About guanazole molecule

Guanazole molecule is of multiple interests - chemical (as a useful synthetic intermediate), theoretical (the tautomeric equilibrium), biological (it is biologically active), medicinal (it is an anti-tumor agent) and space- science related (it is a prebiotic compound that can serve as an alternative base in early genetic systems). At the core of its chemical and biological behavior are the H-bonding ability and the reactivity of its various lone-electron pairs on its nitrogens.

Outline of the experiment

One needs to build each of the tautomers of guanazole, minimize its energy by different computational methods that are available in the Spartan software package, calculate atomic charges, dipole moment, electron densities, and determine bond lengths and dihedral angles. Based on the calculated properties, one hopes to predict a particular chemical behavior of each tautomer.

The Guanazole Tautomers

Five tautomeric forms of guanazole are shown in the following Figure:

Guanazole is 3,5-diamino-1,2,4-triazole, which exists in the crystalline form as tautomer I (see Figure), which is a diamino asymmetric form (Ref. 1). The tautomer I is aromatic.

The tautomer II is also aromatic, and is a diamino symmetric form.

The tautomer III is diimino (two C=N-H groups)-hydrazino (-NH-NH-) form.

The tautomer IV is amino-imino-hydrazino species.

Finally, the tautomer V is amino-imino form.

Some calculations one would want to do are on the type of hybridization of nitrogens and on the aromatic properties: 

- geometry of the hydrazino group: Is H-N-N-H planar (i.e. the dihedral angle 
  of 0^o)? 
- bond lengths in the aromatic forms: do they support the notion of aromaticity?

Basicity of Guanazole

Guanazole's pKa in H2O is 4.43 (Ref. 2), similar to that of aniline (pKa= 4.6, Ref. 3). It is generally known that aliphatic amines have basicity comparable to dilute solutions of NaOH 
(e.g. n-propylamine has pKa of 10.6). Voronkov et al. (Ref. 2) concluded that protonation of guanazole occurs at N-4 of form I, namely at the ring nitrogen, and not on the exocyclic NH2.

Some calculations one would want to do: 

- What are the electrostatic charges on N-4 vs. those on the exocyclic amino group? Are the calculated charges good predictors of the observed basicity?

Nucleophilicity of Guanazole

Fuentes and Lenoir (Ref. 4) found that alkylation of guanazole, which they represented as tautomer I, in non-aqueous medium (CH3OH), with CH3I, with CH3O-Na+ as a base, gives 1-methylguanazole. They also found that when N-1 is substituted, the exocyclic C-3 NH2 group reacts as a nucleophile, and is more reactive than the other, C-5 NH2 group. Chang et al. (Ref. 5) established the order of reactivities of nitrogen centers in guanazole for an acylation reaction in a non-aqueous solvent. They found the following decreasing order: NH, NH2 (C-3), NH2 (C-5), which parallels that of Fuentes and Lenoir.

Some calculations one would want to do:

- Look at the electrostatic charges at N-1, NH2 (C-3), and NH2 (C-5), and determine if they are good predictors of the observed nucleophilic reactivity.

Guanazole as a Prebiotic Molecule

Guanazole has been established as an important prebiotic molecule whose hydrogen-bonding pattern mimics that of diaminopyrimidine (Ref. 6). Based on this, it was hypothesized that guanazole could be a part of an early pre-RNA self-replicating system. In light of the new discovery (Ref. 7-9) that guanazole readily makes complexes with metals, such as Zn, Co, Cd, Ni, Fe, and Ag, it is conceivable that such complexes played role as prebiotic catalysts, perhaps as "proto-enzymes".

In the studies of complexation of guanazole with metals, it was proposed that complexation occurs via ring nitrogens N-2 and also N-4 of tautomer I. The ability to complex was linked to the electron density on these ring nitrogens. (Ref.7,9).

Some calculations one would want to do:

- Repeat the literature calculations, which were done by a PM3 method (Ref. 9), but by using a different, higher-level calculational method. Find out if the relative electrostatic charges on the guanazole nitrogens are dependent on the mode of calculation and if they indeed predict complexation mode at the higher level of calculations.

Guanazole as an Anti Tumor Agent

Guanazole exhibits anti tumor activity against mouse L1210 leukemia (Ref. 10) and K1964, the mast cell tumor P815, Walker 256 carcinosarcoma, and reticulum cell sarcoma A-RCS (Ref. 11). Guanazole's anti tumor activity is based on its inhibition of DNA synthesis. Guanazole inhibits ribonucleotide reductase by inactivating the tyrosine free radical (Ref. 12). Guanazole has been evaluated in the clinic as potential anti tumor agent (Ref. 12,13). The structure-activity studies have been performed on guanazole (in vitro, against cell lines of leukemia L1210; activity = % inhibition of cell growth)(Ref. 11). The following was found: i) elimination of the amino moiety in positions 3 and 5 virtually abolishes activity; ii) elimination of just one amino group (e.g. from position 5, thus giving 3-amino-1,2,4-triazole), markedly reduces, but does not abolish the activity; iii) if NH2 group is at N-4 (4-amino-1,2,4-triazole), there was no cytotoxic activity against leukemia L1210; iv) if NH2 group is added at N-4 of guanazole (3,4,5-triamino-1,2,4-triazole), the cytotoxic activity is less than that of guanazole itself.

Calculations one may want to do:

- One may want to do calculations of the molecular features of guanazole and its less active or inactive derivatives (e.g. electron density, H-bonding ability, the distances between particular amino groups, planarity or non-planarity of the ring, etc.) and get an idea about the "active" space- and electronic features around this molecule. Another approach would be to look at the one-electron transfer donation ability of guanazole and the stability of a radical that remains. The electron-donating ability of guanazole could perhaps explain its inactivation of the tyrosine free radical.

References

1) G. L. Starova, O. V. Frank-Kamenetskaya, V. V. Makarskii, and V. A. Lopyrev, Kristallografiya, 25, 1292-1295 (1980). 

2) M. G. Voronkov, T. V. Kashik, V. V. Makarskii, V. A. Lopyrev, S. M. Ponomareva, and E. F. Shibanova, Dokl. Acad. Nauk SSSR, 227, 282-285 (1976). 

3) A. Streitweiser, Jr., and C. H. Heathcock, "Introduction to Organic Chemistry", Third Ed., Macmillan Publ. Co., New York, 1985, p. 693. 

4) J. J. Fuentes and J. A. Lenoir, Can. J. Chem., 54, 3620-3625 (1976). 

5) H. Chang, K. Kim, M. Ree, and K.-W. Lee, Macromol. Chem. Phys., 200, 422-430 (1999). 

6) V. M. Kolb, J. P. Dworkin, and S. L. Miller, J. Mol. Evol., 38, 549-557 (1994). 

7) M. Gabryszewski and B. Wieczorek, Pol. J. Chem., 73, 2061-2066 (1999). 

8) M. I. Barmin, E. L. Kasatikova, I. B. Karaulova, and V. V. Mel'nikov, Russ. J. Coord. Chem., 22, 434-437 (1996). 

9) M. Gabryszewski and B. Wieczorek, Pol. J. Chem., 72, 2352-2355 (1998). 

10) R. W. Brockman, S. Shaddix, W. R. Laster, Jr., and F. M. Schabel, Jr., Cancer Res., 30, 2358-2368 (1970). 

11) M.-A. Hahn, and R. H. Adamson, J. Natl. Cancer Inst., 48, 738-790 (1972). 

12) W. B. Parker, "Inhibitors of Nucleoside Diphosphate Reductase", in W. O. Foye, Ed., "Cancer Chemotherapeutic Agents", pp. 68-71 (1995), American Chemical Society, Washington, D.C., and the references cited therein. 

13) N. Gerber, R. Seibert, D. Desiderio, R. M. Thomson, and M. Lane, Clinical Pharmacology Ther., 14, 264-270 (1973).

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