- Ionic Probes of Aromaticity in Annelated Rings
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Ionization by the deprotonation of benzene and pyridine, and by the protonation of pyridine, involves lone pairs in the ? plane without significant ? effects.In these cases annelation by a benzene ring increases the acidity or proton affinity by a constant 6+/-1 kcal/mol, ascribed to increased polarizability.In comparison, protonation of benzene and deprotonation of cyclopentadiene disrupts or creates a 6-electron aromatic system, respectively, and in their annelated derivatives, naphthalene and indene, a secondary 4-electron conjugated ? system.These cases involving ? electrons show annelation effects that are substantially larger (13.4 kcal/mol) or smaller (1.0 kcal/mol), respectively, than just the electrostatic effect.Analysis of these data suggests that the stability of secondary 4-electron system in the annelated rings is smaller by 6+/-1 kcal/mol than the aromatic 6-electron systems, in fair agreement with Herndon's structure-resonance values for these species.Annelation effects are reproduced well by Dewar's AM1 semiempirical method.
- Meot-Ner (Mautner), Michael,Liebman, Joel F.,Kafafi, Sherif A.
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p. 5937 - 5941
(2007/10/02)
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- Models for Strong Interactions in Proteins and Enzymes. 1. Enhanced Acidities of Principal Biological Hydrogen Donors
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The acid dissociation energies of several key biological hydrogen donors are found to fall into a narrow range, ΔHoacid=352-355 kcal/mol.The strong acidities of these donor groups enhance the hydrogen bond strengths involved in the protein α-helix, imidazole enzyme centers and DNA.Specifically, the peptide link is modeled by the dipeptide analogue CH3CO-Ala-OCH3.Its acidity is strengthened, i.e. ΔHoacid is decreased by 8 kcal/mol compared with other amides, due to electrostatic stabilization by the second carbonyl in the peptide -CON-CH(CH3)CO- grouping.The acidity of imidazole is also strengthened by 8 kcal/mol compared with that of the parent molecule, pyrrole, primarily due to resonance stabilization of the ion.Hydrogen donor NH2 groups of adenine and cytosine are modeled by 4-aminopyrimidine, and the acidity of this amine group is strengthened by ring aza substitution.An intrinsic acidity optimized for hydrogen bonding strength therefore emerges as a common property of the diverse hydrogen donors in the protein α-helix, enzymes and DNA.This property may therefore be in part responsible for the natural selection of these molecules as principal biological hydrogen donors.
- Meot-Ner (Mautner), Michael
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p. 3071 - 3075
(2007/10/02)
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