Tautomer structures

Hydrogen bonding plays a major role in pyrazolone tautomerism, and the formation of a chelate structure can shift the equilibrium towards the chelated form. Structures (135) and (136) are two representative examples of such stabilized tautomers. Structure (137) is a hypothetical example of stabilization of the NH tautomer. 

The NH-tautomer structure. The solution of the question of internal protons location in each tautomer of the investigated porphyrins with non-symmetrical substitution will result in obtaining definite correlations between tautomer spectra and their structures. Here we present such information for compound 10. 

Near IR spectra—See Electronic absorption spectra New IR spectra, R sphaeroides reaction centers, 207,208f NH-tautomer structure, porphyrins with nonsymmetrical substitutions, 84 NH-tautomerism, porphyrins with nonsymmetrical substitution, 74-93,89-91 Ni(II)-reconstituted hemoglobin—See Nickel-reconstituted heme proteins Nickel isobacteriochlorin... 

There are numerous other modifications of nucleobases possible, many of which are biologically relevant in that they lead to alternations in p/fa values, changes in tautomer structure as well as their base pairing patterns (55). Examples include ligands such as N6-methoxyadenine, which is a promutagen (66, 67) as well as 8-oxoadenine and 8-oxoguanine (68), which are known mutagens. 

Scenario 2 applies to a completely different situation in which a metal ion replaces a proton normally involved in the proper tautomer structure of the... 

FIGURE 14. 5 Example of tautomer structure canonicalization. In reality tautomers can exist as mixtures. 

H atom addition, protonation state, active site definition, His tautomers, structural water, pharmacophore definition etc. 

Usefulness of IR and NMR data in assigning tautomer structure was further validated by structural elucidation of dihydropyrimidine 19 based on Silversmith s data.144 Its structure was assigned as the 1,6-tautomer 19b, based on a reinvestigation of its IR band at 1652 cm-1, similar to the IR spectra of 20b. This was later confirmed by X-ray analysis.23... 

Scheme 12. Structures of different tautomer structures of 1,2-diazoles...

DCC treatment of the thiourea (57 X = CH, N), prepared from the amine (56) by reaction with benzoylisothiocyanate followed by base hydrolysis, affords the tetracyclic derivatives (58) which are claimed to possess considerable antitumour activity in vitro <94H(38)1213>. In an alternative synthesis, (58) may be prepared by methylation of the thiourea group in (57) with methyl iodide followed by treatment with K2CO3 in DMF. The imino-tautomer structure is base upon NMR and infrared spectroscopic evidence. 

PMR measurements show that humulone exists principally as a dienol. Of the possible tautomer structures (la d), structure la is thought to represent the major tautomer structures lb and Id are excluded on the basis of optical rotatory dispersion measurements and la was preferred over Ic by comparison with model compounds [11]. 

Tautomers Structural isomers that differ in the location of their hydrogen and double bonds. 

Thalheim T, Vollmer A, Ebert R-U, Kuhne R, Schuurmann G (2010) Tautomer identification and tautomer structure generation based on the InChl code. J Chem Inf Model 50(7) 1223-1232. doi 10.1021/cil001179... 

Another hydroxy acid has also been prepared (structure 2) but this is less stable and equilibrates in solution with its open-chain tautomer (structure 3)/ ... 

Table 5.1 Numerical values of chemical shifts for tautomer structures...

---