Conformationally rigid neurotransmitters. Acetylcholine analogs in the bicyclo(2.2.2)octane system

Conformation of ligands bound to the muscarinic acetylcholine receptor

Many biogenic amines evoke a variety of physiological responses by acting on G protein-coupled receptors. We have determined the conformation of two acetylcholine analogs, (S)-methacholine and (2S,4R,5S)-muscarine, bound to the M(2) muscarinic acetylcholine receptor (M(2) mAChR) by NMR spectroscopy. The analysis of the transferred nuclear Overhauser effect indicated that the receptor selectively recognized the conformers of (S)-methacholine and (2S,4R,5S)-muscarine with the gauche O-C2-C1-N dihedral angle at +60 degrees. This is distinct from the predominant conformations of these ligands in solution with O-C2-C1-N dihedral angle (+80 to approximately 85 degrees ) in the absence of the M(2) mAChR, as assessed by analyses of the coupling constants and nuclear Overhauser effect spectroscopy. We have also built a molecular model of the M(2) mAChR-(S)-methacholine complex, based on the X-ray crystallographic structure of rhodopsin. This model indicated that the conformation with the gauche O-C2-C1-N dihedral angle at +55.5 degrees, which is similar to the one determined by NMR measurement, is energetically favored in the binding of (S)-methacholine to the receptor. We suggest that this conformation represents the binding of the agonist to the M(2) mAChR in the absence of G protein.

Inhibition of monoamine oxidases by functionalized coumarin derivatives: biological activities, QSARs, and 3D-QSARs

A large series of coumarin derivatives (71 compounds) were tested for their monoamine oxidase A and B (MAO-A and MAO-B) inhibitory activity. Most of the compounds acted preferentially on MAO-B with IC(50) values in the micromolar to low-nanomolar range; high inhibitory activities toward MAO-A were also measured for sulfonic acid esters. The most active compound was 7-[(3, 4-difluorobenzyl)oxy]-3,4-dimethylcoumarin, with an IC(50) value toward MAO-B of 1.14 nM. A QSAR study of 7-X-benzyloxy meta-substituted 3,4-dimethylcoumarin derivatives acting on MAO-B yielded good statistical results (q(2)() = 0.72, r(2)() = 0.86), revealing the importance of lipophilic interactions in modulating the inhibition and excluding any dependence on electronic properties. CoMFA was performed on two data sets of MAO-A and MAO-B inhibitors. The GOLPE procedure, with variable selection criteria, was applied to improve the predictivity of the models and to facilitate the graphical interpretation of results.

2,3-Dihydro-2,5-dihydroxy-4H-benzopyran-4-one: a nonphysiological substrate for fungal melanin biosynthetic enzymes

We have synthesized an alternate substrate for trihydroxynaphthalene reductase (3HNR) and scytalone dehydratase (SD), two enzymes in the fungal melanin biosynthetic pathway. The oxidation of 2,3-dihydro-2,5-dihydroxy-4H-benzopyran-4-one (DDBO) to 4,5-dihydroxy-2H-benzopyran-2-one (DBO) with concomitant reduction of NADP+ is catalyzed by 3HNR. DDBO is dehydrated by SD to 5-hydroxy-4H-1-benzopyran-4-one (HBO). These reactions can be monitored using continuous spectrophotometric assays. DDBO race-mizes rapidly, so chiral synthesis to mimic the natural substrate is not required. DDBO, DBO, and HBO are stable in aerated aqueous solution, in contrast to the rapidly autooxidizing trihydroxynaphthalene, a physiological substrate for 3HNR and product of SD. Unlike the natural substrates, DDBO, DBO, and HBO do not change protonation state between pH's 4 and 9. Oxidation of DDBO is effectively irreversible at pH 7, as DBO deprotonates with a pKa of 2.5. At pH 7.0 and 25 degrees C, the kcat for 3HNR catalyzed DDBO oxidation is 14 s-1 and the K(m) is 5 microM; the kcat for SD catalyzed DDBO dehydration is 400 s-1 and the K(m) is 15 microM. Based on these kinetic constants, DDBO is a better substrate than the natural substrate scytalone for both 3HNR and SD at neutral pH. An explanation for the preference of DDBO over scytalone in the oxidation and dehydration reactions is offered.