1/7/2024 0 Comments Amino functional groupThis is certainly one of the reasons why NMR spectra always give too low activation energies, even in organic solvents which are never free from some traces of water. Otherwise, the rate of the amino rotation k r will be systematically overestimated and the activation energy E a will be, by way of consequence, systematically underestimated. Even if this process is generally low (less than 2.1 s − 1 in the present case) as compared to the hindered amino rotation (about 80–90 s − 1 here), it should be taken into proper account in the line shape analysis. In their careful measurements on the N6 amino group of A by 15N-edited magnetisation transfer experiments, Michalczyk and Russu 420 have seriously stressed upon the spurious contribution from the amino proton exchange with water. 319 Our feeling is founded on the idea that this barrier should not be smaller in Pys than in Pus in view of our previous discussion about the distinct ring current effects in the two kinds of compounds (see above). ![]() It is very difficult to propose a plausible explanation for this divergence, but we are inclined to think that the barrier is certainly greater than that deduced by Martin and Reese. 315 for a series of cytosine derivatives with methyl-substituted amino groups which are also in the previous range (Δ H ≠ = 15–18 kcal mol − 1). This last one is clearly in the calculated range and does not differ much from that data reported by Shoup et al. 315 An indication to this disagreement should be found in the result given by Jordan 320 for 4-aminopyrimidine in water at pH 1.56 and 12 ☌ and for which a value Δ H ≠ = 14.6 kcal mol − 1 is reported. 422 The only divergence seems to appear with the Pus for which some reported data are largely outside this range, being sometimes greater by a factor of 2. It is noticeable that this interval nicely encompasses the one which is predicted by semi-empirical SCF-MO-CNDO/2 and EHT theoretical calculations (12–16 kcal mol − 1). There is large agreement for the activation energy of the Pus, where all experimental results, 316–319,419,420 generally obtained in water and by the classical line shape analysis, 421 are in the range Δ H ≠ = 11–18 kcal mol − 1. ![]() ![]() It is then of special interest to know the activation energy of this restricted rotation. The non-planarity of the amino groups with the base plane has also been recognised for a long time and water can influence both its non-planarity and the rotation around the C–N bonds. Their exposure in the grooves of double-helical structures makes them privileged targets for binding with drugs as well for specific interaction with water. Bernard Ancian, in Annual Reports on NMR Spectroscopy, 2010 6.4 Rotation barrier about the exocyclic amino bondĮxocyclic amino groups of adenine, guanine and cytosine are key sites in the formation of hydrogen bonding for maintaining canonical Watson–Crick pairing of the bases in DNA and RNA.
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