In the title compound [systematic name: 4-amino-7-(β-
D-ribofuranosyl)-7
H-pyrazolo[3,4-
d][1,2,3]triazine], C
9H
12N
6O
4, the torsion angle of the N-glycosylic bond is high
anti [χ = −83.2 (3)°]. The ribofuranose moiety adopts the C2′-
endo–C1′-
exo (
2T1) sugar conformation (
S-type sugar pucker), with
P = 152.4° and τ
m = 35.0°. The conformation at the C4′—C5′ bond is +
sc (
gauche,
gauche), with the torsion angle γ = 52.0 (3)°. The compound forms a three-dimensional network that is stabilized by several hydrogen bonds (N—H
O, O—H
N and O—H
O).
Supporting information
CCDC reference: 616130
Compound (I) (Source?) was crystallized from water as colourless crystals (m.p. 481–482 K).
In the absence of suitable anomalous scattering, refinement of the Flack parameter (Flack, 1983) led to inconclusive values. Therefore, Friedel equivalents were merged before the final refinement. The known configuration of the parent molecule was used to define the enantiomer employed in the refined model. All H atoms were found in a difference Fourier synthesis. In order to maximize the data/parameter ratio, H atoms were placed in geometrically idealized positions (C—H = 0.93–0.98 Å, O—H = 0.82 Å and N—H = 0.86 Å) and constrained to ride on their parent atoms, with Uiso(H) = 1.2Ueq(C), 1.5Ueq(O) or 1.2Ueq(N). The OH groups were refined as rigid groups allowed to rotate but not tip.
Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: SHELXTL (Sheldrick, 1997); program(s) used to solve structure: SHELXTL; program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2003).
4-amino-7-(
β-
D-ribofuranosyl)-7
H-pyrazolo[3,4-
d][1,2,3]triazine
top
Crystal data top
C9H12N6O4 | Dx = 1.581 Mg m−3 |
Mr = 268.25 | Mo Kα radiation, λ = 0.71073 Å |
Trigonal, P3221 | Cell parameters from 40 reflections |
Hall symbol: P 32 2" | θ = 4.8–12.3° |
a = 9.7859 (7) Å | µ = 0.13 mm−1 |
c = 20.3813 (14) Å | T = 293 K |
V = 1690.3 (2) Å3 | Transparent block, colourless |
Z = 6 | 0.3 × 0.2 × 0.2 mm |
F(000) = 840 | |
Data collection top
Siemens P4 diffractometer | Rint = 0.045 |
Radiation source: fine-focus sealed tube | θmax = 30.0°, θmin = 2.4° |
Graphite monochromator | h = −1→13 |
2θ/ω scans | k = −13→1 |
4386 measured reflections | l = −28→1 |
1910 independent reflections | 3 standard reflections every 97 reflections |
1587 reflections with I > 2σ(I) | intensity decay: none |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.044 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.125 | H-atom parameters constrained |
S = 1.01 | w = 1/[σ2(Fo2) + (0.0781P)2 + 0.1318P] where P = (Fo2 + 2Fc2)/3 |
1910 reflections | (Δ/σ)max < 0.001 |
175 parameters | Δρmax = 0.59 e Å−3 |
0 restraints | Δρmin = −0.30 e Å−3 |
Crystal data top
C9H12N6O4 | Z = 6 |
Mr = 268.25 | Mo Kα radiation |
Trigonal, P3221 | µ = 0.13 mm−1 |
a = 9.7859 (7) Å | T = 293 K |
c = 20.3813 (14) Å | 0.3 × 0.2 × 0.2 mm |
V = 1690.3 (2) Å3 | |
Data collection top
Siemens P4 diffractometer | Rint = 0.045 |
4386 measured reflections | 3 standard reflections every 97 reflections |
1910 independent reflections | intensity decay: none |
1587 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.044 | 0 restraints |
wR(F2) = 0.125 | H-atom parameters constrained |
S = 1.01 | Δρmax = 0.59 e Å−3 |
1910 reflections | Δρmin = −0.30 e Å−3 |
175 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
N1 | 1.0451 (3) | 0.9361 (3) | 0.09555 (11) | 0.0388 (5) | |
N2 | 1.0616 (3) | 0.8124 (3) | 0.10549 (11) | 0.0397 (5) | |
N3 | 0.9454 (3) | 0.6640 (3) | 0.10555 (10) | 0.0356 (5) | |
C4 | 0.8019 (3) | 0.6471 (3) | 0.09600 (11) | 0.0272 (4) | |
C5 | 0.7669 (3) | 0.7672 (3) | 0.08898 (10) | 0.0283 (5) | |
C6 | 0.9003 (3) | 0.9211 (3) | 0.08776 (11) | 0.0325 (5) | |
N6 | 0.8940 (3) | 1.0518 (3) | 0.07817 (13) | 0.0475 (6) | |
H6A | 0.9797 | 1.1417 | 0.0771 | 0.057* | |
H6B | 0.8044 | 1.0467 | 0.0731 | 0.057* | |
C7 | 0.6010 (3) | 0.6887 (3) | 0.07929 (13) | 0.0336 (5) | |
H7 | 0.5429 | 0.7392 | 0.0733 | 0.040* | |
N8 | 0.5403 (2) | 0.5336 (2) | 0.07985 (11) | 0.0317 (4) | |
N9 | 0.6642 (2) | 0.5079 (2) | 0.09112 (9) | 0.0284 (4) | |
C1' | 0.6525 (3) | 0.3562 (3) | 0.08146 (11) | 0.0272 (4) | |
H1' | 0.7386 | 0.3543 | 0.1052 | 0.033* | |
C2' | 0.4964 (3) | 0.2146 (3) | 0.10230 (11) | 0.0277 (4) | |
H2' | 0.4086 | 0.2323 | 0.0922 | 0.033* | |
O2' | 0.5005 (2) | 0.1873 (2) | 0.17016 (8) | 0.0363 (4) | |
H2'1 | 0.4259 | 0.1876 | 0.1884 | 0.054* | |
C3' | 0.4894 (3) | 0.0831 (3) | 0.05859 (11) | 0.0297 (5) | |
H3' | 0.3801 | 0.0038 | 0.0483 | 0.036* | |
O3' | 0.5711 (2) | 0.0125 (2) | 0.08815 (9) | 0.0369 (4) | |
H3'1 | 0.5139 | −0.0526 | 0.1154 | 0.055* | |
C4' | 0.5790 (3) | 0.1704 (3) | −0.00325 (11) | 0.0304 (4) | |
H4' | 0.6529 | 0.1343 | −0.0151 | 0.036* | |
C5' | 0.4767 (4) | 0.1513 (4) | −0.06224 (13) | 0.0431 (6) | |
H5'A | 0.5432 | 0.2202 | −0.0970 | 0.052* | |
H5'B | 0.4297 | 0.0436 | −0.0780 | 0.052* | |
O5' | 0.3560 (3) | 0.1854 (3) | −0.04965 (13) | 0.0544 (6) | |
H5'1 | 0.3842 | 0.2757 | −0.0615 | 0.082* | |
O4' | 0.6682 (2) | 0.3363 (2) | 0.01341 (8) | 0.0385 (4) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
N1 | 0.0299 (10) | 0.0306 (11) | 0.0475 (11) | 0.0089 (9) | −0.0064 (9) | −0.0012 (9) |
N2 | 0.0297 (11) | 0.0387 (12) | 0.0484 (12) | 0.0153 (10) | −0.0051 (9) | −0.0008 (10) |
N3 | 0.0295 (10) | 0.0350 (11) | 0.0437 (10) | 0.0171 (9) | −0.0050 (8) | −0.0020 (9) |
C4 | 0.0277 (11) | 0.0254 (10) | 0.0285 (9) | 0.0133 (9) | −0.0017 (8) | −0.0002 (8) |
C5 | 0.0303 (11) | 0.0237 (10) | 0.0311 (10) | 0.0136 (9) | −0.0057 (8) | −0.0033 (8) |
C6 | 0.0345 (12) | 0.0265 (11) | 0.0335 (10) | 0.0128 (10) | −0.0052 (9) | −0.0004 (9) |
N6 | 0.0432 (14) | 0.0234 (10) | 0.0711 (16) | 0.0130 (10) | −0.0076 (12) | 0.0047 (10) |
C7 | 0.0309 (12) | 0.0272 (11) | 0.0466 (12) | 0.0174 (10) | −0.0070 (10) | −0.0046 (10) |
N8 | 0.0263 (10) | 0.0269 (10) | 0.0452 (11) | 0.0157 (8) | −0.0034 (8) | −0.0023 (8) |
N9 | 0.0271 (10) | 0.0236 (9) | 0.0361 (9) | 0.0140 (8) | −0.0015 (8) | 0.0005 (7) |
C1' | 0.0269 (10) | 0.0235 (10) | 0.0319 (9) | 0.0132 (8) | 0.0030 (8) | 0.0024 (8) |
C2' | 0.0277 (11) | 0.0239 (10) | 0.0347 (10) | 0.0153 (9) | 0.0067 (8) | 0.0049 (8) |
O2' | 0.0444 (10) | 0.0391 (10) | 0.0341 (8) | 0.0275 (8) | 0.0127 (7) | 0.0082 (7) |
C3' | 0.0289 (11) | 0.0220 (10) | 0.0388 (11) | 0.0131 (9) | 0.0035 (9) | 0.0007 (9) |
O3' | 0.0454 (11) | 0.0309 (9) | 0.0451 (9) | 0.0271 (8) | 0.0113 (8) | 0.0101 (8) |
C4' | 0.0308 (11) | 0.0254 (10) | 0.0357 (10) | 0.0146 (9) | 0.0043 (9) | −0.0006 (8) |
C5' | 0.0400 (14) | 0.0459 (16) | 0.0402 (12) | 0.0190 (13) | −0.0020 (11) | 0.0016 (11) |
O5' | 0.0359 (10) | 0.0464 (13) | 0.0774 (15) | 0.0179 (10) | −0.0043 (10) | 0.0102 (11) |
O4' | 0.0459 (11) | 0.0245 (8) | 0.0337 (8) | 0.0090 (8) | 0.0122 (7) | 0.0032 (6) |
Geometric parameters (Å, º) top
N1—N2 | 1.315 (4) | C1'—H1' | 0.9800 |
N1—C6 | 1.359 (4) | C2'—O2' | 1.413 (3) |
N2—N3 | 1.323 (3) | C2'—C3' | 1.538 (3) |
N3—C4 | 1.343 (3) | C2'—H2' | 0.9800 |
C4—N9 | 1.359 (3) | O2'—H2'1 | 0.8200 |
C4—C5 | 1.387 (3) | C3'—O3' | 1.426 (3) |
C5—C6 | 1.416 (3) | C3'—C4' | 1.529 (3) |
C5—C7 | 1.420 (3) | C3'—H3' | 0.9800 |
C6—N6 | 1.325 (4) | O3'—H3'1 | 0.8200 |
N6—H6A | 0.8600 | C4'—O4' | 1.448 (3) |
N6—H6B | 0.8600 | C4'—C5' | 1.515 (4) |
C7—N8 | 1.325 (3) | C4'—H4' | 0.9800 |
C7—H7 | 0.9300 | C5'—O5' | 1.402 (4) |
N8—N9 | 1.374 (3) | C5'—H5'A | 0.9700 |
N9—C1' | 1.444 (3) | C5'—H5'B | 0.9700 |
C1'—O4' | 1.419 (3) | O5'—H5'1 | 0.8200 |
C1'—C2' | 1.522 (3) | | |
| | | |
N2—N1—C6 | 121.3 (2) | O2'—C2'—C1' | 109.6 (2) |
N1—N2—N3 | 125.2 (2) | O2'—C2'—C3' | 113.76 (18) |
N2—N3—C4 | 114.0 (2) | C1'—C2'—C3' | 101.81 (17) |
N3—C4—N9 | 125.9 (2) | O2'—C2'—H2' | 110.5 |
N3—C4—C5 | 126.7 (2) | C1'—C2'—H2' | 110.5 |
N9—C4—C5 | 107.5 (2) | C3'—C2'—H2' | 110.5 |
C4—C5—C6 | 114.6 (2) | C2'—O2'—H2'1 | 109.5 |
C4—C5—C7 | 104.8 (2) | O3'—C3'—C4' | 108.34 (19) |
C6—C5—C7 | 140.3 (2) | O3'—C3'—C2' | 111.06 (19) |
N6—C6—N1 | 117.5 (3) | C4'—C3'—C2' | 103.74 (18) |
N6—C6—C5 | 124.5 (3) | O3'—C3'—H3' | 111.1 |
N1—C6—C5 | 118.0 (2) | C4'—C3'—H3' | 111.1 |
C6—N6—H6A | 120.0 | C2'—C3'—H3' | 111.1 |
C6—N6—H6B | 120.0 | C3'—O3'—H3'1 | 109.5 |
H6A—N6—H6B | 120.0 | O4'—C4'—C5' | 108.4 (2) |
N8—C7—C5 | 110.8 (2) | O4'—C4'—C3' | 106.76 (17) |
N8—C7—H7 | 124.6 | C5'—C4'—C3' | 115.2 (2) |
C5—C7—H7 | 124.6 | O4'—C4'—H4' | 108.8 |
C7—N8—N9 | 106.2 (2) | C5'—C4'—H4' | 108.8 |
C4—N9—N8 | 110.7 (2) | C3'—C4'—H4' | 108.8 |
C4—N9—C1' | 124.7 (2) | O5'—C5'—C4' | 114.1 (2) |
N8—N9—C1' | 122.7 (2) | O5'—C5'—H5'A | 108.7 |
O4'—C1'—N9 | 108.42 (18) | C4'—C5'—H5'A | 108.7 |
O4'—C1'—C2' | 105.99 (18) | O5'—C5'—H5'B | 108.7 |
N9—C1'—C2' | 115.19 (19) | C4'—C5'—H5'B | 108.7 |
O4'—C1'—H1' | 109.0 | H5'A—C5'—H5'B | 107.6 |
N9—C1'—H1' | 109.0 | C5'—O5'—H5'1 | 109.5 |
C2'—C1'—H1' | 109.0 | C1'—O4'—C4' | 109.73 (16) |
| | | |
C6—N1—N2—N3 | 3.8 (4) | C4—N9—C1'—O4' | −83.2 (3) |
N1—N2—N3—C4 | −1.7 (4) | N8—N9—C1'—O4' | 79.6 (3) |
N2—N3—C4—N9 | 177.0 (2) | C4—N9—C1'—C2' | 158.2 (2) |
N2—N3—C4—C5 | −2.6 (4) | N8—N9—C1'—C2' | −38.9 (3) |
N3—C4—C5—C6 | 4.4 (4) | O4'—C1'—C2'—O2' | 155.32 (18) |
N9—C4—C5—C6 | −175.3 (2) | N9—C1'—C2'—O2' | −84.8 (2) |
N3—C4—C5—C7 | 179.3 (2) | O4'—C1'—C2'—C3' | 34.6 (2) |
N9—C4—C5—C7 | −0.4 (3) | N9—C1'—C2'—C3' | 154.44 (19) |
N2—N1—C6—N6 | 179.8 (3) | O2'—C2'—C3'—O3' | −31.9 (3) |
N2—N1—C6—C5 | −1.6 (4) | C1'—C2'—C3'—O3' | 85.9 (2) |
C4—C5—C6—N6 | 176.4 (2) | O2'—C2'—C3'—C4' | −148.0 (2) |
C7—C5—C6—N6 | 4.2 (5) | C1'—C2'—C3'—C4' | −30.3 (2) |
C4—C5—C6—N1 | −2.1 (3) | O3'—C3'—C4'—O4' | −101.7 (2) |
C7—C5—C6—N1 | −174.3 (3) | C2'—C3'—C4'—O4' | 16.4 (2) |
C4—C5—C7—N8 | −0.5 (3) | O3'—C3'—C4'—C5' | 138.0 (2) |
C6—C5—C7—N8 | 172.3 (3) | C2'—C3'—C4'—C5' | −104.0 (2) |
C5—C7—N8—N9 | 1.1 (3) | O4'—C4'—C5'—O5' | −67.5 (3) |
N3—C4—N9—N8 | −178.5 (2) | C3'—C4'—C5'—O5' | 52.0 (3) |
C5—C4—N9—N8 | 1.1 (3) | N9—C1'—O4'—C4' | −149.78 (19) |
N3—C4—N9—C1' | −13.9 (4) | C2'—C1'—O4'—C4' | −25.6 (2) |
C5—C4—N9—C1' | 165.8 (2) | C5'—C4'—O4'—C1' | 130.1 (2) |
C7—N8—N9—C4 | −1.4 (3) | C3'—C4'—O4'—C1' | 5.5 (3) |
C7—N8—N9—C1' | −166.4 (2) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N6—H6A···O5′i | 0.86 | 2.13 | 2.973 (3) | 166 |
N6—H6B···O3′ii | 0.86 | 2.16 | 2.993 (3) | 164 |
O2′—H2′1···N2iii | 0.82 | 1.91 | 2.727 (3) | 178 |
O3′—H3′1···O2′iv | 0.82 | 1.99 | 2.756 (3) | 156 |
O5′—H5′1···N8v | 0.82 | 2.28 | 3.071 (3) | 162 |
Symmetry codes: (i) y+1, x+1, −z; (ii) x, y+1, z; (iii) x−y, −y+1, −z+1/3; (iv) x−y, −y, −z+1/3; (v) y, x, −z. |
Experimental details
Crystal data |
Chemical formula | C9H12N6O4 |
Mr | 268.25 |
Crystal system, space group | Trigonal, P3221 |
Temperature (K) | 293 |
a, c (Å) | 9.7859 (7), 20.3813 (14) |
V (Å3) | 1690.3 (2) |
Z | 6 |
Radiation type | Mo Kα |
µ (mm−1) | 0.13 |
Crystal size (mm) | 0.3 × 0.2 × 0.2 |
|
Data collection |
Diffractometer | Siemens P4 diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4386, 1910, 1587 |
Rint | 0.045 |
(sin θ/λ)max (Å−1) | 0.703 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.044, 0.125, 1.01 |
No. of reflections | 1910 |
No. of parameters | 175 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.59, −0.30 |
Selected geometric parameters (Å, º) topN1—N2 | 1.315 (4) | C4—C5 | 1.387 (3) |
N1—C6 | 1.359 (4) | C6—N6 | 1.325 (4) |
N2—N3 | 1.323 (3) | C7—N8 | 1.325 (3) |
N3—C4 | 1.343 (3) | N8—N9 | 1.374 (3) |
C4—N9 | 1.359 (3) | N9—C1' | 1.444 (3) |
| | | |
N2—N1—C6 | 121.3 (2) | C4—N9—C1' | 124.7 (2) |
N1—N2—N3 | 125.2 (2) | N8—N9—C1' | 122.7 (2) |
N2—N3—C4 | 114.0 (2) | O4'—C1'—N9 | 108.42 (18) |
N3—C4—N9 | 125.9 (2) | N9—C1'—C2' | 115.19 (19) |
C4—N9—N8 | 110.7 (2) | C1'—O4'—C4' | 109.73 (16) |
| | | |
C6—N1—N2—N3 | 3.8 (4) | N3—C4—N9—C1' | −13.9 (4) |
N1—N2—N3—C4 | −1.7 (4) | C5—C4—N9—C1' | 165.8 (2) |
N2—N3—C4—N9 | 177.0 (2) | C4—N9—C1'—O4' | −83.2 (3) |
N2—N3—C4—C5 | −2.6 (4) | N8—N9—C1'—O4' | 79.6 (3) |
N9—C4—C5—C6 | −175.3 (2) | O3'—C3'—C4'—O4' | −101.7 (2) |
N3—C4—C5—C7 | 179.3 (2) | C2'—C3'—C4'—C5' | −104.0 (2) |
N9—C4—C5—C7 | −0.4 (3) | C3'—C4'—C5'—O5' | 52.0 (3) |
N2—N1—C6—N6 | 179.8 (3) | N9—C1'—O4'—C4' | −149.78 (19) |
C4—C5—C6—N6 | 176.4 (2) | C2'—C1'—O4'—C4' | −25.6 (2) |
C5—C4—N9—N8 | 1.1 (3) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N6—H6A···O5'i | 0.86 | 2.13 | 2.973 (3) | 166.3 |
N6—H6B···O3'ii | 0.86 | 2.16 | 2.993 (3) | 164.1 |
O2'—H2'1···N2iii | 0.82 | 1.91 | 2.727 (3) | 178.0 |
O3'—H3'1···O2'iv | 0.82 | 1.99 | 2.756 (3) | 155.7 |
O5'—H5'1···N8v | 0.82 | 2.28 | 3.071 (3) | 162.1 |
Symmetry codes: (i) y+1, x+1, −z; (ii) x, y+1, z; (iii) x−y, −y+1, −z+1/3; (iv) x−y, −y, −z+1/3; (v) y, x, −z. |
The aza and deaza derivatives of purine nucleosides attract attention because they are valuable tools in chemistry and biology. Many of them show antifungal, antiviral and anticancer activity (Montgomery & Thomas, 1972; Montgomery et al., 1975; Bennett et al., 1976). They also show unusual base-pairing properties when they are constituents of nucleic acids. 7-Deaza-2,8-diaza-2'-deoxyadenosine forms strong base pairs with 2'-deoxyguanosine and weaker ones with 2'-deoxythymidine in duplex DNA (Seela et al., 2004). We now report the single-crystal X-ray structure of the ribonucleoside 7-deaza-2,8-diazaadenosine, (I).
The title nucleoside was synthesized from 8-aza-7-deazaadenosine, (II), via its 1,N6-etheno derivative (Lin et al., 2005) (purine numbering is used throughout this manuscript). The structure of compound (I) is depicted in Fig. 1. Selected geometric parameters are shown in Table 1.
The N-glycosylic bond torsion angle χ (O4'—C1'—N9—C4) of (I), which describes the orientation of the base relative to the sugar moiety, shows a high anti conformation [χ = −83.2 (3)°] (IUPAC-IUB Joint Commission on Biochemical Nomenclature, 1983). The glycosylic bond conformation of compound (II) is also high anti [χ = −77.6 (3)°; 102.4–180° Significance of this range of angles?; Sprang et al., 1978]. In contrast, 2-azaadenosine, (III), and adenosine, (IV), adopt the anti conformation, with χ = −166.2 and −171.1°, respectively (Singh & Hodgson, 1979; Lai & Marsh, 1972). Thus, the introduction of an additional N atom in the six-membered ring of the purine nucleoside [(VI) → (III)] or in the corresponding pyrazolo[3,4-d]pyrimidine analogue [(II) → (I)] does not lead to significant changes in the conformation of the glycosylic bond. On the other hand, the shift of the imidazole N atom from position 7 to 8 [(III) → (I) or (IV) → (II)] changes the conformation towards syn, as is found for the related nucleosides (Sprang et al., 1978; Singh & Hodgson, 1979; Lai & Marsh, 1972). The length of the glycosylic bond (C1'—N9) of (I) is 1.444 (3) Å, which is shorter than those of the nucleosides (II) [1.460 (5) Å], (III) [1.470 (4) Å] or (IV) (1.466 Å) (Sprang et al., 1978; Singh & Hodgson, 1979; Lai & Marsh, 1972).
The sugar moiety of (I) shows a pseudorotation phase angle P of 152.4° and an amplitude τm of 35.0°, which indicates S-conformation (2T1) (Altona & Sundaralingam, 1972; Rao et al., 1981). This is similar to compound (II) (P = 141.9°, τm = −41.9°, 1T2; Sprang et al., 1978). This S-conformation is rather uncommon for ribonucleosides. The conformation of the sugar moiety of compounds (III) and (IV) is N [P = 20.8°, τm = 39.5°, 3T4 for (III), and P = 6.9°, τm = 36.8°, 3T2 for (IV)]. These data are not included in the manuscripts (Singh & Hodgson, 1979; Lai & Marsh, 1972) but are available from the Cambridge Structural Database (CSD, Version?; Allen, 2002). They indicate that the major conformational change results from the alternation of the nitrogen pattern in the five-membered ring (Singh & Hodgson, 1979; Lai & Marsh, 1972). The C3'—C4'—C5'—O5' torsion angle of compound (I) is 52.0 (3)°, which shows that the exocyclic hydroxyl group prefers a gauche,gauche (+sc) conformation. This is similar to compound (III) (γ = 42.28°, +sc; CSD Refcode?), but different from compound (II) (γ = 179.5°, +ap; Sprang et al., 1978) and (IV) (γ = 177.0°, +ap; Lai & Marsh, 1972).
Compound (I) forms a three-dimensional network which is stabilized by several intermolecular hydrogen bonds listed in Table 2 and shown in Figs. 2 and 3. Whereas hydrogen bonds 1, 2 and 5 (numbers relate to entries in Table 2 [Please check added text]) lead to double layers perpendicular to the c axis (Fig. 2), hydrogen bonds 3 and 4 connect the sugar moieties (O3'—H3'1···O2') or the sugar moiety with the base (O2'—H2'1···N2) (Fig. 3). The hydrogen-bond acceptor properties of N2 have been already suggested to be involved in the base pairing of 7-deaza-2,8-diaza-2'-deoxyadenosine with 2'-deoxyguanosine within duplex DNA (Seela et al., 2004).