Abstract
Substituent effects at the C2-, C8- and N-positions of adenine and purine in their four the most stable tautomers are studied by means of B97D3/aug-cc-pvdz computation applying substituents of varying electronic properties: NO2, CN, CHO, Cl, F, H, Me, OMe, OH and NH2. The substituent effect is characterized by the substituent effect stabilization energy (SESE) and substituent Hammett constant σ. For adenine, SESE is obtained with purine as the reference system. Additionally, for both adenine and purine, SESE characteristics are estimated with benzene, imidazole and amino-pyrimidine as reference systems, when possible, taking into account substitution in topologically equivalent positions. The role of a C6–NH2 group in adenine in modifying the substitution effect is observed and discussed. Additionally, the proximity effect for some asymmetric substituents (e.g. CHO, OMe) is recognized and meticulously analyzed.
Highlights
Adenine is one of ve building blocks of DNA and RNA nucleic acids[1] and is a simple amino derivative of purine
Adenine and purine are closely related to each other structurally, differing only in the amino group in the position 6 of purine. Their four most stable tautomers are realized by migration of the proton between the nitrogen atoms in positions 9, 7, 3 and 1 and are named 9H, 7H, 3H and 1H, respectively (Scheme 1)
The energy changes due to the presence of the most electronaccepting and electron-donating substituents (NO2 and NH2, respectively) in purine and adenine are presented in Fig. 1; the data are shown in the order: 9H, 7H, 3H and 1H
Summary
Adenine is one of ve building blocks of DNA and RNA nucleic acids[1] and is a simple amino derivative of purine. There are possibilities of tautomeric forms in which the exocyclic amino-group or carbon atoms are involved, but these tautomeric rearrangements are highly energetically unfavorable.[3] Proton transfers in tautomeric rearrangements are always accompanied by the migration of double bonds, that is, changes in p-electron structure.[4] As a result, it can be expected that the different sensitivity of a given position to the substituent effect depends on the type of tautomer considered This is the reason why the impact of the substituent effect on various properties of purine or adenine systems has been studied. This allowed to determine the association constants between variously substituted 1-cyclohexyluracil derivatives and 9-
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