We report the 2.4 A resolution X-ray structure of a complex in which a small molecule flips a base out of a DNA helical stack. The small molecule is a metalloporphyrin, CuTMPyP4 [copper(II) meso-tetra(N-methyl-4-pyridyl)porphyrin], and the DNA is a hexamer duplex, [d(CGATCG)]2. The porphyrin system, with the copper atom near the helical axis, is located within the helical stack. The porphyrin binds by normal intercalation between the C and G of 5' TCG 3' and by extruding the C of 5' CGA 3'. The DNA forms a distorted right-handed helix with only four normal cross-strand Watson-Crick base pairs. Two pyridyl rings are located in each groove of the DNA. The complex appears to be extensively stabilized by electrostatic interactions between positively charged nitrogen atoms of the pyridyl rings and negatively charged phosphate oxygen atoms of the DNA. Favorable electrostatic interactions appear to draw the porphyrin into the duplex interior, offsetting unfavorable steric clashes between the pyridyl rings and the DNA backbone. These pyridyl-backbone clashes extend the DNA along its axis and preclude formation of van der Waals stacking contacts in the interior of the complex. Stacking contacts are the primary contributor to stability of DNA. The unusual lack of van der Waals stacking contacts in the porphyrin complex destabilizes the DNA duplex and decreases the energetic cost of local melting. Thus extrusion of a base appears to be facilitated by pyridyl-DNA steric clashes.

We have determined the x-ray structure of a DNA fragment containing 7,8-dihydro-8-oxoguanine (Go).

In crystallographic structures of biological macromolecules, one can observe many hydration rings that originate at one water molecule, pass via hydrogen bonds through several others, and return to the original water molecule. Five-membered water rings have been thought to occur with greater frequency than other ring sizes. We describe a quantitative assessment of relationships between water ring size and frequency of occurrence in the vicinity of nucleic acid interfaces. This report focuses on low-temperature X-ray crystallographic structures of two anthracyclines, adriamycin (ADRI) and daunomycin (DAUN), bound to d(CGATCG) and on several DNA structures published previously by others. We have obtained excellent low-temperature (-160 degrees C, LT) X-ray intensity data for d(CGATCG)-adriamycin and d(CGATCG)-daunomycin with a multiwire area detector. The LTX-ray data sets contain 20% (daunomycin, LT-DAUN) and 35% (adriamycin, LT-ADRI) more reflections than were used to derive the original room-temperature (15 degrees C) structures [Frederick, C.A., Williams, L.D., Ughetto, G., van der Marel, G. A., van Boom, J.H., Rich, A., & Wang, A.H.-J. (1990) Biochemistry 29, 2538-2549]. The results show that five-membered water rings are not preferred over other ring sizes. This assessment is consistent with our observation of broad dispersion W-W-W angles (sigma = 20 degrees). In addition, we report that the thermal mobility, distinct from the static disorder, of the amino sugar of daunomycin and adriamycin is significantly greater than that of the rest of the complex. This mobility implies that if the central AT base pair is switched to a CG base pair, there should be a low energy cost in avoiding the guanine amino group. The energy difference (for the sugar-binding preference) between d(CGTACG) and d(CGCGCG) could be considerably less than 20 kcal/mol, a value proposed previously from computation.

The bis-intercalators Flexi-Di and ditercalinium are synthetic dimers that bis-intercalate into DNA and cause cell death in prokaryotes from futile and abortive repair of DNA. Each is composed of two 7H-pyridocarbazole units and a linker. Flexi-Di has a flexible spermine-like linker while ditercalinium has a rigid bis(ethylpiperidinium) linker. This report, describing the 2.5-A X-ray structure of Flexi-Di complexed with [d(BrCGCG)]2, appears to be the first report of a three-dimensional structure of a DNA complex with a bis-intercalator with a flexible linker. DNA complex formation with a ditercalinium analog having a flexible linker was not anticipated to yield unstacked and bent DNA as was observed in the previously reported ditercalinium.[d(CGCG)]2 complex. Surprisingly, the DNA in the Flexi-Di complex is bent to a degree exceeding that of the ditercalinium complex. A comparison of the DNA complexes of Flexi-Di and ditercalinium has allowed us to propose a mechanism by which these bis-intercalators distort DNA. We propose that this class of bis-intercalators pulls the internal base pairs into the major groove and pushes the external base pairs into the minor groove. The result is a bend toward the minor groove. It appears that hydrogen bonds between the linker and the internal guanines effectively pull the central base pairs of the complex out into the major groove. At the external regions of the complex, stacking interactions between the chromophores and terminal base pairs effectively push the terminal base pairs into the minor groove. The result of this push/pull combination is to bend the DNA.

The substitution of methionines with leucines within the interior of a protein is expected to increase stability both because of a more favorable solvent transfer term as well as the reduced entropic cost of holding a leucine side chain in a defined position. Together, these two terms are expected to contribute about 1.4 kcal/mol to protein stability for each Met + Leu substitution when fully buried. At the same time, this expected beneficial effect may be offset by steric factors due to differences in the shape of leucine and methionine. To investigate the interplay between these factors, all methionines in T4 lysozyme except at the amino-terminus were individually replaced with leucine. Of these mutants, M106L and M120L have stabilities 0.5 kcal/mol higher than wild-type T4 lysozyme, while M6L is significantly destabilized (-2.8 kcal/mol). M102L, described previously, is also destabilized (-0.9 kcal/mol). Based on this limited sample it appears that methionine-to-leucine substitutions can increase protein stability but only in a situation where the methionine side chain is fully or partially buried, yet allows the introduction of the leucine without concomitant steric interference. The variants, together with methionine-to-lysine substitutions at the same sites, follow the general pattern that substitutions at rigid, internal sites tend to be most destabilizing, whereas replacements at more solvent-exposed sites are better tolerated.

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We characterize intercalative complexes as either "high charge" and "low charge". In low charge complexes, stacking interactions appear to dominate stability and structure. The dominance of stacking is evident in structures of daunomycin, nogalamycin, ethidium, and triostin A/echinomycin. By contrast in a DNA complex with the tetracationic metalloporphyrin CuTMPyP4 [copper (II) meso-tetra(N-methyl-4-pyridyl)porphyrin], electrostatic interactions appear to draw the porphyrin into the duplex interior, extending the DNA along its axis, and unstacking the DNA. Similarly, DNA complexes of tetracationic ditercalinium and tetracationic flexi-di show significant unstacking. Here we report x-ray structures of complexes of the tetracationic bis-intercalator D232 bound to DNA fragments d(CGTACG) and d(BrCGTABrCG). D232 is analogous to ditercalinium but with three methylene groups inserted between the piperidinium groups. The extension of the D232 linker allows it to sandwich four base pairs rather than two. In comparison to CuTMPyP4, flexi-di and ditercalinium, stacking interactions of D232 are significantly improved. We conclude that it is not sufficient to characterize intercalators simply by net charge. One anticipates strong electrostatic forces when cationic charge is focused to a small volume or region near DNA and so must consider the extent to which cationic charge is focused or distributed. In sum, ditercalinium, with a relatively short linker, focuses cationic charge more narrowly than does D232. So even though the net charges are equivalent, electrostatic charges are expected to be of greater structural significance in the ditercalinium complex than in the D232 complex.