Guanine-Cytosine Base Pairs in Parallel-Stranded DNA:An ab Initio Study of the Keto-Amino Wobble Pair versus the Enol-Imino Minor Tautoemr Pair

摘要

Doubly hydrogen bonded,"reverse Watson-Crick" thymine-adenine base pairs make possible the formation of parallel-stranded DNA double helices.Although the presence of guanine and cytosine reduces the stability of parallel-stranded NDA,the rather modest experimentally determined reduction in stability (less than 1 kcal/mol for each C-G pair)has been ascribed separately to favorable amino-amino contacts could yield an interaction energy (gas phase)of about-5 kcal/mol for a twisted,reverse Watson-Crick C-G base pair.It is shown here that either a minor tuatomer pairing or a wobble pairing can much more strongly stabilize reversed C-G gaxe pairs.The calculated gas-phase interaction energies of-14kcal/mol ech are comparable to the gas-phase stability of a T-A base pair.Aqueous-phase calculations,however,greatly favor the wobble pair geometry by 9 cal/mol.