetic and structural parameters of this and other related complexes are compared in order to know how the type of NO bridging conformation affects the magnetic properties.

Very strong hydrogen bonds in hydrated 1:1 adducts of an olefinic double betaine with perchloric and nitric acids: L·HClO4·H2O and L·HNO3·H2O [L=cis-(p-Me2N+C5H4N)2C2(COO−)2

The hydrated 1:1 adducts of an olefinic double betaine with perchloric and nitric acids, L·HClO4·H2O I and L·HNO3·N2O2 [L=cis-(p-Me2N+C5H4N)2C2(COO−)2], have been shown to be nearly isostructural by X-ray crystallography and infrared spectroscopy. The common cationic unit HL+ of both compounds exhibits significant semi-quinonoid character and possesses symmetrym, ist two coplanar carboxylate groups being linked by a very strong, nearly linear intramolecular O−H−O hydrogen bond [O...O 2.397(9)A and O−H−O 167.1° for1; 2.388(7)A and 167.4° for2]. The HL+ units are further connected by intermolecular O−H...O hydrogen bonds between the exocyclic carboxyl oxygen atoms and the water molecules to generate polymeric zigzag chains running parallel to thec axis, and the disordered ClO4 or NO3− anions occupy channels running parallel to thea axis in the resulting host lattice.

A convenient strategy for replacement of the anomeric hydroxyl group by difluoromethyl functionality in carbohydrate derivatives

A series of carbohydrate gem-difluoroenol ethers are readily prepared by the reaction of the corresponding lactones with dibromodifluoromethane, tris(dimethylamino)phosphine and zinc. Subsequent catalytic hydrogenation provides difluoromethyl C-glycoside analogues in which the exocyclic oxygen atom has been replaced by a difluoromethylene unit.

Cell-permeable non-hydrolyzable cAMP derivatives as tools for analysis of signaling pathways controlling gene regulation in Dictyostelium.

A novel class of cAMP derivatives were tested for binding to surface cAMP receptors (CAR), protein kinase A (PKA), and cAMP-phosphodiesterase (PDE) and for induction of three classes of cAMP regulated genes in Dictyostelium discoideum. These derivatives carry sulfur substitutions for either the axial (Sp) or equatorial (Rp) exocyclic oxygen atoms, while further modifications were introduced to provide specificity for binding to either CAR or PKA, and/or to increase lipophilicity and render the derivatives membrane-permeable. All derivatives bind weakly to PDE and are almost not degraded during incubation with Dictyostelium cells. One cAMP derivative, 6-thioethyl-purineriboside 3',5'-monophosphorothioate, Sp-isomer (Sp-6SEtcPuMPS), fulfills the criteria for selective activation of PKA in vivo. The compound enters Dictyostelium cells and reaches an intracellular concentration of 1 microM, sufficient to activate PKA, at an extracellular concentration of 30 microM, which is insufficient to activate CAR. Expression of cAMP-regulated prespore and prestalk genes and the aggregative PDE gene are effectively induced by CAR agonists and very poorly by PKA agonists. Even Sp-6SEtcPuMPS is ineffective to induce gene expression. These data not only indicate that surface cAMP receptors are the first targets for cAMP-induced gene expression, but argue against direct induction of expression of these genes by cAMP-induced PKA activation.

Mapping of the epitope/paratope interactions of a monoclonal antibody directed against adenosine 3',5'-monophosphate.

A series of systematically modified cyclic AMP (cAMP) analogues, including newly synthesized benzimidazole ribofuranosyl 3',5'-monophosphates was used to map the essential molecular interactions between cAMP and the monoclonal antibody 4/2C2 (mab 4/2C2) directed against 2'-O-succinoyl cAMP [Colling, Gilles, Nass, Moka & Jaenicke (1988) Second Messengers Phosphoproteins 12, 123-133]. Its paratope binds the purine base in syn conformation by dipole-dipole interactions and hydrophobic forces and/or stacking interactions. The ribose phosphate moiety is recognized by a combination of charge interactions and H-bonds to the exocyclic and the 5'-oxygen atoms and a hydrophobic interaction at the 2'-position. There is no regioselectivity for the exocyclic oxygen atoms. Compared with the known types of binding, mab 4/2C2 thus shows a new combination of molecular interactions which may be the basis of its strikingly specific recognition and binding of the cyclic adenylates. On this account mab 4/2C2 may become an important tool in studies on cAMP metabolism.

Crystal structures of [AgHgL2(NO3)]n, where LH = 2-oxazolidone and β-propiolactam: Formation of helical chains and new 12-membered Ag/Hg metallamacrocyclic rings in the propiolactam complex

Reaction of 2-oxazolidone (C3H5NO2) or β-propiolactam (C3H5NO) with mercury(II) acetate and silver(I) nitrate in methanol leads to the formation of the polymeric heterometallic complexes [AgHg(C3H4NO2)2(NO3)]n (1) and [AgHg(C3H4NO)2(NO3)]n (2), respectively; the structures of which have been determined by X-ray diffraction methods. In each case, the deprotonated organic ligands bind to the mercury via nitrogen with nearly linear NHgN bonding and to the silver atom via oxygen. Despite the similar nature of the ligands, totally different three-dimensional assemblies are produced. Complex 1 is dimeric with a folded Ag2O2 ring in which the bridging oxygen atoms are provided by the exocyclic oxygen atoms of the 2-oxazolidone anions. These dimers are further linked in pairs to produce a “stepped” Ag4O4 species. Each silver atom is six-coordinate. Weak electrostatic, anion-mediated interact ions bridge these tetrameric units so as to produce loosely linked polymeric sheets. In compound 2 the silver atoms are linked by twisted Hg(β-propiolactam)2 bridges to form a helical chain. Each turn of the helix is bridged by a nitrate anion linking between adjacent silver atoms forming a 12-membered metallamacrocycle. Here the silver atoms are tetrahedrally coordinated.

Lactam and acid amide acetals 57. Synthesis, protonation, and acid-base properties of condensed derivatives of 4-oxo-1,4-dihydro-1,8-naphthyridine

A number of 4-oxo-1,4-dihydro-1,8-naphthyridine derivatives that differ with respect to the sizes of the azaand carbocycles were synthesized by the reaction of 3-amino-4-ethoxycarbonyl-5,6-dihydro-7H-pyrindine and 3-amino-4-ethoxycarbonyl-5,6,7,8-tetrahydroisoquinoline with N,N-dimethylacetamide diethylacetal and N-methylbutyrolactam, N-methylvalerolactam, and N-methylcaprolactam diethylacetals and subsequent cyclization of the intermediate amidines. It was established by UV and 1H and 13C NMR spectroscopy that the protonation of these compounds takes place at the exocyclic oxygen atom. The dependence of the ionization constants of the compounds obtained in 70% DMFA on the size of the saturated cyclic fragments of the molecules was established.

An MNDO molecular orbital study of the reactions of protonated oxirane derivatives (XCHCH2OH+, X = CN, Cl, CH3, Ph) with simple nucleophiles. Implications for regioselectivity in the reactions of electrophiles with nucleic acid bases

AbstractThe gas‐phase analogs of the classical SN1 and SN2 reactions of nucleophiles with a series of protonated oxiranes bearing the substituents CN, Cl, Me, and Ph were studied using MNDO semiempirical molecular orbital theory. The transition states for nucleophilic attack on the intact rings are calculated to become progressively more “SN1‐like,” as the ability of the substituent group to stabilize the corresponding ring opened carbenium ion increases. At the same time the activation barriers become progressively smaller and their relative values for different nucleophiles approach the order expected on the basis of a purely electrostatic attraction between the reacting moieties. Exactly the opposite trends are calculated for the transition states for nucleophilic attack on the intermediate carbenium ions. As the stabilities of the latter increase the extent of bond formation, and transfer of charge to the incoming nucleophile also increase. Here, the relative barriers for attack by different nucleophiles approach the order expected on the basis of a superposition of both covalent and electrostatic interactions in which the former dominate. These results support a previously suggested rationalization for the enhanced reactivities of certain alkylating agents towards the exocyclic oxygen atoms, rather than the ring nitrogens, of the nucleic acid bases. They also suggest a new explanation for the tendencies of aralkylating electrophiles to modify the exocyclic amino groups of the nucleic acid bases: sites which are unreactive towards simple alkylating agents.

Hydrogen-bonding effects and 13C-NMR of the DNA double helix.

13C-nmr chemical shifts of the nucleotides in DNA are sensitive to hydrogen bonding, especially for three of the carbons immediately bonded to exocyclic oxygen or nitrogen atoms acting as H-bond acceptors or donors. GuoC2, GuoC6 and ThdC4 are strongly deshielded (about 1 ppm) upon Watson-Crick pairing in oligodeoxynucleotide duplexes, regardless of the base sequence. Deshielding at these sites may be useful to distinguish bases involved in Watson-Crick pairs from unpaired bases.