Tautomerism in nucleotide bases is one of the possible mechanisms of DNA mutation. In spite of numerous studies on the structure and energy of protonated cytosine tautomers, little information is available on the process of their intra- and intermolecular tautomerizations. The catalytic ability of HO, HCOOH, and the HCOOHHO group to facilitate the tautomerism of the Cyt2t to CytN3 isomer has been studied. It is shown that the activation free energies of tautomerism in the gas phase are 161.17, 58.96, 26.06, and 15.69 kJ mol, respectively, when the reaction is carried out in the absence and presence of HO, HCOOH, or the HCOOHHO group. The formation of a doubly hydrogen bonded transition state is central to lowering the activation free energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. In the aqueous phase, although the solvent effects of water significantly decrease the activation free energy of intramolecular tautomerization, the isomerization of the Cyt2t to CytN3 isomer remains unfavorable, and the HCOOH and HCOOHHO group mediated mechanisms are still more favorable. Meanwhile, conventional transition state theory (CTST) followed by Wigner tunneling correction is then applied to estimate the rate constants. The rate constant with Wigner tunneling correction for direct tautomerization is obviously smaller than that of HCOOH-mediated tautomerization, which is the most plausible mechanism. Finally, another important finding is that the product complex (CytN3HCOOH) is in the rapid tautomeric equilibrium with the reaction complex (Cyt2tHCOOH) (τ = 3.84 × 10 s), which is implemented by the mechanism of the concerted synchronous double proton transfer. Its lifetime of the formed CytN3HCOOH complex (τ = 8.33 × 10 s) is almost one order of magnitude larger than the time required for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication (several ns), which is further dissociated into the CytN3 and HCOOH monomers. The results of the present study demonstrate the feasibility of acid catalysis for DNA base isomerization reactions that would otherwise be forbidden.
Graphyne (GY) and functionalized GY have become the cutting edge research among the scientific community. The rare bases -cytosine (Cyt), 5-methylcytosine (5-meCyt), 5-hydroxymethylcytosine (5-hmCyt), 5-formoxylcytosine (5-fCyt), and 5-carboxylcytosine (5-caCyt) adsorbing...
Eutrophication often leads to the periodic proliferation of harmful cyanobacterial blooms (HCBs), which threaten the sustainability of freshwater ecosystems and lead to serious environmental, health and economic damage. Hence, it is vitally important to take effective measures to manage HCBs and associated problems. In this study, vertical flow constructed wetlands (CWs) were operated under different hydraulic loading rates (HLRs) to treat a hyper-eutrophic water body with HCBs. Six sampling ports (representing different layers) were evenly distributed along the water flow direction to study the purification processes of CWs. With HLRs ranging from 0.2 m/d to 0.8 m/d, total nitrogen (TN), total phosphorus (TP), COD, total suspended solid (TSS) and Chlorophyll a (Chl.a) were efficiently treated by CWs, and they were mainly removed at the second layer of CWs. The concentrations of two cyanobacterial metabolites (geosmin and β-cyclocitral) in the effluent were mostly below their odorous threshold concentrations. As the HLRs increased, the treatment efficiencies of the CWs decreased gradually. There was no removal of TP, Chl.a, geosmin, or β-cyclocitral at an HLR of 1.0 m/d. Under suitable HLRs, this type of CW could provide a promising way to control HCBs and associated odorous problems in hyper-eutrophic water bodies.
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