1.2.4- Triazines protonation sites

Experimental data concerning protonation sites for monocyclic 1,2,4-triazines are still not sufficient to make any generalization of this phenomenon. On the basis of ultraviolent (UV) spectroscopic studies it has been... 

A further ring nitrogen atom in the 5-position in 2,4-diamino-1,3,5-triazine leads to a considerable reduction in basic strength (pATj = 3 91) and the site of protonation remains N-1 (or the equivalent N-5) (Roth and Strelitz, 1969). The effect of 6-substituents on pAf, correlates with Hammett s a -constants, and it is claimed that calculations of TT-electron densities and nmr spectra indicate protonation at N-3, rather than at N-1 as in the unsubstituted compound (Marimoto, 1966). 

The chemistry and physical properties of various -triazines are summarized in Tables A1 and A2 of the Appendix. The triazines are Lewis bases as the ring nitrogen (N) atoms may donate electron pairs for the formation of covalent bonds. In aqueous systems, triazines exist as either neutral or protonated (cationic) forms depending on the pKa of the compound and the pH of the system. The most basic ring N and the most likely site of protonation are located in the 5 position between the electron-rich alkylamino side chains (Figure 21.1). 

A precise description of bonding between triazines and humic substances is complicated by the extreme heterogeneity of humic substances. However, it is clear that all of the above mechanisms contribute to the sorption of triazines and that two or more mechanisms may contribute to the interaction energy for a given molecule. The stereo chemistry of each potential binding site determines which mechanisms are involved. Figures 21.2 and 21.3 summarize the types of interactions that may contribute to the retention of chloro-.v-triazines and protonated-keto-triazines, respectively. 

The abiotic hydrolysis of triazines in soil environments is catalyzed by acidic sites on the surfaces of both organic and inorganic soil constituents. The surfaces of soil constituents have both Lewis acid sites (which accept electron pairs) and Bronsted acid sites (which donate protons). However, triazines are not competitive with water and OH groups for complexation with Lewis acid sites, so in soil environments hydrolysis is catalyzed primarily by Bronsted acid sites. Four types of Bronsted acid sites are found on soil surfaces (Mortland, 1970) ... 

The CNDO/2 method has been used for the calculation of the sites of the protonation and alkylation of 1,2,4-triazines <88KGS525>. It was also applied to calculate the two-photon tensor invariants proportional to two-photon absorptivities for transitions to the lowest singlet states, Lb (B2 -) and L/ (B,/) <84CPL(107)125>. 

This also suggests that each triazine molecule can undergo hydrogen bonding with two sites. The N of the alkyamino groups are repulsive to the proton, whereas the N atom in position 5 on the ring is somewhat attractive. Nitrofen, which is not a DCMU-type inhibitor, presents a different case the molecule is somewhat twisted, so the interaction map depends upon the relative position between point charge and the nitrofen molecule. 

The acid-sensitive l,4,5,6-tetrahydro-l,2,3-triazine 11 and its N-l-protonated form (although not stabilized by charge delocalization) were investigated within the HF approximation. Total energies of —279.351 69 (SCF 3-21G) and —280.943 554 (SCF 6-31G ) hartree for the neutral and —279.713 64 hartree (SCF 3-21G) for the protonated form were calculated. N-1 is the preferred site of protonation. Optimized geometrical parameters for the neutral and the N-l-protonated species were given <1991JA7893>. 

For 1,2,3-triazine 1 and 1,2,3-benzotriazine 2, N-2 as the preferred site of protonation has been predicted from theoretical investigations (see Section 9.01.2.12). The preparation of stable 4- and 5-phenyl-l,2,3-triazin-2-ium tetrafluoroborates and their spectral characterization have been described <2003S413>. Regarding the protonation of the acid-sensitive l,4,5,6-tetrahydro-l,2,3-triazine 11, see Section 9.01.2.15. 

In Rps. viridis no site-directed mutants are available. However, a number of herbicide-resistant mutants have been characterized (Sinning et al., 1989 Ewald et al., 1990). Herbicides of the triazine type compete with Qb for the binding pocket and their binding is controlled by nearby residues which also are potential candidates for proton donors to reduced Qb- Recently, we have characterized the kinetics and pH-dependence of the electron transfer in the quinone acceptor complex in Rps. viridis wild type (Wt). To get further insight into the mechanisms of quinone reduction we studied these reactions in herbicide-resistant mutants. In this contribution we present a first glance on these experimental results. 

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