Electronic and structural effects in muscular relaxants: Riparin I and Riparin III

Two muscular relaxants (MR), methyl ethers of N-benzoyl tyramine (Riparin I) and N-(1,6-dihydroxybenzoyl) tyramine (Riparin III) were studied with X-ray diffraction (XRD), optical spectroscopy and semi-empirical calculations. The XRD data show that the CO group in Riparin I is 34° out of plane from the closest benzene ring. The abinitio calculations using RHF/6-31G* suggest that the dihydroxybenzoyl (DHB) group in Riparin III forms intramolecular hydrogen bond between the H of the O-H group and the O of the CO group, and gives a relatively planar structure between DHB group and CO group. The spectral analysis together with the excitation energy calculations using the ZINDO program showed that the absorption band at 309 nm in Riparin III, absence in Riparin I, is assigned to the π→π* transition associated mainly from the DHB. A long range charge-transfer (CT) transition, as the low-lying excited electronic state, S1 has been calculated for both relaxants involving the non-bonding electron on carbonyl and/or amine group and the antibonding orbitals of aromatic ring. The planar structure of Riparin III and its extensive electron delocalization may have an important role on the pharmacological action mode and on the potency this MR as compared to those of Riparin I.