外文资料Conational Analysis of Epiquinine and Epiquinidine.pdf

P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 Structural Chemistry Vol 12 No 6 December 2001 c 2001 Conformational Analysis of Epiquinine and Epiquinidine Thais H A Silva 1Ala de B Oliveira 1H elio F Dos Santos 2and Wagner B De Almeida3 4 Received April 19 2001 revised May 16 2001 accepted May 18 2001 The conformational potential energy surfaces for the epiquinine and epiquinidine molecules were analyzed in gas phase and water solution using semiempirical and ab initio levels of theory The results obtained showed that the main conformation of the nonactive threo epimers is distinct from those observed for the active parent compounds quinine and quinidine This result might be used on a qualitative way to understand the loss of activity of the threo epimers and allow selecting important conformations to be considered in molecular modeling quantitative studies addressing the drug receptor interactions KEY WORDS Conformational analysis antimalarial epiquinine epiquinidine INTRODUCTION Malariaisoneofthegreatesthumanhealthproblems becauseofitshighincidence Over40 oftheworldpop ulation in 99 countries are living under risk of contract ing the parasitosis In spite of existing useful antimalar ial agents the prevalence of resistant and multiresistant infections decreases the effi cient chemotherapeutic and chemoprophylatic alternatives 1 The Cinchona alkaloids are natural compounds ex tracted from the barks of Cinchona trees native of Peru ThemostabundantconstituentsoftheCinchonabarksare twopairsoferythrodiastereoisomers quinine1andquini dine2 whichareactiveantimalarials Theirthreoepimers epiquinine 3 and epiquinidine 4 are practically inactive 2 Fig 1 Sincetheactivitydifferencesappeartodepend upon the absolute stereochemistry of their amine and hy droxy C8 and C9 groups a conformational analysis of the Cinchona alkaloids is certainly of interest consider ingthatitcanshowimportantstructuralcharacteristicsfor 1DepartamentodeProdutosFarmac euticos FaculdadedeFarm acia Uni versidade Federal de Minas Gerais Belo Horizonte MG Brazil 2N ucleo de Estudos em Qu mica Computacional NEQC Departa mentodeQu mica ICE UniversidadeFederaldeJuizdeFora Campus Universit ario Martelos 36036 330 Juiz de Fora MG Brazil 3Laborat orio de Qu mica Computacional e Modelagem Molecular LQC MM DepartamentodeQu mica ICEx UFMG BeloHorizonte MG 31 270 901 Brazil 4To whom all correspondence should be addressed the antimalarial activity 3 The three dimensional struc tures of the Cinchona alkaloids have been examined in order to explain the differences in activities between the 9 epialkaloids 3 4 and quinine 1 and quinidine 2 Thebioactiveconformationisonesingleuniquecon formation among all the low energy conformations that can bind the active site of the receptor It is widely ac cepted that the bioactive conformation is not necessarily identical to the lowest energy conformation However it cannot be a conformation that is so high in energy that is excluded from the conformational equilibrium in solution 4 Totheconstructionofapharmacophore thebioactive conformationsofaseriesofanalogswiththesamemecha nismofactionmustbecomparedtoverifytheirsimilarity Once the pharmacophore has been determined it can be tested against by the inclusion of low active and inactive congeners 4 In this article we aim to show how a detailed con formational analysis is important for understanding the biological behavior of this class of compounds avoiding that some important structural features of molecules of biological interest relevant for structure activity relation ships may be overlooked Several studies on the structure and conformation of the Cinchona alkaloids have been reported Prelog and Wilhelm in 1954 5 proposed a confi guration for the Cinchona alkaloids In addition 1H NMR data were used insomeconformationalstudiesofthesealkaloids 6 9 A conformationalanalysisstudy usingmolecularmechanics 431 1040 0400 01 1200 0431 19 50 0C 2001Plenum Publishing Corporation P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 432Silva Oliveira Dos Santos and De Almeida Fig 1 Specifi cationoftheatomiclabelsforthequinine1 quinidine2 epiquinine3 andepiquinidine4molecules and defi nition of the dihedral angles MM and quantum mechanical semiempirical methods were described for the epiquinidine molecule 10 The results of the crystal structure analysis for the epiqui nine hydrochloride dihydrate 2 and for the epiquinidine hydrochloride monohydrate 11 were also reported In the theoretical study of the epiquinidine molecule 9 re ported so far the search for minimum energy structures on the potential energy surface PES was not fully car riedout withonlythreelocalminimabeinglocated Some geometrical constraints were introduced in order to sim plify the computational task In two previous studies we presented the conformational analysis of the quinine 12 and quinidine 13 using MM and semiempirical meth ods Karle and Bhattacharjee 4 used crystallographic data of Cinchona alkaloids as the initial point for energy minimization using Hartree Fock HF ab initio quantum mechanical calculation HF 3 21G with the purpose to provideaprofi leoftheelectroniccharacteristicsnecessary for potent antimalarial activity In the present article an exploratory conformational study of the PES for the epiquinine 3 and epiquinidine 4 molecules was carried out using semiempirical PM3 calculations All the minima obtained at PM3 level were confi rmed using ab initio HF 6 31 G calculations METHODOLOGY A comprehensive search for stationary points on the multidimensional PESs for epiquinine and epiquinidine free base molecules was carried out using the PM3 quantum mechanical semiempirical method 14 Three dimensional 3D PESs were constructed by varying the dihedral angles 1and 2 Fig 1 and using a step size of 30 degrees For sake of simplicity the bond distances and bondangleswerekeptunchangedattheiroptimizedvalues corresponding to a local minimum Only the remaining dihedral angles were optimized for each calculated point onthe3Dsemiempiricalsurface Thisprocedurehasbeen used in previous theoretical studies involving antimalarial agents 12 13 By looking at the contour map a rough geometry of the likely minimum energy structures may be guessed and by this procedure fi ve minima could be located on the PM3 surface These fi ve stationary points were used as initial data for full geometry optimizations using the PM3 method The optimized geometry obtained using the PM3 method and the epiquinine and epiquinidine crystal lographic structures were also used as initial guesses for geometry optimization at the ab initio quantum mechani cal level The following split valence basis set was used 6 31G for C and H and 6 31 G for O and N The sta tionary points found at the Hartree Fock HF level were characterized as the true minimum through harmonic fre quencies analysis By using this consistent procedure we can be sure that there is no relevant molecular structure missing in the conformational search In order to mimic the biological conditions the sol vent effect was included in the calculations The Polar izable Continuum Mode PCM 15 16 was used in the P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 Conformational Analysis of Epiquinine and Epiquinidine433 new formulation 17 18 implemented in the Gaussian 98 program 19 where the solute cavity is constructed using the United Atoms UA procedure Details of the methodology and the quality of the results have been re ported elsewhere 17 18 All calculations in solution werecarriedoutconsideringthedielectricconstantofwa ter 78 39 Semiempiricalcalculationswerecarriedoutwiththe MOPAC version 6 0 package 20 and the ab initio cal culationswereperformedusingtheGaussian 98program 19 RESULTS AND DISCUSSION The epiquinine 3 and epiquinidine 4 molecules Fig 1 are composed of two rigid cyclic systems a het eroaromatic quinoline ring and a quinuclidinic bicyclic system connected via a carbon atom carrying a hydroxy group These molecules possess four asymmetric centers C3 C4 C8 and C9 Three groups in the molecule are important for the investigation of internal rotation the vinyl hydroxy and methoxy groups However the ro tation around the C9 C16 and C8 C9 bonds determines themainconformationalchangesinthesemolecules since these rotations specify the relative positions of the two Table I Ab Initio Structural Data Dihedral Angles in Degrees Relative Energy 1Egin kcal mol 1 and Relative Gibbs Free Energy 1Ggat 298 K and 1 atm in kcal mol 1 Calculated in the Gas Phase for the Stable Conformers Located on the PES for the Epiquinine and Epiquinidine Molecules Optimized dihedral angles 1 2 4 5 3 6 7 81Eg1Gg Epiquinine 3 EQNA171 71 131172175 3 114 1580 00 0 EQNB179103421771802 114 1570 50 3 EQNC68 90 14267681 114586 76 4 EQND 69 86 140 66 620 115577 57 5 EQNXR1691681101711753 114 1541 31 2 X raya 177 152 94 179 9 71 Epiquinidine 4 EQDA 17172132 172 1753 1181600 00 0 EQDB 171 104 43 177 180 3 1211590 50 3 EQDC 6690143 65 66 1 177 606 25 8 EQDD66 93 3463570 166758 17 9 EQDXR 169 101 41 175 178 11401570 81 2 X rayb 177 107 48 175 4 130 O Ac 131 171 O Bc 21 45 O Cc 51 175 aFrom Ref 2 bFrom Ref 11 cFrom Ref 10 rings The analysis of the PM3 3D PESs generated by rotations around the C9 C16 and C8 C9 single bonds revealed the presence of fi ve distinct minimum energy structures The fi ve structures obtained by direct inspec tion of the 3D surfaces were further fully optimized using the PM3 method Subsequent optimization with ab initio calculation of each of the fi ve conformers of epiquinine and epiquinidine leads to four distinct conformers of each molecule The main structural parameters and the relative Gibbs free energy 1Gg calculated in the gas phase are given in Table I The experimental values for some dihe dral angles from crystallographic structures 11 2 and from previous theoretical studies 10 are also quoted in Table I for comparison The conformations are labeled as EQN and EQD followed by the labels A B C D and XR X ray for the epiquinine and epiquinidine respec tively The gas phase optimized structures for the global minimum A and XR conformers are depicted in Fig 2 The Newman projections in relation to the C9 C8 and C9 C16 bonds for the epiquinine and epiquinidine conformers are represented in Figs 3 6 Three types of rotamers alternatedwithrespecttotheC8 C9bond were observed see Figs 3 and 4 The most stable conforma tions of the epiquinine and epiquinidine molecules in the gas phase are the ones where H8 H9 are in the antiperi planar position Figs 3a and 4a the least stable are the P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 434Silva Oliveira Dos Santos and De Almeida Fig 2 Ab initio gas phase optimized geometries for the global minimum A and XR conformers ones where H8 H9 are in synclinal position Figs 3b 3c 4b and 4c Intheconformationswherethe 1angleisnear180 thehydroxyhydrogenisanadequatepositiontoformahy drogen bond with the quinuclidine nitrogen N1 In these conformers EQNA EQNB EQNXR EQDA EQDB and EQDXR the distances between the hydroxy hydrogen and the quinuclidine nitrogen N1 are 2 1 2 2 A and the 8dihedral angle is near 160 Table I In these confor mations the charge density on the quinuclidine nitrogen Fig 3 Newman projections with respect to the C9 C8 bond for the epiquinine molecule determined by ab initio calculations are higher more negative than on the other one Table II In the other conformations where there is no possibility of the forma tion of the hydrogen bond EQNC EQND EQDC and EQDD the distances between the hydroxy hydrogen and thequinuclidinenitrogen N1 ishigherthan3 8 Aandthe 8dihedral angle is about 60 In the crystal structure of the epiquinine hydrochloride monohydrate 2 and of the epiquinidine hydrochloride monohydrate 11 no in tramolecular hydrogen bond between the hydroxy group Fig 4 Newman projections with respect to the C9 C8 bond for the epiquinidine molecule P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 Conformational Analysis of Epiquinine and Epiquinidine435 Fig 5 Newman projections with respect to the C9 C16 bond for the epiquinine molecule andthequinuclidinenitrogenatomwasfound Onlyinter molecularhydrogenbondswereobserved 2 11 because the nitrogen is protonated Thecrystallinestructureofepiquininehydrochloride monohydrate 2 presents H8 H9 in antiperiplanar posi tion This conformation corresponds to that of the con formers A and B here reported see Fig 2 Dijkstra et al determined the conformation of the epialkaloids by in terring nuclear overhauser effect NOE and by 3JH8H9 coupling constants For epiquinine it was observed that the presence of the NOEs interactions between H5 H9 and H11 and a 3JH8H9 of 9 9 Hz corresponds to an anti relationship between H8 and H9 Dijkstraetal 9 reporteda 3JH8H9of10 1Hz which corresponds to the anti arrangement of H8 and H9 of the epiquinidine as expected They also determined the con formation of epiquinidine molecule in solution by inter ring NOEs The presence of NOE between H5 H9 H10 and H18 shows that the epiquinidine has an open con formation which is similar to the conformer B here de scribed Theantiperiplanar arrangementbetweenH8 H9 is also observed in the crystal structure of epiquinidine hydrochloride monohydrate 11 It is worth remember ing that the conformers A and B where H8 and H9 are in antiperiplanar position are the principal minima of the epiquinine and epiquinidine molecules in the gas phase see 1Ggvalues in Table I By analyzing the Newman projections with respect to the C9 C16 bond Figs 5 and 6 two types of alter nated rotamers were observed where the hydroxy and the Fig 6 Newman projections with respect to the C9 C16 bond for the epiquinidine molecule TableII AbInitioSolvationFreeEnergy 1Gsolv kcalmol 1 Relative Gibbs Free Energy in Aqueous Solution 1Gaq kcal mol 1 and Elec tronic Properties Electric Dipole Moments and Atomic charges q Calculated for the Distinct Conformers of the Epiquinine and Epiquini dine Molecules 1Gsolv1Gaq g aqq O12 q N1 EQNA 13 930 05 6968 581 0 920 0 986 EQNB 12 981 35 6838 249 0 918 0 977 EQNC 13 886 54 3096 640 0 989 0 813 EQND 11 509 92 6034 953 0 982 0 881 EQNXR 12 642 53 4998 179 0 820 0 979 EQDA 15 610 05 6628 314 0 917 1 005 EQDB 14 191 75 6018 251 0 913 1 000 EQDC 13 957 54 1836 775 0 994 0 834 EQDD 13 979 54 6777 772 0 912 0 899 EQDXR 11 825 06 0888 458 0 918 1 004 a1Gij aq 1Gijg 1Gsolv j 1Gsolv i beingtheconformer i theform A 1Gijgfrom Table I H9 atom are on the same side but opposite to the quinu clidinic ring in relation to the plane of the quinolinic ring Figs 5a 5b and 6b The crystal structure of epiquinine hydrochloride di hydrate 2 presents a conformation where H9 and the quinuclidinicringwereatthesamesideandattheopposite side to hydroxy in relation to the plane of the quinolinic ring Fig 5c This conformation was observed only in the conformer EQNXR that was obtained from geometry optimization of the crystal structure of the epiquine The EQNXR form was found to be only 1 3 kcal mol higher in energy than the global minimum EQN3A In order to better understand the plausible conforma tional interconversions the MMX 21 potential energy curves for rotation around 1and 2torsion angles were constructed with all remaining geometrical parameters being fully optimized for each point on the curve From the one dimensional MMX PES generated by rotation through the 1angle of the epiquinine and epiquinidine molecules itwasverifi edthattheinterconversionsthatin volve the rotation through the 1angle are easier as long as the barriers are lower than 5 kcal mol while rotations through the 2 angle are diffi cult involving high barriers Oleksyn et al 10 reported a molecular mechanics MM2 conformational analysis study for the epiquini dine molecule where three minimum energy structures were predicted Table I The conformers O A and O C correspond respectively to the conformers EQDA and EQDB here described The conformer O B is similar to one that was obtained in the PM3 PES and converged to the conformation EQDA at the ab initio HF level They alsoreportedthattheconformersO BandO Cwereen ergetically preferred This is in partial disagreement with P1 MRM FYXP2 MRM RKPQC MRM Structural Chemistry STUC PP260 344027September 13 200112 27Style file version Nov 07 2000 436Silva Oliveira Dos Santos and De Almeida our fi ndings where conformers EQDA and EQDB are en ergetically preferred The relative concentrations in gas phase are 57 7 EQDA and 34 7 EQDB As it was observed for epiquinine and epiquinidine the preferred orientation of the methoxy in all conformers here described is that observed in the crystal structures 2 11 i e 00in relation to the dihedral angle C24 O23 C19 C18 Thevinylangleisca 114to 121 TableI Thepositionofthehydroxygroupdependsonthe 1angle Table I In the last part of this work the solvent effect on the conformational equilibrium was analyzed using the PCM continuum model The relative Gibbs free energy in aque ous solution 1Gaq is presented in Table II The results obtained are similar to those obtained in the gas phase as long as the conformers A and B are the preferred ones In general the conformational equilibrium is shifted toward form A with the relative concentration found 57 7 gas and 88 8 water EQNA and 57 7 gas and 94 6 wa ter EQDA Therefore it can be seen that in this case the solvent does not play a signifi cant role for the deter mination of the relative stability order of the conformers as far as Hartree Fock solvation energies are concerned Then gas phase quantum mechanical calculations can be of great utility for understanding the behavior and mode of interaction of these molecules of biological in