Effect of stabilizing groups on the extent of long range .pi. electron participation in carbanions

Origins of Stabilization and Evidence for Charge Delocalization in the Bicyclo[3.2.1]octadienyl Anion and Related Species

Bridged bicyclic allylic anions with ethene and allyl units in proximity possess enhanced stability over analogues lacking the ethene bridge. Experimental observations have encouraged much computational effort, and for many years claims for homoaromatic behaviour in anions were disputed. More recently, and especially with the advent of quantum mechanics methods that assess magnetic susceptibility, homoaromaticity has become accepted. This paper provides a review of work on this topic to date, and additional density functional theory calculations with the purpose of providing a general overview. The presence of homoaromaticity in anions of this class is supported, and their stability is augmented by inductive and counterion effects.

Dimethylsulfate methylation of guanine residues in mammalian DNA: inverse correlation between methylation susceptibility and mutagenesis by bromodeoxyuridine and thymidine

We have previously demonstrated that mutagenesis by bromodeoxyuridine (BrdU) and thymidine (dT) in mammalian cells occurs with a high degree of sequence specificity within runs of multiple adjacent guanine residues. To determine whether there is a structural component to this sequence specificity, we have analyzed stereochemical properties of guanine residues in different sequence contexts. Stereochemical differences were assessed by measuring the susceptibility of individual guanine residues to methylation by the agent dimethylsulfate (DMS). The results from this study suggest that there is a strong inverse correlation between susceptibility of various guanine residues to DMS methylation and the susceptibility of those residues to mutagenesis by BrdU and dT. These results suggest that the stereochemical attributes of guanine residues in different sequence contexts affect the susceptibility of those guanine residues to mutagenesis by BrdU and dT.