Comparative biophysical studies of sartan class drug molecules losartan and candesartan (CV-11974) with membrane bilayers.

The interactions of the antihypertensive AT(1) antagonists candesartan and losartan with membrane bilayers were studied through the application of DSC, Raman, and solid state (31)P NMR spectroscopies. (1)H and (13)C NMR resonances of candesartan were assigned on the basis of 1D and 2D NMR spectroscopy. A (31)P CP NMR broadline fitting methodology in combination with ab initio computations was implemented and, in conjunction with DSC and Raman results, provided valuable information regarding the perturbation, localization, orientation, and dynamic properties of the drugs in membrane models. In particular, results indicate that losartan anchors in the mesophase region of the lipid bilayers with the tetrazole group oriented toward the polar headgroup, whereas candesartan has less definite localization spanning from water interface toward the mesophase and upper segment of the hydrophobic region. Both sartan molecules decrease the mobilization of the phospholipids alkyl chains. Losartan exerts stronger interactions compared with candesartan, as depicted by the more prominent thermal, structural, and dipolar (1)H-(31)P changes that are caused in the lipid bilayers. At higher concentrations, candesartan strengthens the polar interactions and induces increased order at the bilayer surface. At the highest concentration used (20 mol %), only losartan induces formation of microdomains attributed to the flexibility of its alkyl chain. These results in correlation to reported data with other AT(1) antagonists strengthen the hypothesis that this class of molecules may approach the active site of the receptor by insertion in the lipid core, followed by lateral diffusion toward the binding site. Further, the similarities and differences of these drugs in their interactions with lipid bilayers establish, at least in part, their pharmacological properties.

EPR line-shape anisotropy and hyperfine shift of methyl radicals in solid Ne, Ar, Kr, and p-H2 gas matrices.

Earlier studies have shown that pure quantum mechanical effects on the "light" methyl radical at low temperature minimize the anisotropy of CW EPR spectra to a high resolution character, and new experiments under different conditions display a small additional electron paramagnetic resonance (EPR) line-shape anisotropy. In this work the effects of the solid H(2) quantum matrix and three other typical solid noble-gas matrices on the spectral anisotropy and the hyperfine interaction (hfi) constant of trapped methyl radicals presented as matrix shifts (deviation from the value in free space) are studied in some detail. Experimental EPR data at liquid-He temperatures were used to explore the dependence of the additional broadening and the spectral anisotropy of the hosted methyl radicals and to correlate the experimental spectral anisotropy to the matrix-radical interaction. Models correlating the spectral anisotropy and the matrix shift of the hyperfine (hf) coupling constant to the van der Waals (vdW) attraction and/or to the repulsive Pauli exclusion (RPE) forces between the host-matrix molecules and the methyl radical were constructed. It was shown that both vdW and RPE forces must be involved to explain these matrix effects, but while the RPE is the major source for the extra anisotropy, its contribution to the hf shift was also important but not dominant.

Development of a CP 31P NMR broadline simulation methodology for studying the interactions of antihypertensive AT1 antagonist losartan with phospholipid bilayers.

A cross-polarization (CP) (31)P NMR broadline simulation methodology was developed for studying the effects of drugs in phospholipids bilayers. Based on seven-parameter fittings, this methodology provided information concerning the conformational changes and dynamics effects of losartan in the polar region of the dipalmitoylphosphatidylcholine bilayers. The test molecule for this study was losartan, an antihypertensive drug known to exert its effect on AT(1) transmembrane receptors. The results were complemented and compared with those of differential scanning calorimetry, solid-state (13)C NMR spectroscopy, Raman spectroscopy, and electron spin resonance. More specifically, these physical chemical methodologies indicated that the amphipathic losartan molecule interacts with the hydrophilic-head zone of the lipid bilayers. The CP (31)P NMR broadline simulations showed that the lipid molecules in the bilayers containing losartan displayed greater collective tilt compared to the tilt displayed by the load-free bilayers, indicating improved packing. The Raman results displayed a decrease in the trans/gauche ratio and increased intermolecular interactions of the acyl chains in the liquid crystalline phase. Additional evidence, suggesting that losartan possibly anchors in the realm of the headgroup, was derived from upfield shift of the average chemical shift sigma(iso) of the (31)P signal in the presence of losartan and from shift of the observed peak at 715 cm(-1) attributed to C-N stretching in the Raman spectra.