Stacking Energies in DNA

Variations in base mono- and dipoles result in variations in stacking energies for the 10 unique neighbor pairs in DNA. Stacking energies for pair M on N, expressed as TMN, were derived by matrix decomposition of a large set of linear algebraic expressions relating the measured Tm for subtransitions emanating from large polymeric DNAs, and the fractional neighbor frequencies, fMN, for the domains responsible for the transitions, Tm = sigma fMNTMN. Tm were determined for subtransitions that dissociate in approximately all-or-none fashion in high resolution melting profiles of partially deleted and recombinant forms of pBR322 DNA. Three different analytical maneuvers were undertaken to resolve subtransitions: site-specific cleavage of domains; deletion of domains; and addition of domains. Three dozen domains of widely divergent, quasi-random neighbor frequencies were identified and assigned, resulting in a unique set of values for TMN with standard deviation, sigma = +/- 0.23 degree C. The average difference between calculated and experimental Tm for domains is only +/- 0.17 degree C, indicating that the thermodynamic properties of these domains are not in any way unusual. Assuming delta S to be constant for all pairs, the corresponding delta HMN are found to have a precision of +/- 10 calories.mol-1 and an accuracy of +/- 606 calories.mol-1. TMN used to calculate melting curves by statistical mechanical analysis of sequences of the different plasmid specimens in this study were in quantitative agreement with observed curves for most sequences. These TMN differ significantly from those determined previously and also correlate poorly with values determined by quantum chemical analysis. Stabilities of neighbor pairs, expressed as the difference in free energy between that for a given pair (MN) and that for the average of like pairs (M, N), depend on the relationship of stacked purines and pyrimidines as follows. delta delta Gpu-py(-466 cal) greater than delta delta Gpu-pu(+52 cal) greater than delta delta Gpy-pu(+335 cal) Differences between experimental Tm and Tm calculated with TMN for the isolated neighbor pairs in the B-conformation are useful in the identification of altered structures and unusual modes of dissociation of helixes. A significantly higher Tm is observed for the highly biased repeated sequence synthetic helixes dA.dT, d(AGC).d(GCT), and d(GAT).d(ATC), reflecting auxiliary sources of stability such as bifurcated hydrogen bonds and/or altered structures for these helixes.