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Relevant academic research and scientific papers

Ionic Probes of Aromaticity in Annelated Rings

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.

Models for Strong Interactions in Proteins and Enzymes. 1. Enhanced Acidities of Principal Biological Hydrogen Donors

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.