Conformational stability in dinucleoside phosphate crystals. Semiempirical potential energy calculations for uridylyl-3'-5'-adenosine monophosphate (UpA) and guanylyl-3',5'-cytidine monophosphate (GpC)

A Monte Carlo method for generating structures of short single-stranded DNA sequences

A Monte Carlo method has been developed for generating the conformations of short single-stranded DNAs from arbitrary starting states. The chain conformers are constructed from energetically favorable arrangements of the constituent mononucleotides. Minimum energy states of individual dinucleotide monophosphate molecules are identified using a torsion angle minimizer. The glycosyl and acyclic backbone torsions of the dimers are allowed to vary, while the sugar rings are held fixed in one of the two preferred puckered forms. A total of 108 conformationally distinct states per dimer are considered in this first stage of minimization. The torsion angles within 5 kcal/mole of the global minimum in the resulting optimized states are then allowed to vary by +/- 10 degrees in an effort to estimate the breadth of the different local minima. The energies of a total of 2187 (3(7)) angle combinations are examined per local conformational minimum. Finally, the energies of all dinucleotide conformers are scaled so that the populations of differently puckered sugar rings in the theoretical sample match those found in nmr solution studies. This last step is necessitated by limitations in the theoretical methods to predict DNA sugar puckering accurately. The conformer populations of the individual acyclic torsion angles in the composite dimer ensembles are found to be in good agreement with the distributions of backbone conformations deduced from nmr coupling constants and the frequencies of glycosyl conformations in x-ray crystal structures, suggesting that the low energy states are reasonable. The low energy dimer forms (consisting of 150-325 conformational states per dimer step) are next used as variables in a Monte Carlo algorithm, which generates the conformations of single-stranded d(CXnG) chains, where X = A, T and n = 3, 4, 5. The oligonucleotides are built sequentially from the 5' end of the chain using random numbers to select the conformations of overlapping dimer units. The simulations are very fast, involving a total of 10(6) conformations per chain sequence. The potential errors in the buildup procedure are minimized by taking advantage of known rotational interdependences in the sugar-phosphate backbone. The distributions of oligonucleotide conformations are examined in terms of the magnitudes, positions, and orientations of the end-to-end vectors of the chains. The differences in overall flexibility and extension of the oligomers are discussed in terms of the conformations of the constituent dinucleotide steps, while the general methodology is discussed and compared with other nucleic acid model building techniques.

Base sequence effects in double helical DNA. I. Potential energy estimates of local base morphology

A series of potential energy calculations have been carried out to estimate base sequence dependent structural differences in B-DNA. Attention has been focused on the simplest dimeric fragments that can be used to build long chains, computing the energy as a function of the orientation and displacement of the 16 possible base pair combinations within the double helix. Calculations have been performed, for simplicity, on free base pairs rather than complete nucleotide units. Conformational preferences and relative flexibilities are reported for various combinations of the roll, tilt, twist, lateral displacement, and propeller twist of individual residues. The predictions are compared with relevant experimental measures of conformation and flexibility, where available. The energy surfaces are found to fit into two distinct categories, some dimer duplexes preferring to bend in a symmetric fashion and others in a skewed manner. The effects of common chemical substitutions (uracil for thymine, 5-methyl cytosine for cytosine, and hypoxanthine for guanine) on the preferred arrangements of neighboring residues are also examined, and the interactions of the sugar-phosphate backbone are included in selected cases. As a first approximation, long range interactions between more distant neighbors, which may affect the local chain configuration, are ignored. A rotational isomeric state scheme is developed to describe the average configurations of individual dimers and is used to develop a static picture of overall double helical structure. The ability of the energetic scheme to account for documented examples of intrinsic B-DNA curvature is presented, and some new predictions of sequence directed chain bending are offered.

Conformational characteristics of dimeric subunits of RNA from energy minimization studies. Mixed sugar-puckered ApG, ApU, CpG, and CpU

Following the procedure described in the preceding article, the low energy conformations located for the four dimeric subunits of RNA, ApG, ApU, CpG, and CpU are presented. The A-RNA type and Watson-Crick type helical conformations and a number of different kinds of loop promoting ones were identified as low energy in all the units. The 3E-3E and 3E-2E pucker sequences are found to be more or less equally preferred; the 2E-2E sequence is occasionally preferred, while the 2E-3E is highly prohibited in all the units. A conformation similar to the one observed in the drug-dinucleoside monophosphate complex crystals becomes a low energy case only for the CpG unit. The low energy conformations obtained for the four model units were used to assess the stability of the conformational states of the dinucleotide segments in the four crystal models of the tRNAPhe molecule. Information on the occurrence of the less preferred sugar-pucker sequences in the various loop regions in the tRNAPhe molecule has been obtained. A detailed comparison of the conformational characteristics of DNA and RNA subunits at the dimeric level is presented on the basis of the results.

Conformations of dinucleoside phosphates in aqueous solution

The conformations of dinucleoside phosphates have been reexamined by semiempirical potential energy calculations. Conformations I, II, and III, proposed by Lee & Tinoco [Lee, C. H., & Tinoco, I., Jr. (1977) Biochemistry 16, 5403], are possible species after refinement of their structures by potential energy minimization. These three conformers can represent three types of dinucleoside phosphate species in solution. Dhingra et al. [Dhingra, M. M., Sarma, R. H., Giessner-Prettre, C., & Pullman, B. (1978) Biochemistry 17, 5815] had concluded that conformations of type II and III were unlikely or impossible. They favored conformations g-g- (equivalent to I), g+g+,g+t, and tg+; the last three conformations have little stacking and are calculated to be energetically less favorable by more than 5 kcal/mol. Common structures of the types I, II, and III are found for dinucleoside phosphates with different purine-pyrimidine sequences. The sequence dependence of the potential energy of these three conformers has been calculated. The experimental nuclear magnetic resonance data of dinucleoside phosphates are consistent with these three conformations.

'A' forms of RNAs in single strands, duplexes and RNA-DNA hybrids

Helical parameters have been calculated for the 'A' form minimum energy conformations of ApA, CpC, GpG, UpU, GpC and UpA. The helix geometries are base sequence dependent. The single strands are narrower and more tightly wound than that duplex RNA-11 form. 9-12 kcal./mole are needed to convert these single strands to the RNA-11 conformation. However, in some sequences other 'A' type conformers capable of complementary base pairing may be formed at lower energetic cost. There is substantially more base stacking in the calculated single strands than in the RNA-11 conformation. Calculated intrastrand base stacking energies reflect these differences, and also are sequence dependent. The 'A' form RNA subunits differ from the analogous DNAs in possessing a larger rise per residue, needed to accomodate the 2'-OH. RNA-DNA hybrids are consequently more likely to be in the 'A-RNA than in the 'A'-DNA conformation, although the base sequence determines the extent of the preference.