Structural Studies and Investigation of NMR Shielding Tensors in Coordination of Magnesium Hydrate to Purine Nucleotide 5'-Monophosphates (AMP, GMP, IMP)

M. Monajjemi", M. A. Seyed Sajadi, R. Sayadia, M. Kia, G. Ghasemi.

Department of Chemistry, Islamic Azad University, Science and Research Campus, Poonak, Hesarak, Tehran, Iran, P. O. Box: 14515-775.

ABSTRACT:

The interaction of magnesium hydrate at the phosphate oxygen atoms of the purine nucleotides (AMP, GMP, IMP) were studied at the Hartree-Fock level Theory. We used LANL2DZ basis set for Mg and 6-3 lg* basis set for other atoms. The basis set superposition error (BSSE) begins to converge for used method/basis set. NBO calculations were performed to the second-order; perturbative estimates of donor-acceptor (bonding-antibonding) interactions have been done. The gauge-invariant atomic orbital (GIAO) method was employed to calculate isotropic atomic shielding of the nucleotides using HF/6-31g** level.

Keywords: Ab initio Calculation, NMR shielding Tensor, Purine Nucleotide,Magnesium Complexes.

INTRODUCTION:

All of the reactions involving nucleic acids in biological systems, such as those involving ATP or dinucleotide coenzymes, are mediated by metal ions. Metal ions are known to have an effect on the stability of both DNA and RNA in vivo. Physiologically, the important complexes are those involving Mg+2. There is, however considerable discord as to the structure of these metal-nucleotide complexes /1-6/. A nucleotide consists of three main subunits, the nucleobase residue (purine or pyrimidine ), the sugar part and the phosphate group(s). The structures of the three common purine-nucleoside 5'-monophosphates are shown in Fig 1 /7,8/. Nuclear magnetic resonance techniques applied to poly- or mononucleotides can be directly used to detect the effects caused by the binding of a metal ion on these molecules in aqueous solutions. Hammes, Maciel, and Waugh found in 1961 that addition of Mg+2 and Ca+2 did not change the H(2) and H(8) resonance positions and therefore the metals did not bind to the ring III. Happe and Morales concluded from 1SN NMR chemical shifts that Mg+2 does not bind to the ring 161.

The NMR shielding tensor σ can be all written as second derivatives of the energy σ =(<52E/dB0m)B.o,m-o,

* Corresponding author: e-mail:m_monajjemi@yahoo. com

71 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine where Ε is the energy of the molecule, Β external magnetic field, Ε electric intensity, and n, m nuclear magnetic moments. Scalar parameter and σ observable for an isotropic medium are defined as 1/3 of the traces of the tensors J and σ, respectively /7/. Electronic structures were analyzed with the natural bond order method /9/ (NBO). In the NBO method, the aim is to optimally transform a given wave function into localized form, corresponding to the one-center (lone pair) and two-center (bond) elements of the chemist's Lewis structure picture. This is carried out by examining all possible interactions between filled (donor) Lewis type NBOs and empty (acceptor) non Lewis NBOs and estimating their energetic importance by two second-order perturbation theory.

SLLJ-/ Η I * I Η η OΓH OHl· Guanosine 5'-monophosphates(GMP2-) lnosine 5'-monophosphaies(lMP2-)

NH2 7

Ι Ο

-Ο-

O- y OH OH

Adenosine 5'-monophosphates(AMP2-) Fig. 1: Chemical structures of the purine-nucleoside 5'-monophosphates (AMP, GMP, IMP).

We describe in detail the structures of magnesium hydrate complexes of purine nucleotide 5'- monophosphate, using ab initio quantum-chemical calculation made at the Hartree-Fock (HF) theoretical level with 6-3lg* basis set predicted effects of the magnesium hydrate on the phosphate coordination. Lanl2DZ for Mg and purine nucleotide 5'-monophosphates have been carried out. Hydrogen bonding will be discussed in term of observable properties such as geometry. The structures were supported by comparing the measured 'H-NMR spectra to the results of ab initio gauge-invariant atomic orbital (GIAO) /8/ computation of chemical shifts. The basis set superposition error (BSSE) was computed, through the counterpoise method /10/ implemented in the Gaussian 98 code, for the most stable complexes.

72 Μ Monajjemi et al. Main Group Metal Chemistry

METHOD:

The structures of all the systems were optimized using the framework of the Hartree-Fock and LANL2DZ basis set for magnesium and 6-31g* basis set /ll,12/ for other atoms. A natural bond orbital (NBO) /9/ analysis, which localizes the many-electron wave function into Lewis-type electron-pairs, was carried out at the HF level and 6-3 lg* basis set of theory to determine donor-acceptor interactions. Finally, All NMR analysis have been performed using 6-31g** basis set and the HF level. The GIAO /8/ methods were used to calculate the isotropic NMR shielding at the HF/6-31g** of theory. The interaction energies of the counterparts were estimated as the energy difference between the complex and the isolated components and were corrected for the basis set superposition error (BSSE). The Boys- Bernardi counterpoise method /13/, applied at the magnesium hydrate nucleotide complexes geometry, is used to account for BSSE. According to this method:

Ec.orr• =Einlerac,ion+AEßssK

where ECon. is corrected-interaction energies, and:

BSSE ~ * Magnesium Hydrate ΕMagnesium Hydrate (Complex)^ •

* Purine nucleotide Purine nucleotide (Complex)^

where E* indicates that the energy of components at complex geometry is calculated from Methods/Basis set of complex geometry.

RESULTS:

Theoretical results of the calculated optimized geometries for structures of purine nucleotide complexes are given in Table 1 and optimized structures obtained in the HF/6-31g*:LANL2DZ are shown in Figure 2.

Table 1 Optimized bond length ( A") of purine nucleotide complexes in the 6-31g*:LANL2DZ basis set

Bond Length Mg(H20)4AMP Mg(H20)5GMP Mg(H20)5IMP Mg-0(H20) 2.035,2.165,2.058, 2.060,2.094, 2.164,2.180,2.149 2.089 2.028,2.167 2.088,2.106 Mg—0(P04) 1.912 1.912 1.93 01 (P04)—H(H20) 1.561 1.577 1.616 02(P04)—H(H20) 1.510 1.526 1.592 N7—H(H20) 1.871 1.819 2.063,2.239 P-Ol 1.505 1.496 1.505 P-03 1.557 1.554 1.556 P-02 1.504 1.511 1.496

In HF-SCF study, it was found that the magnesium ion is coordinated with water and phosphate oxygens

73 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine

at distances ranging form 1.912 to 2.150, values that were in agreement with the available experimental results (2.02A°and 2.11A°) /14/. The latter kind of structure was also described for the Mg(II)-hydrate complexes of the 5'- monophosphates. Interaction with the phosphate group was suggested to be either a direct one or mediated through water of the molecule, depending on the conditions in which the complexes were prepared /15,16/.

HFMg(H20)4AMP HFMg(H20)4GMP

HFMg(H20)5IMP

Fig. 2: Optimized structures of Mg-nucleotide complexes in the HF/6-31g*:LANL2DZ Indirect (outer- sphere) phosphate coordination probably occurs between phosphate oxygen atoms and water molecules of Mg-coordination.

74 Μ. Monajjemi et al. Main Group Metal Chemistry

The conformation of the sugar ring in nucleotides and nucleosides can be examined by using a concept of pseudorotation, which utilizes a quantitative description of puckering and conformation in terms of the maximum torsion angle (τ„,) and the "Phase angle" of pseudorotation (P), which is a function of the interrelationship between the five torsion angles(T0-T4) in the nonplanar five-membered ring. The phase angle, P, and the maximum pucker, tm, are calculated with eq 1 and 2 /17/.

tanP=(T4+T1)-(T3-h:oy2T2(sin36+sin72) (1)

T2=Tm cosP (2)

All the possible conformations are grouped into two categories. Type N(P=0±90°) or 3' endo and type S(P=180±90°) or 2' endo/Cl' exo /18/. A majority of the ribose and deoxyribose Ρ values fall in the ranges 0-36° and 144-180° /19/. We have theoretically computed the Ρ values for all nucleotides using HF/6-31g* level on the optimized structures (Table 2 ). The ribofuranose rings of nucleosides and nucleotides are puckered, usually into one of two preferred conformations, described as C3'-endo or C2'-endo 1201. A more precise description of the ribofuranose conformation is given by the torsion angles about each bond. The orientation of the ribose rings relative to the purine base is given by torsion angle χ™ about the glycosidic bond for the sequence of atoms C4-N9- Cl'-Ol' /21-24.

Table2 Dihedral Angles complexes at 6-31g* basis set. Dihedral Angles Mg(H20)4AMP Mg(H20)4GMP Mg(H20)5IMP MgOPOS' 172.0 -177.2 176.1 P05'C5'C4 -145.4 -150.8 -146.3 05,C5'C4'C3,(\|/) 179.8 -179.7 178.3 OI'Cl'N9C4 179.2 179.3 -154.1 01'C1'N9C8 -2.1 3.0 26.4 05,C5'C4'01· 62.4 63.0 61.2

C3'C4Orcr(T4) -15.4 -18.9 -13.3 CSTTCrol'Cr,, 26.9 24.3 29.7 , , , 01 C4'C3 C2 (T3) 31.4 33.1 31.2 C4'OrCl'C2'(To) -7.5 -3.6 -10.6 C4,C3'C2'CI'(Ti) -34.6. -34.2 -36.5

Table 3 Ρ Angles complexes in HF/6-31g* level. Complexes Ρ Angle Mg(H20)4AMP -6.6 Mg(H20)5GMP 12.9 Mg(H20)5IMP 2.1

75 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine

These structures are compared with those predicted from an HF/6-31g* basis in Tables 1-3. The agreement between the theoretical structures predicted in both levels and the experimental geometry is very good for magnesium-nucleotides. The results of these calculations showed that metal-nucleotide sugar conformations fall into categories C3' endo, anti. The computed energies of the complexes for non-metalated and metalated purine nucleotides are compared by HF/6-31g* method (Table 4). Geometry optimization and NMR analysis AMP, IMP have been formed with charge and multipilicity -2,3, but geometry optimization GMP was used at the UHF/6-31g* level of theory with charge and multiplicity -2,1. Metalation of purine nucleotides has been found to gie higher stability than in the nonmetalated molecule.

Table 4 Stability energies(Hartree) for Mg-Purine nucleotide complexes in gas phase

Complexes HF/6-31g* AMP -1522..592 GMP -1597.553 IMP -1542.523 Mg(H20)4AMP -2026.665 Mg(H20)5GMP -2101.543 Mg(H20)5IMP -2122/534

Table 5 shows the value of BSSE and Ε I+BSSE for the structures. Clearly for the all complexes, values of BSSE are rather small. Therefore, for these cases BSSE is negligible.

Table 5

E| (Interaction energies), BSSE and Ε I+BSSI: (Hartree) for complexes in HF/6-31g* level

Complexes E4 BSSE -L+BSSE Mg(H20)4AMP -0.36437 -0.01306 -0.3774 Mg(H20)5GMP -0.3757 -0.01422 -0.3899 Mg(H2Q)5IMP -0.3896 -0.01774 -0.3896

Ν BO calculation shows the π-bonding contribution in the base moiety of nucleotides (Table 6). A filled bonding or lone pair orbital can act as a donor and an empty or filled bonding, antibonding or lone pair orbital can act as acceptor. These interactions can strengthen and weaken bonds. For example, a lone pair donor —»antibonding acceptor orbital interaction will weaken the bond associated with the antibonding orbitals. Conversely, an interaction with a bonding pair as the acceptor will strengthen the bond. Strong electron derealization in a best Lewis structure will also show up as donor-acceptor interactions. Table 7 shows the interactions that give the strongest stabilization. NBO calculation and optimization GMP have been formed with charge and multiplicity -2,1 at UHF method.

76 Μ. Monajjemi et al. Main Group Metal Chemistry

Table 6 Hybridation coefficient of bonds calculated by NBO method in HF/6-31g* level

Bond Mg(H20)4AMP Mg(H20)5GMP P-03 σ 0.4373SP 303+0.8993SP 155 0.4379SP 301 + 0.8990SP 154

C8-N7 σ 0.6296SP +0.777SP 166 0.6283SP 199 + 0.7779SP 163 π 0.5687SP 100 +0.8226SP 100 0.5924SP 100 + 0.8057 SP 100 C4-C5 σ 0.7078SP 17 +0.7064SP 203 0.7113SP 161 +0.7029SP 192 λ 0.6339SP 100 +0.7734SP '00 C6-N1 σ 0.7644SP 176 +0.6447SP 212 π 0.8455SP 100 +0.5339SP 100 C4-N9 σ 0.6119SP2 51 + 0.7909SP 203 π 0.4277SP 100 +0.9039 SP 100 N3-C2 σ 0.6236SP "·91 +0.7817SP 167 0.6390SP 175 +0.7692SP 163 π 0.5524SP 100 +0.8335 SP 100 0.5220SP 100 +0.8529SP 100 C60 σ 0.5884SP 193 +0.8086SP 122 π 0.5109SP 100 +0.8596SP 100 Bond Mg(H20)5IMP P-03 σ 0.4353SP303 + 0.9003SP 152 C8-N7 σ 0.6300SP2 06 + 0.7766SP 170 π 0.5814SP 100 + 0.81367SP 100 C4-C5 σ 0.7099SP 163 + 0.7043SP 189 π 0.6494SP 100 + 0.7604SP 100 N3-C2 σ 0.6347SP 179 + 0.7728SP 158 π 0.5844SP 100 + 0.8115SP 100 C60 σ 0.5895SP 192 + 0.8077SP 123 π 0.5129SP 100 + 0.8585SP 100

The Lewis NBOs in Table 8 describe percentage of the total density, with the remaining non-Lewis density found primarily in the valence-shell antibonding. Also, analysis of the atomic charges is done by the natural bonding orbital (NBO) method. It was found that the charge on 03 in metalated purine nucleotides is higher than in nonmetalated purine nucleotides(03 coordinated to Mg) (Table 9). As shown above, metalation of the purine nucleotides strongly influences the electronic structure of the purine nucleotides, and this leads to energetic stabilization of the structures. Therefore, the increasing 03 basicity of the metalated nucleotides can be attributed to the relative stabilization and increased stability of Mg-nucleotide complexes.

77 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine

Table 7 The stabilization energy E(2) associated with derealization for interactions that give the strongest stabilization in HF/6-31G* level.

Complex Donor NBO Acceptor NBO E(2)Kcal mol1

AMP LP(1)N9 BD*(2)C4-C5 58. 9 BD*(2)C8-N7 27. 26 BD*(1)C2'-C3' 0. 28 BD*(1)C2'-C1' 3.79 BD*(l)Cl'-Or 0. 58 GMP LP(1)N9 BD*(2)C4-C5 41. 96 BD*(2)C8-N7 28. 97 BD*(1)C3'-C2' 0.31 BD*(1)C2'-C1' 4. 02 BD*(l)Cl'-Or 1. 11 BD*(1)C1'-Hr 1. 41 IMP LP(1)C5 BD*(1)N3-C4 2. 29 BD*(2)N3-C4 66.45 BD*(l)C6-09 49. 34 BD*(2)C8-N7 32. 85 Mg(H20)4AMP LP(1)C5 BD*(1)01-H29 0. 06 BD*(2)N1-C6 162. 73 BD*(2)C4-N9 367. 51 BD*(2)C8-N7 46.00 BD*(2)C2-N3 Mg(H20)4GMP LP(1)N9 BD*(2)C4-C5 76. 68 BD*(2)C8-N7 77. 33 BD*(1)C2'-Cr 5. 92 BD*(1)C1'-Hr 5.03 Mg(H20)5IMP LP(1)N9 BD*(2)C4-C5 69. 47 BD*(2)C8-N7 83. 38 BD*(1)C2'-C3' 0. 53 BD*(1)C2'-Cr 7. 75 BD*(l)Cl'-Or 2. 54 BD*(1)C1'-H 2. 20

78 Μ. Monajjemi et al. Main Group Metal Chemistry

Table 8 %Total Lewis, highest energy Lewis NBO and lowest occupancy of complexes in HF/ 6-3 lg* level

Complexes % Total Lewis Lowest occupancy Highest energy Lewis NBO (a.u.)

Mg(H20)4AMP 98.1120 1.15546 -0.11163

Mg(H20)5GMP 98. 3841 1. 61358 -0. 3587 Mg(H20)5IMP 98. 3927 1.61118 -0. 36909

Table 9 Natural population of phosphate oxygen atoms and P,Mg in metalated purine nucleotides in HF/ 6-31G* basis set.

Complexes Ol 02 03 Ρ Mg

Mg(H20)4AMP -1. 28627 -1. 27437 -1. 43508 2. 85548 1. 82657

Mg(H20)5GMP -1. 27014 -1. 29181 -1. 43422 2. 85512 1. 82848

Mg(H20)5IMP -1. 29264 -1. 27385 -1. 43725 2. 85301 1. 81970

NMR CHEMICAL SHIFT ASSIGNMENTS

We first optimized the geometry of Mg-nucleotide complexes with the HF/6-31g* level. Then, we calculated isotropic spectroscopic shielding for all atoms in nucleotides, using HF/6-31g** level. In this work, we use the GIAO method which is implemented in thfc GAUSSIAN 98 program. The isotropic part oiso of σ is measured by taking the average of σ with respect to the orientation to the magnetic field,i. e. ,aiso=(a, 1+σ22+σ;1;1)/3. The results calculated are summarized in Tables 10 and 11.

The anisotropy is ζ=| σ 33 -σ iso|, and the asymmetry is η=(σ 22 - σΐ 1)/ζ 1211. Ab initio calculations yield the data in Tables 10 and 11, showing that the values for the isotropic shielding of C2,C6 (strongly), N3, H2 atoms in metalated AMP, C2 atom in metalated GMP and C2,N1 (strongly),N9,C4,H8,H2 atoms in metalated IMP have been decreased, but the isotropic shielding of N7, H8, C4, C5 atoms in metalated AMP and H8, C8, N7, N3 atoms in metalated GMP and 06 (strongly), N7 atoms in metalated IMP have been increased compared to nonmetalated nucleotides, while the isotropic shielding of other atoms did not change importantly.

79 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine

Table 10

HF/6-31 g** calculation of the oiso for atoms in purine nucleotides of AMP, IMP in GIAO method with charge and multiplicity -2,3 and the GMP at UHF level with charge and multiplicity -2,1.

Atoms AMP GMP IMP C6 73. 4423 48. 1187 45. 8608 C4 41.0316 54.4096 38. 8281 C5 77. 9742 90. 7034 84. 0715 C2 100. 7886 54. 5839 113. 6832 C8 45.0491 49. 3810 54. 2788 N1 25. 1994 134. 5586 161. 1006 N6 219. 0271 N3 101. 2866 102. 4924 56. 4251 N7 -1. 1396 25. 1780 14. 2635 N9 116. 0116 104. 8883 117. 1578 N2 218. 4288 06 22. 5441 -185. 3328 HI' 27. 7218 28. 6363 27. 8366 H8 17. 5050 18.3746 23. 7885 H5' 29. 8267 29. 8138 28. 3611 H5" 28. 8172 28. 4663 29. 4726 H2 27. 2772 26.8111

80 Μ. Monajjemi et αϊ. Main Group Metal Chemistry

Table 11 HF/6-3 lg** calculations of the oiso and η in ppm for atoms in Mg-nucleotides of AMP, GMP, IMP in GIAO method.

Mg(H20)4AMP Mg(H20)5GMP Mg(H20)5 IMP Atom Liso η oiso η oiso Η C5' 144. 5061 -9. 7789 145. 1925 -24. 3251 145. 0024 -7. 5581 C4' 123. 6273 0. 8465 123. 4419 -1. 4184 125. 2550 1. 1774 C3' 136. 9526 -0. 3746 136. 9227 0. 1897 137. 2476 -0. 8155 C2' 130. 8855 -1. 5054 129. 8743 -2. 3015 129. 2311 -0. 1918 cr 120. 2653 -1. 5456 120. 6305 -4. 4421 123. 3485 -13. 552 C6 44. 7806 -0. 2605 49. 4235 2. 2264 49.3871 1. 5051 C4 50. 8171 -0. 1410 54. 4096 0. 3329 54. 8476 0. 2411 C5 90. 3460 0. 2368 92. 2234 -0. 1922 84. 8698 0. 0008 C2 47. 9058 0. 8595 49. 6163 0. 3129 57. 4516 0. 9705 C8 54. 0725 0. 8294 58. 7669 0. 9481 54. 5663 0. 7496 HI' 26. 8355 -0. 5131 26. 9491 0. 6369 26.5481 -0. 4191 H8 22. 1625 -0. 2758 22. 4809 -0. 3742 20. 8858 -0. 5599 H5' 29. 1005 -4. 1671 29. 0996 -10. 2421 29. 1470 -4. 8204 H5" 29. 0301 -0. 9583 29. 3994 -2. 6977 29. 0076 0. 0029 H2 23. 6760 0. 0624 24. 3844 -0. 3525 N1 42. 0518 -0. 5860 136. 9376 -2. 4658 119. 3550 -1.5707 N2 213.5558 -6. 9605 N6 211.0508 6. 6683 N3 57. 8929 -0. 3825 118.6376 0. 3808 58.0880 0. 3871 N7 66. 1338 0. 3621 49. 4510 0.3177 47. 9794 0. 2714 N9 109. 2485 -0. 8786 105. 5702 -0. 7983 102. 3851 -0. 7024 03 245. 0582 0. 4330 247. 0149 -0. 2211 245. 7825 0. 4388 02 237. 0036 -1. 2467 241.1591 1.1360 242. 6505 -0. 2717 01 242. 5392 0. 9889 240. 1460 -3. 7735 245. 0212 16667 05' 288. 3617 -0. 6090 288. 4288 -0. 1532 288. 6309 -1. 0037 or 312.0232 0. 8686 310. 2797 0. 9766 309. 3895 2. 3049 06 22. 5441 -0. 1571 21. 0505 0. 0087 Ρ 413. 0295 -1. 0998 413. 8954 -1.3417 414. 0891 -0. 8282

81 Vol. 28, No. 2, 2005 Structural Studies and Investigation of NMR Shielding Tensors Coordination of Magnesium Hydrate to Purine

Table 12 Relative (to TMS)shifts in ppm for 13 C-NMR of AMP,GMP,IMP, using GIAO method at HF/6-31g**.

The 13C-NMR chemical shift(6=oiso TMS -aiso Sample). TMS: Isotropic carbon shielding tensor=203. 1557 and isotropic hydrogen shielding tensor=32. 3361 at HF/6-31g** and GIAO method.

Atoms AMP GMP IMP C6 129. 7134 155. 037 157. 2949 C4 162.124 148. 7461 164. 3276 C5 125.1815 112. 4523 119. 0842 C2 102. 3671 148. 5718 89. 4725 C8 158. 1066 153. 7747 148. 8769 HI' 4. 6143 3. 6998 4. 4995 H8 14. 8311 13. 9615 8. 5476 H5' 2. 5094 2. 5223 3. 9758 HS" 3.5189 3. 8698 2. 8635 H2 5. 5089 5.525

Table 13 Relative (to TMS)shifts in ppm for 13 C-NMR of purine complexes using GIAO method at HF/6-31g**. The

13C-NMR chemical shift(5=oiso TMS -oiso Sample). TMS: Isotropic carbon shielding tensor=203. 1557 and isotropic hydrogen shielding tensor=32. 3361 at HF/6-31g** and GIAO method.

Atoms Mg(H20)4AMP Mg(H20)4GMP Mg(H20)5IMP C5' 58. 6496 57. 9632 58.1533 C4' 79. 5284 79. 7138 77. 9007 C3' 66. 2031 66. 233 65. 9081 C2' 72. 2702 73. 2814 73. 9246 CI' 82. 8904 82. 5252 79. 8072 C6 158. 3751 153. 7322 153. 7686 C4 152. 3386 148. 7461 148. 3081 C5 112. 8097 110. 9323 118. 2859 C2 155.2499 153. 5394 145. 7041 C8 149. 0832 144. 3888 148. 5894 ΗΓ 5. 5006 5.387 5. 788 H8 10. 1736 9. 8552 11.4503 H5' 3. 2356 3. 2365 3. 1891 H5" 3. 306 2. 9367 3. 3285 H2 8.6601 7. 9517

82 Μ. Monajjemi et al. Main Group Metal Chemistry

We have found a deshielding of the H-8 proton of purine nucleotides by the 5'-phosphate group, but H8 at metalated AMP, GMP have been decreased and the metalated IMP have been increased (Tables 12 and 13). Chemical shifts to TMS of the C6,C2 atoms for metalated AMP and C4,C2 for metalated IMP are larger than nonmetalated.

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