Abstract
Intramolecular hydrogen bonding interactions in 1,4-dihydroxy-5,8-naphthoquinone imine ( 1a), its tautomers ( 2aâ 4a), and the rotamers of 1aâ 4a, and the utility of the SAM1 semiempirical method, in comparison to ab initio HartreeâFock (HF) (as well as AM1 and PM3) methods, for the study of such interactions were evaluated. The NâŻHâO hydrogen bonds can be characterized as strong and were found to be stronger than, or comparable to, the strength of the OâŻHâO hydrogen bonds. This finding is in contrast with an earlier report based on NMR experimental data on 5-iminodaunomycin ( II) in which the OâŻHâO hydrogen bonds were reported to be the strongest (G.L. Tong, D.W. Henry, E.M. Acton, J. Med. Chem., 22 (1979) 36; J.W. Lown, H.H. Chen, J.A. Plambeck, Biochem. Pharma., 28 (1979) 2563). The relative stabilities of the tautomers (all local minima) predicted by SAM1 and HF/6-31G ââ were: 4a> 2a> 1a> 3a; 4a> 1a> 2a> 3a, respectively). 2a and 3a were not found to be saddle points whereas the structures analogous to 2a and 3a in the naphthazarin system were reported to be so. All the methods used predicted the imidine form ( 4a) to be the most stable tautomer. The above findings suggest the interpretation of the experimental data on the biochemical reactivity of II should be re-examined. The SAM1 barrier energies for tautomerization by intramolecular proton transfer were substantially lower than, or comparable to the HF/6-31G ââ ones. The SAM1 trend in stability of the tautomers and some of the SAM1 barrier energies and other hydrogen-bond parameters are found to be qualitatively more in line with preliminary DFT results, and thus, the SAM1 trend in stability is probably correct. However, the overall accuracy of the HF/6-31G ââ and the SAM1 methods for the study of hydrogen bonding in these systems appears to be unsatisfactory.
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