Tautomers preferential stabilization

Fig. 13. The Ne-H tautomer of histidine is favored for the free amino acid, but within a protein structure either tautomer may be preferentially stabilized due to hydrogen bond arrangements and other environmental factors. For example, the NS-H tautomer is a frequently observed zinc ligand (Chakrabarti, 1990c).

Crampton KT, Rathur AI, Nei YW, Berden G, Oomens J, Rodgers MT (2012) Protonation preferentially stabilizes minor tautomers of the halouracils IRMPD action spectroscopy and theoretical studies. J Am Soc Mass Spectrom 23 1469-1478... 

Hydroxy-1-benzoselenophene (22) and its 2,2-disubstituted derivatives exist preferentially as the oxo tautomer (23) (Scheme 3). A similar effect was observed for 3-hydroxyselenophene. Introduction of the acetyl group at the 2-position promotes enolization and for 2-acetyl-3-hydroxy-1-benzoselenophene the enol (24) not oxy form (25) is preferred, presumably by virtue of the extra stability by intramolecular hydrogen bonding. The broad IR absorption band oOH... Q = 3005-1930 cm 1 and the band of the exocyclic carbonyl group oc=0 = 1583 cm-1 both shifted to lower frequencies, as well as the signal for the acid proton (S = 12.86 ppm) in the H NMR spectrum provide evidence for structure 24. The same effects were observed for other 2-acyl-3-hydroxy-l-benzoselenophenes [47, 48],... 

A polar solvent like water introduces a dramatic shift in the tautomeric preferences of cytosine, leading to a stabilization of the keto-amino tautomer. Thus, experimental data precludes the existence of significant amount of imino or enol tautomers of cytosine in aqueous solution [144]. MC-FEP and SCRF calculations [118] indicate that the preferential solvation of the keto-amino tautomer in front of the keto-imino and enol-amino tautomers amounts to around 4 and 7 kcal/mol. Therefore, the enol and imino tautomers of cytosine are predicted to be 6-7 kcal/mol less stable than the canonical keto-amino tautomer [118]. 

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