Arylstibonic acids: novel inhibitors and activators of human topoisomerase IB.

Human topoisomerase IB (hTopo) forms a covalent phosphotyrosyl linkage with the DNA backbone, and controls genomic DNA topology by relaxing DNA supercoils during the processes of DNA replication, transcription, chromosome condensation and decondensation. The essential role of hTopo in these processes has made it a preeminent anticancer drug target. We have screened a small library of arylstibonic acids for their effects on plasmid supercoil relaxation catalyzed by hTopo. Despite the similar structures of the library compounds, some compounds were found to be effective competitive inhibitors, and others, nonessential activators. Some arylstibonic acids show selectivity in their action against hTopo and the related enzyme from poxvirus (vTopo). Structure-activity relationships and structural modeling suggest that competitive inhibition may result from positioning of the negatively charged stibonic acid and carboxylate groups of the inhibitors into DNA phosphate binding pockets on hTopo. The hTopo activators act by a surprising allosteric mechanism without interfering with DNA binding or binding of the widely used hTopo poison camptothecin. Arylstibonic acid competitive inhibitors may become useful small molecules for elucidating the cellular functions of hTopo.

Cross-polarization schemes for peptide samples oriented in hydrated phospholipid bilayers.

Continuous-wave, ramped amplitude, and frequency modulated cross-polarization schemes (abbreviated as CWCP, RACP, and FMCP, respectively) are evaluated for static samples in anisotropic phases, such as peptides oriented in lipid environments. It is shown experimentally that both RACP and FMCP give rise to 20% higher polarized signal intensity in comparison to CWCP. The CP matching bandwidths for CWCP and RACP are about the same. Because of its adiabaticity, FMCP has a much broader CP matching bandwidth than CWCP and RACP. In addition, the (15)N RF amplitude used at the center of the FMCP matching profile is much lower than that of the CWCP and RACP matching profiles. A sample of [(15)N]Leu(4) labeled gramicidin A oriented in lipid bilayers was used to demonstrate these experiments.

The effect of Hartmann-Hahn mismatching on polarization inversion spin exchange at the magic angle.

The effect of the Hartmann-Hahn mismatch delta = omega(eff)-omega(1S) during polarization inversion spin exchange at the magic angle (PISEMA) has been investigated, where omega(eff) and omega(1S) represent the amplitudes of the 1H effective spin-locking field at the magic angle and the 15N RF spin-locking field, respectively. During the PISEMA evolution period, the exact Hartmann-Hahn match condition (i.e., delta = 0) yields a maximum dipolar scaling factor of 0.816 for PISEMA experiments, while any mismatch results in two different effective fields for the first and second half of each frequency switched Lee-Goldburg (FSLG) cycle. The mismatch effect on the scaling factor depends strongly on the transition angle from one effective field to the other within each FSLG cycle as well as on the cycle time. At low RF spin-lock amplitudes in which the FSLG cycle time is relatively long, the scaling factor rapidly becomes smaller as omega(1S) becomes greater than omega(eff). On the other hand, when omega(1S) < omega(eff), there is relatively little effect on the scaling factor with variation in delta. As a result, the presence of RF inhomogeneities may significantly broaden the line-width in the dipolar dimension because of the mismatch effect. Higher RF spin-lock amplitudes result in a relatively small variation for the scaling factor. Furthermore, ramped amplitude of the 15N RF spin-lock field in synchronization with the flip-flop of the FSLG sequence minimizes the transition angle between the two effective fields within the FSLG cycle. It is shown experimentally that such a ramped amplitude not only gives rise to the same scaling factor but also results in a narrower dipolar line-width in comparison with the rectangular amplitude.