Geometric consequences of electron delocalization for adenine tautomers in aqueous solution

Purinergic system in cancer stem cells

Accumulating evidence supports the idea that cancer stem cells (CSCs) are those with the capacity to initiate tumors, generate phenotypical diversity, sustain growth, confer drug resistance, and orchestrate the spread of tumor cells. It is still controversial whether CSCs originate from normal stem cells residing in the tissue or cancer cells from the tumor bulk that have dedifferentiated to acquire stem-like characteristics. Although CSCs have been pointed out as key drivers in cancer, knowledge regarding their physiology is still blurry; thus, research focusing on CSCs is essential to designing novel and more effective therapeutics. The purinergic system has emerged as an important autocrine-paracrine messenger system with a prominent role at multiple levels of the tumor microenvironment, where it regulates cellular aspects of the tumors themselves and the stromal and immune systems. Recent findings have shown that purinergic signaling also participates in regulating the CSC phenotype. Here, we discuss updated information regarding CSCs in the purinergic system and present evidence supporting the idea that elements of the purinergic system expressed by this subpopulation of the tumor represent attractive pharmacological targets for proposing innovative anti-cancer therapies.

Influence of the Solvent on the Stability of Aminopurine Tautomers and Properties of the Amino Group

Amino derivatives of purine (2-, 6-, 8-, and N-NH2) have found many applications in biochemistry. This paper presents the results of a systematic computational study of the substituent and solvent effects in these systems. The issues considered are the electron-donating properties of NH2, its geometry, π-electron delocalization in purine rings and tautomeric stability. Calculations were performed in ten environments, with 1 < ε < 109, using the polarizable continuum model of solvation. Electron-donating properties were quantitatively described by cSAR (charge of the substituent active region) parameter and π-electron delocalization by using the HOMA (harmonic oscillator model of aromaticity) index. In aminopurines, NH2 proximity interactions depend on its position and the tautomer. The results show that they are the main factor determining how solvation affects the electron-donating strength and geometry of NH2. Proximity with the NH∙∙∙HN repulsive interaction between the NH2 and endocyclic NH group results in stronger solvent effects than the proximity with two attractive NH∙∙∙N interactions. The effect of amino and nitro (previously studied) substitution on aromaticity was compared; these two groups have, in most cases, the opposite effect, with the largest being in N1H and N3H purine tautomers. The amino group has a smaller effect on the tautomeric preferences of purine than the nitro group. Only in 8-aminopurine do tautomeric preferences change: N7H is more stable than N9H in H2O.

Effect of the Solvent and Substituent on Tautomeric Preferences of Amine-Adenine Tautomers

Adenine is one of the basic molecules of life; it is also an important building block in the synthesis of new pharmaceuticals, electrochemical (bio)sensors, or self-assembling molecular materials. Therefore, it is important to know the effects of the solvent and substituent on the electronic structure of adenine tautomers and their stability. The four most stable adenine amino tautomers (9H, 7H, 3H, and 1H), modified by substitution (C2- or C8-) of electron-withdrawing NO2 and electron-donating NH2 groups, are studied theoretically in the gas phase and in solvents of different polarities (1 ≤ ε < 109). Solvents have been modeled using the polarizable continuum model. Comparison of the stability of substituted adenine tautomers in various solvents shows that substitution can change tautomeric preferences with respect to the unsubstituted adenine. Moreover, C8 substitution results in slight energy differences between tautomers in polar solvents (<1 kcal/mol), which suggests that in aqueous solution, C8-X-substituted adenine systems may consist of a considerable amount of two tautomers-9H and 7H for X = NH2 and 3H and 9H for X = NO2. Furthermore, solvation enhances the effect of the nitro group; however, the enhancement strongly depends on the proximity effects. This enhancement for the NO2 group with two repulsive N···ON contacts can be threefold higher than that for the NO2 with one attractive NH···ON contact. The proximity effects are even more significant for the NH2 group, as the solvation may increase or decrease its electron-donating ability, depending on the type of proximity.

Impact of the Substituents on the Electronic Structure of the Four Most Stable Tautomers of Purine and Their Adenine Analogues

Substituent effects at the C2-, C8-, and N-positions of adenine and purine on the structural and π-electronic changes in their four tautomers were studied using the B97D3/aug-cc-pvdz computational level. The effect of various substituents (NO2, CN, CHO, Cl, F, H, Me, OMe, OH, and NH2) was characterized by the charge of the substituent active region (cSAR) approach and Hammett substituent constants σ. It has been found that for both adenine and purine derivatives, substituents from the C8-X position have a stronger influence on their electronic structure than from the C2-X and N-X positions. The presence of the amino group in adenine enhances the substituent effect compared to that which occurs in purine. In addition, its electronic structure is more sensitive to the effect of the substituent in 3H and 1H than in the 9H and 7H adenine tautomers. For a given substituent, a large variation in cSAR(X) values is observed, strongly dependent on the substitution position. For 7H and 9H adenine tautomers for C8-X systems, substituents reduce the aromaticity of the five-membered rings but increase the aromaticity of the six-membered rings.

Whether 2-aminopurine induces incorporation errors at the DNA replication? A quantum-mechanical answer on the actual biological issue

In this paper, we consider the mutagenic properties of the 2-aminopurine (2AP), which has intrigued molecular biologists, biophysicists and physical chemists for a long time and been widely studied by both experimentalists and theorists. We have shown for the first time using QM calculations, that 2AP very effectively produces incorporation errors binding with cytosine (C) into the wobble (w) C·2AP(w) mispair, which is supported by the N4H⋯N1 and N2H⋯N3 H-bonds and is tautomerized into the Watson-Crick (WC)-like base mispair C*·2AP(WC) (asterisk denotes the mutagenic tautomer of the base), that quite easily in the process of the thermal fluctuations acquires enzymatically competent conformation. 2AP less effectively produces transversions forming the wobble mispair with A base - A·2AP(w), stabilized by the participation of the N6H⋯N1 and N2H⋯N1 H-bonds, followed by further tautomerization A·2AP(w) → A*·2AP(WC) and subsequent conformational transition A*·2AP(WC) → A*·2AP_syn thus acquiring enzymatically competent structure. In this case, incorporation errors occur only in those case, when 2AP belongs to the incoming nucleotide. Thus, answering the question posed in the title of the article, we affirm for certain that 2AP induces incorporation errors at the DNA replication. Obtained results are consistent well with numerous experimental data.

Whether the amino–imino tautomerism of 2-aminopurine is involved into its mutagenicity? Results of a thorough QM investigation

2AP* mutagenic tautomer is able to induce only one incorporation error – transversion – by pairing through the H-bonds into the G·2AP* mispair.

Geometric and energetic consequences of prototropy for adenine and its structural models – a review

Prototropy for adenine and its convenient models causes parallel changes of geometric (HOMED) and energetic (ΔE) parameters for neutral tautomers.