J. CHEM. SOC. DALTON TRANS. 1995 1395Ranitidine Bismuth(iii) CitratePeter J. Sadler" and Hongzhe SunDepartment of Chemistry, Birkbeck College, University of London, Gordon House andChristopher lngold Laboratories, 29 Gordon Square, London WC I H OPP, UKA variety of amines have been shown to solubilize bismuth(ll1) citrate, [Bi(Hcit)], and the natureof the adduct 1 between it and ranitidine [N,N-dimethyl-5- (3-nitromethylene-7-thia-2,4-diazaoctyl)furan-2-methanamine], which is currently on clinical trial as an antiulcer drug, has beeninvestigated by 'H and 13C NMR spectroscopy and polarography. Complex 1 undergoes a structuraltransition in aqueous solution with an associated pK, of 6.2. Ranitidine appears to be involved insecond-co-ordination-sphere interactions with polymeric bismuth(iii) citrate species via the HN Me,'group for which the pK, is raised from 8.64 to 8.90, whereas the pK, of the diaminonitroethene groupof ranitidine (2.2) is unaffected.In solutions of I in (CD,),SO this interaction increases the rate ofNH exchange compared to free ranitidine. The chemical properties of 1 in aqueous solution differfrom those previously reported for the potassium ammonium adduct, colloidal bismuth subcitrate, adrug in current clinical use. Complexation of both citrate and ranitidine to Bi"' in acidic solutions (pH2.5-3) was detected by polarography, which demonstrated the existence of rapid deprotonationequilibria for bismuth(iri) citrate complexes in the range pH 1-5.8. Since antiulcer drugs are subjectedto low-pH environments in the stomach, such equilibria may be relevant to the biological activity ofranitidine bismuth citrate.Bismuth compounds have been used for treating gastrointestinaldisorders for more than 200 years.' Recently the effectiveness ofbismuth has been attributed to its bacteriocidal activity towardsHelicubacter pyluri, a microorganism which has beenimplicated in the pathogenesis of gastric and duodenal ulcerdisease. 'There has been particular interest for drug use in forms ofbismuth(Ir1) citrate solubilized by ammonium and potassiumhydroxides. They are the basis of so-called colloidal bismuthsubcitrate present, for example, in the drugs Telen (BykGulden) and De-No1 (Gist Brocades)., The crystal structures ofseveral adducts in this class have been Theycontain stable dinuclear units [(cit)BiBi(cit)]* - (H,cit = citricacid = 3-carboxy-3-hydroxypentane-2,5-dioic acid) with ad-ditional o'-, OH- and H,O ligands, and these subunitsaggregate further oia bridging citrate ligands and a network ofhydrogen bonds involving citrate, ammonium ions and water.In these compounds, Bi"' has a high co-ordination number of 9,and there are short bismuth-alkoxide (C-0- of citrate) bondsof ca.2.13 A. The stereochemical role played by the bismuth(Ir1)lone pair of electrons is particularly notable. The high solubilityof these bismuth citrate compounds in water has beenattributed to the presence of channels in the aggregates.'In addition to the well established organic histamine(imidazole-4-ethanamine) H,-receptor antagonists such ascimetidine and ranitidine [N,N-dimethyl-5-(3-nitromethylene-7-thia-2,4-diazaoctyl)furan-2-methanamine], bismuth com-pounds can also be efficacious in the treatment of peptic ulcers,and recently an adduct? of ranitidine with bismuth citrate(ranitidine bismuth citrate) has entered clinical trials.' Thiscompound is highly water-soluble, but no chemical studies of ithave yet been published.In this paper we consider thesolubilization of bismuth citrate by a variety of amines and alsoinvestigate the properties of ranitidine bismuth citrate (referredmRanitidineto as complex 1). Solution 'H and NMR spectroscopy werechiefly employed, but we also demonstrate that polarographyprovides a very sensitive method for detection of interactions ofboth citrate and ranitidine with Bi"'ExperimentalMaterials. -Bismuth citrate [Bi(Hcit)], ranitidine, ranitidinehydrochloride and ranitidine bismuth citrate 1 were supplied byGlaxo.The salt Bi(NO,),.SH,O (Aldrich, A. R.), iron(II1)citrate (Fluka-Garantie), citric acid monohydrate (Aldrich, A.R.), trisodium citrate (Aldrich, A. R.), NaNO, (Aldrich,99.99973, NaOD (Goss) and DCl (Aldrich) were used asreceived.NMR Spectroscopy.-Proton and "C-{ 'H) NMR spectrawere recorded on JEOL GSX270, GSX5OO and Bruker AM400spectrometers at 270, 500 and 400 MHz, respectively, for 'H,and 67.5, 125 and 100 MHz, respectively, for 13C, using 5 mmtubes, ca. 0.6 cm3 of solution, at ambient temperature (ca.297K) unless otherwise stated. Typical 'H spectral accumulationconditions were 16 K points, 32 scans, 45-60" pulses, andrelaxation delay of 2-3 s, and for 13C, 16 K points, relaxationdelay 2.0 s, 6000-30000 scans and 45" pulses. The chemicalshift reference for 'H and 13C was internal sodium 2,2,3,3-tetradeuterio-3-trimethylsilyl propionate for D,O solutions,and tetramethylsilane for dimethyl sulfoxide (dmso) solutions.t We use the term 'adduct' to cover possible first- and second-co-ordination-sphere interactions between ranitidine and bismuth citrate;since the interactions turn out to be second sphere, the term 'complexsalt' could also be used.$ Ranitidine bismuth citrate is an amorphous solid containingranitidine, bismuth and citrate in the approximate molar ratio0.84 : 1 : 0.941396 J.CHEM. soc. DALTON TRANS. 1995There was no evidence for binding of the reference to Bi"' underthe conditions used (checks carried out using dioxane assecondary reference).For determination of pK, values, Henderson-Hasselbalchcurves were fitted to experimental data using the KALEIDA-GRAPH numerical analysis program (Sinergy Software) onan Apple Macintosh SE/30 computer. Adjustments of pH*(see below) in D20 solutions were made using NaOD or DCl.To investigate the extent of solubilization of bismuth citrateby amines, known amounts of the amines were added to asuspension of [Bi(Hcit)] (8 mg, 20 pmol) in D20 (1 cm3) whichwas stirred and heated if necessary at a temperature betweenambient and 368 K for 2-60 min until a clear solution wasobtained.To determine the apparent equilibrium constant, therelative amounts of citrate and amine in the equilibriumsolution were measured by integration of 'H NMR peaks.Po1arography.-Differential pulse polarograms were re-corded on either a model 264 EG & G polarographic analyzer/stripping voltameter or a Metrohm instrument at ca. 289 Kusing a three-electrode system with a Ag-AgC1 referenceelectrode, dropping mercury electrode and auxiliary platinumelectrode, and 30 pmol dm-, bismuth solutions. The ionicstrength was maintained at ca. 0.5 mol dm-, with NaNO, as thesupporting electrolyte. We report potentials relative to Ag-AgCl which is + 0.222 V relative to the normal hydrogenelectrode.Infrared Spectroscopy.-Spectra were recorded as Nujolmulls between KBr plates on a Perkin-Elmer 1700 IR FTspectrometer.pH Measurements.-These were made using a Corning pHmeter 145 equipped with an Aldrich micro combinationelectrode calibrated with Aldrich buffer solutions at pH 4,7 and10. The pH meter readings for D 2 0 solutions are recorded aspH* values, i.e. uncorrected for the effect of deuterium on theglass electrode.Results and DiscussionSofubifization of [Bi(Hcit)].-We determined the amounts ofvarious amines which were required to solubilize [Bi(Hcit)],Table 1. From the integration of the peaks in 'H NMR spectraof the supernatants from mixtures of amines and bismuthcitrate, apparent equilibrium constants were calculated on theassumption that 1 : 1 complexes are formed [equation (l)].It[Bi(Hcit)](s) + amine [Bi(cit)]-Hamine (1)can be seen (Table 1) that the constants correlate to some extentwith the pK, values of the amines, the strongest bases being themost effective solubilizing agents. Ethylenediamine is moreeffective than its pK, values would predict, suggesting that otherfactors such as first- or second-co-ordination sphere binding ofthe amine may also be involved. Although we were able tosolubilize [Bi(Hcit)] with Na,C03 and NaHCO, (mol ratios ca.1 : 2) in water, the resulting solutions were unstable, giving riseto precipitates within a few hours. This contrasts with solutionsobtained using the above amines which remained clear andstable for > 1 week, again suggesting that the amine is involvedin interactions in solution.Infrared Spectroscopy.-The solid-state IR spectra of[Bi(Hcit)] and ranitidine bismuth citrate, complex 1, are shownin Fig.1. The most notable difference is the presence of a sharpband at 3455 cm-' in the spectrum of [Bi(Hcit)] assignable tothe C(3)O-H stretch which is absent in the spectrum of 1. Such aband is also present for Na,(Hcit) and a mechanical admixtureof bismuth citrate and ranitidine but absent for iron(II1) citrate(Fig. 1). These data show that 1 is not simply a mixture ofranitidine and [Bi(Hcit)] and suggest that solubilization ofTable 1 Correlation between the solubilization of [Bi(Hcit)] byvarious amines and their pK, values. An apparent equilibrium constant(K) for formation of a 1 : l complex was determined by NMRmeasurements of the citrate: amine mol ratio in solutionAmine Ratio" pHb K/dm3 mol-' pK,'Pyridine 1:5 6.9 0.3 5.24Imidazole 1 :2 6.5 0.5 7.03Ranitidine 1 :2 6.9 1.5 8.64Triethylamine d d 1.94 10.751,2-Diaminoethane 1 : 0.5 8.4 e 9.89, 7.08Dimethylamine 1 : 2 10.7 e 10.80a Molar ratio bismuth : amine required for complete solubilization of[Bi(Hcit)].The amine was added in 0.5 mol ratio steps. bOf clearsolution, except pH* for imidazole, ranitidine and pyridine. Valuesfrom ref. 11. Not determined, poor solubility of amine in water.Citrate and amine resonances overlapped.2I1 I I I3500 2500 1500 500Wavenumber/cm-IFig. 1 Infrared spectra of (a) [Bi(Hcit)], (b) complex 1, (c) [Fe"'(Hcit)]and (6) Na,(Hcit).The absence of a sharp OH stretch in (b) and (c) isnotable, and the peak at 1719 cm-' in (c) suggests the presence ofuncomplexed carboxyl groups in the iron(In) complex[Bi(Hcit)] by ranitidine leads to deprotonation and co-ordination of the citrate C(3)OH group allowing the formationof a six-membered chelate ring, as also expected for Fe"', butnot for Na + .NMR Spectroscopy.-Comparison of pH* Dependence ofComplex 1 and Ranitidine. The 'H NMR spectrum ofcomplex 1 in D20 is shown in Fig. 2 together with the resonanceassignments12 (the atom labelling is the same as that usedpreviously'2). The chemical shifts of resonances for Ha, H,, He,H, and citrate protons of 1 are pH*-dependent, and the pH*dependence of the chemical shift of the 'H NMR resonance foJ.CHEM. SOC. DALTON TRANS. 19952.9-2.7-ca 2.5-2.3-1397- 1 I I I I6:s 6:s 6.4 6.2 4.5 4.0 3.5 3.0 2.56Fig. 2 The 270 MHz 'H NMR spectrum of complex 1 in D,O, pH*4.5. One of the broadened AB doublets of citrate is overlapped with theHa, H, and H, resonances of ranitidine. Resonance H, is absent due toexchange with deuterium. The quartet at 6 3.65 is due to ethanolimpurity. The labelling scheme is shown in the text2.1 I I I I 13 5 7 9 11 13PH*Fig. 3 Dependence of the 'H NMR chemical shift of the NMez (a)protons of complex 1 (0) and ranitidine (O), both as 20 mmol dm-3solutions in D,O, showing the increase in pK, of this group in 1the NMe,(a) group of ranitidine alone is compared to that forcomplex 1 in Fig.3. The curves were fitted using pKa values of8.64 5 0.01 and 8.90 5 0.01 for ranitidine and 1, respectively.The increase in pKa was not due to ionic strength effects sincethe pKa of ranitidine in the presence of a ten-fold molar excessof sodium nitrate was found to be the same as that in wateralone (data not shown).Thus, there is no clear evidence for an inner-sphereinteraction between ranitidine and bismuth under theseconditions, but the stabilization of the protonated groupNMe,H+ in solutions of complex 1 relative to ranitidine alonecan be attributed to hydrogen-bonding interactions with boundcitrate. It is clear that such outer-sphere interactions can playa major role in determining the structure of bismuth(rr1)carboxylate complexes.For example, Breeze et al. haverecently shown in their X-ray crystallographic work on ammineand amine adducts of tetrakis(trifluoroacetato)bismuthate(Irr)complexes that outer-sphere hydrogen-bonding interactionsbetween NH and carboxylate oxygens determine both thestability of the complexes and the geometry around Bi'". Incrystal structures of ammonium adducts of bismuth(1n) citratesuch as [NH3]4[Bi(~it)(H~it)(H20)2]-H20 there is close contactbetween NH4+ ions and citrate and water oxygen atoms in thel a t t i ~ e . ~Fig. 4 shows the 'H NMR chemical shifts of the citrateresonances of complex 1 over the pH* range 4-12. Below pH*3.5 a precipitate formed which is likely to contain BiOCl as hasbeen observed for colloidal bismuth subcitrate solutions.2 .4 4 ,1 3 5 7 9 11 13Fig. 4 Dependence of the chemical shifts of the major citrate CH, 'HNMR resonances of complex 1 (+) and citrate alone (0) on pH*.Above pH* CQ. 7.5 the shifts are almost identical, suggesting theseresonances from solutions of 1 represent unbound citrate. At low pH*values ( < 4,20 mmol dm-3) solutions of 1 give rise to precipitates. Thelow-field citrate peaks for 1 are overlapped with ranitidine peaks at pH*< ca. 6PH"Above pH* 7.5 it can be seen that the shifts of the citrateresonances are almost identical to those of citrate alone. Fromthis it might be reasoned that at pH* > 7.5 citrate is not boundto Bi"' but is displaced by OH- or 0,-, a conclusion whichothers have made for ammonium adducts of bismuth citratebased on similar However, there is evidence from I3CNMR and polarographic data (see below) that, in the case of 1,bismuth citrate complexes are also present in solution at highpH* implying that their 'H resonances are too broad toobserve.This illustrates a difference between 1 and ammoniaadducts with bismuth citrate. The broadening of the citrate 'HNMR resonances is likely to be due to the presence of differentforms of bound citrate in intermediate exchange on the 'HNMR time-scale.The I3C NMR spectra of complex 1 in D,O at various pH*values are shown in Fig. 5(a). Marked changes are seen forcitrate resonances in the region 6 175-200 [CO,-, C(1), C(5)and C(6)], 75-90 [CO-, C(3)] and 47-55 [CH,, C(2) and C(4)].At pH* > 5.8 the C0,- [C(l), C(5) and C(6)], CO [C(3)] andCH, [C(2), C(4)] regions suggest that there is a major form ofbound citrate present plus at least two other minor forms, suchthat at pH* 8.6 the major form accounts for about 50% of thecitrate and has chemical shifts close to that of unbound citrateat the same pH*, Fig.5(b). The chemical shifts of the minorcitrate peaks were largely unaffected by pH* changes, whereasthose for the major peaks of bound citrate were fitted by pK,values of 6.22 k 0.05 [C(6)], 6.21 k 0.05 [C(3)] and 6.21 k0.05 [C(2), C(4)] respectively. The value calculated for the C( l),C(5) curve was slightly higher (6.66) but is subject to greatererror because the shift change is much smaller.These can becontrasted with pK, for citrate alone of 5.64 (pK, and pK,being 2.89, 4.34, re~pectively).'~ The measured pKa couldrepresent deprotonation of bound citrate, but it seems unlikelythat this would give rise to shifts which are the same as forunbound citrate. One possibility is that the deprotonation of Bi-bound H20 or OH- is occurring, giving rise to additionalBi-O(H)-Bi bridges, displacement of citrate from Bi, and rapidexchange (on the NMR time-scale) of unbound and boundcitrate. The presence of substantial amounts of Bi-bound citratein solutions of 1 at high pH* is quite distinct from the reportedbehaviour of colloidal bismuth subcitrate, for which the citrateligands are 'hardly co-ordinated' at pH > 7.' This can beattributed to the presence of different cations (ranitidine versusammonium) and to the different Bi : citrate mol ratios in thesetwo preparations (greater than or approximately equal to 1 : 1for complex 1, and < 1.0: 1 for the subcitrate)1398 J.CHEM. SOC. DALTON TRANS. 1995(6)o + y * yPH"8.3I I I I I 200 160 120 80 40n(06hu) qF h rL !- I cd1821787 6 1tf,\k I in * U 0 h gA- T 1 I180 1 40 1006I60Fig. 5 (a) The 67.5 MHz 13C-(1H} NMR spectra of 0.2 mol dm-3 complex 1 in D,O at various pH* values. The labelling scheme for ranitidine peaksis as before; resonances for bound citrate are labelled *C( l), *C(5), *C(2), *C(4), *C(3) and *C(6). The broadening of resonance k is due to exchangeof the associated proton with deuterium, and that of 1 is due to restricted rotation around the double bond.(b) Com-parison of the pH* dependences of the I3C NMR chemical shifts of the major (most intense) citrate resonances of 1 (0) together with thoseof citrate alone (0). (c) The 100 MHz 13C-{1H} NMR spectrum of 50 mmol dm-3 complex 1, pH 6.2. The peaks marked * were not observed inreported spectra of the ammine adduct K4,,,[NH4]o,zs[Biz(cit)z(Hcitj]-l 4H,O under similar conditionsThe aggregation properties of complex 1 are also differentfrom those of colloidal bismuth subcitrate. In order to compare13C NMR spectra of 1 directly with published spectra of theammine adducts K4,,,[NH4],~2,[Bi2(cit)2(Hcit)]~14H20 andcolloidal bismuth subcitrate under similar conditions4 we alsorecorded the I3C-{ 'H) NMR spectrum of 50 mmol dm-3 1 inD20, pH* 6.2 [Fig.5(c)]. This clearly showed the presence ofpeaks for different forms of bound citrate at 6 180.4 [C(6)]J. CHEM. SOC. DALTON TRANS. 1995 1399175-196 [C(l), C(5), a band of broad peaks], 80-84 [C(3), atleast three-peaks] and 51 [C(2), C(4)], which are similar to thepeaks for 1 at 0.2 mol drn-,. In contrast ammine adducts ofbismuth(rI1) citrate have been reported to give rise to only oneset of peaks (over the range pH* 4-9) and, even at very highconcentration (220 mg cm-,, cu. 0.4 mol drn-,), colloidalbismuth subcitrate does not exhibit additional peaks for othertypes of bound citrate at pH* 6.2.7 Again this shows that theammine/amine plays a role in determining the structural formsof bismuth citrate which are present in the adducts.These formsare likely to be different for different adducts and thereforetheir biological activities may differ.The I3C NMR spectra of a 0.7 mol dm-3 solution of complex1 at 293,3 13 and 343 K, pH 8.6 (data not shown), gave evidencefor chemical exchange between the different forms of boundcitrate with broadening and shifting of peaks (at highertemperatures). In contrast, over this temperature range theCH, and CH2 peaks of ranitidine 1, h, i became sharper due tothe known increase in rotation rate about the ethene bond. 'In order to investigate possible interactions of the NMe,H + group of ranitidine with bismuth and citrate in solutions ofcomplex 1 we lyophilized solutions of 1 in water at various pHvalues, and of ranitidine at the same pH value, and redissolvedthe solids in (CD,),SO.A comparison of spectra for pH 4.3solutions is shown in Fig. 6; those for pH 5.7 solutions weresimilar. The most notable differences are for peaks fromprotons at the dimethylaminomethylfuran end of ranitidine.The NMe,N+ proton does not give rise to an observable signalfor 1, and the peaks for Ha,b,e,f are shifted by > 0.3 ppm. Theabsence of the NH peak did not appear to be due to the presenceof more water in the sample of 1 since addition of further water(up to 1 mol dmP3) to the ranitidine sample in dmso still allowedobservation of this peak (data not shown). It can be concludedthat outer-sphere hydrogen-bonding occurs between theNMe2H+ group and bismuth citrate species including boundH20, OH- or 0,- ligands.Such hydrogen-bonding might beexpected to decrease the rate of NH exchange and thereforesharpen the NH 'H NMR resonance, but in the present casethere is presumably a dynamic equilibrium between thedifferent types of hydrogen-bonded structures and betweenhydrogen-bonded and non-hydrogen-bonded ranitidine whichleads to line broadening.b g . . I l l laI I I I I I12 10 0 6 4 26Fig. 6 Comparison of the 'H NMR spectra of samples of (a) complex1 and (b) ranitidine, which had been prepared by dissolving in water atpH 4.3, followed by lyophilization and redissolution in (CD,),SO. Theabsence of an observable NMe,H+ peak for 1 is notable, as are theshifts of Ha,b,e,f suggesting that the dimethylaminomethylfuran end ofranitidine is involved in interactions with bismuth citrate species.TheH,O peak is larger in (a) but probably containscontributions from H,Oor OH ligands. Addition of water to the sample in (b) (to 1 mol dm-3)still allowed observation of NMe,H+. The protons NH, and NH, areinvolved in internal hydrogen bonding, and the resonances for the 2and E isomers are labelled with unprimed and primed letters (k, k', m,m', n, n')16PoZurogruphy.-We studied solutions containing Bi(N03)3and citric acid in a 1 :10 mol ratio, Bi(N03), and ranitidinehydrochloride in a 1 : 10 mol ratio, and Bi(N03)34H20, citricacid and ranitidine in a 1 : 10: 10 mol ratio at various pH valuesin the range 1-8 with NaNO, (0.5 mol dm-,) as a supportingelectrolyte.Bismuth nitrate alone at pH 1 gave rise to singlepeak at -0.03 V. Above pH 2 this peak decreased in intensityand a second peak appeared at -0.13 V, and by pH 4 boththese peaks were broad and weak, consistent with the knownI I I I0 4 . 2 -0.4 -0.6EN vs. Ag-AgCl-0.15-2Lu"-0.25 -PH7.286.296.015.484.063.022.661.05-0.351 3 5PHFig. 7 (a) Differential pulse polarograms for an aqueous solutioncontaining Bi3+ and citric acid in a 1 : 10 mol ratio as a function of pH.(b) Linear dependence of peak shift on pH showing that reduction isaccompanied by the uptake of one proto1400 J . CHEM. soc. DALTON TRANS. 1995tendency2-' for hydrolysis to give a mixture of hydroxo speciessuch as [Bi(OH),] +, Bi(OH),, [Bi(OH),]- and [Bi6-(OH),,16+.In the presence of 10 mol equivalents of citric acidonly a single peak was observed from pH 1 to 5.5, which showedan approximate linear shift of 59 mV per pH unit, Fig. 7. Thesedata suggest that reduction is accompanied by the uptake of oneproton, and that there is a rapid deprotonation equilibriumbetween at least two species which are being reduced, e.g.[Bi(Hcit)] [Bi(cit)]- + H'. At pH > 5.8 a second peakappeared at ca. -0.55 V and the initial peak decreased inintensity, Fig. 7. These data suggest that at least two types ofbismuth(m) citrate complexes exist under these conditions.Polarograms from a mixture of Bi(NO,), and ranitidinehydrochloride in a 1 : 10 mol ratio contain three peaks at pH1.73, assignable to reduction of Bi3+ (-0.06 V) and ofRN02H+ (-0.47 V) and RNO, (-0.80 V) of ranitidine.Thelatter two peaks have a similar pH dependence to that for freeranitidine, with a corresponding pKa value of 2.2 k 0.05,showing that the nitro group is not involved in bismuth(m)complexation. At pH 2.7 two 'Bi3+' peaks are detected, Fig.8(a), one assignable to Bi3+ ions (-0.08 V, I), and the other(-0.22 V) to a bismuth(II1)-ranitidine complex (11). By pH 4.7both these peaks are very broad and barely detectable.It is known that Bi"' has a high affinity for both nitrogen andsulfur as ligands, as well as oxygen.I7 Thus the thioether S,amine N, and furan 0 of ranitidine are all potential donors forBi"'.The strongest interaction may involve formation of anNMe,-furan 0 five-membered chelate ring. Comparativestudies of citrate and ranitidine complexation to Bi"' cannot bemade by NMR spectroscopy under the same conditions asthose used for polarography since at the higher concentrationsneeded for NMR hydrolysis is dominant (precipitation ofBiOCl).Polarograms obtained from solutions of Bi(NO,),, citric acidand ranitidine in a mol ratio 1 : 10: 10, Fig. 8(b), suggest that atpH 2.7 both bismuth-citrate and -ranitidine species are present(overlapping peaks at -0.16 and -0.21 V). The differentialpulse polarogram of a 0.1 mmol dmP3 solution of complex 1 in0.1 mol dm-3 NaNO, (pH 5.80) showed a peak at -0.28 V,similar to that of bismuth citrate alone under similar conditions(-0.3 V), and also similar to the polarogram of a Bi3+4itratemixture in a 1 : 10 mol ratio at pH 5.8, as described above.Thusranitidine does not bind directly to Bi"' in solutions of 1 underthese conditions, in agreement with NMR data.ConclusionRanitidine is one of many amines, including ammonia, whichcan solubilize [Bi(Hcit)]. The primary function of the amineseems to be to deprotonate the citrate hydroxyl group whichallows formation of a six-membered chelate ring. We have notbeen able to crystallize the adduct with ranitidine, complex 1,but the infrared data are consistent with hydroxyl deprotona-tion. From recently reported crystal structures of bismuth(II1)citrate complexes stabilized by ammonium ions as countercations it seems likely that the species in solution are dinuclearunits of the type [(cit)BiBi(cit)I2 - with additional oxide,hydroxide and water ligands depending on the pH, and furtheraggregation into networks via hydrogen bonding by bridgingcitrates, water and counter cations.The structures of theaggregates are important in determining the properties, e.g.solubility and long-term stability, of solutions of the adducts.Therefore the chemical (and biological) properties of bismuthcitrate adducts containing different counter cations are likely todiffer significantly. Complex 1 appeared to release about half ofthe bound citrate at high pH in a process with an apparent pK,of ca. 6.2. This may be due to the formation of complexes suchas [Bi604(oH)(cit),(H,o)3]3 - which has a lower bismuth:cit-rate ratio of 1 :0.5, and is known to be stable at alkaline P H .~This behaviour is quite distinct from that of the ammoniumadduct colloidal bismuth subcitrate which is reported to releasen TI I I I0 -0.2 -0.4 4.6EN vs. Ag-AgClFig. 8 Differential pulse polarograms for solutions containing (a) Bi3 +and ranitidine hydrochloride, rnol ratio 1 : 10, pH 2.70, (b) Bi3+, citricacid and ranitidine hydrochloride, rnol ratio 1 : 10: 10, pH 2.69, and (c)Bi3+ and citric acid, rnol ratio 1 : 10, pH 2.66. Peak assignments for (a):I, Bi3+; 11, Bi3+-ranitidine; 111, RNO, (ranitidine). The Bi3+ peak atca. -0.15 V in (b) appears to contain contributions from both Bi3+-citrate and -ranitidine speciesnearly all of the bound citrate at pH > 7.7 The increase in thepKa of the terminal NMe,H+ group of ranitidine by 0.26 unitsin 1 compared to ranitidine alone provided evidence for second-sphere hydrogen-bonding interactions between ranitidine andbismuth citrate species in solutions of complex 1, and suchinteractions were also apparent from 'H NMR studies of theNH resonances of 1 in dmso compared to ranitidine alone.Using differential-pulse polarography, which is a powerfulmethod for investigating complexation of Bi"' in aqueouss o l ~ t i o n ' ~ at low concentrations where NMR measurementsare difficult, we were able to detect inner-sphere complexationof ranitidine to Bi"' at low pH (2) even in the presence of citrate,but not at higher pH, in agreement with NMR data.Twodistinct types of bismuth(n1) citrate species were detectableover the range pH 1-7, and the lower-pH form was found to beinvolved in a rapid deprotonation equilibrium. Since antiulcerdrugs are subjected to low-pH environments in the stomach,such protonation equilibria may be relevant to the biologicalactivity of ranitidine bismuth citrate.AcknowledgementsWe thank Glaxo for their support for this work, Universityof London Intercollegiate Research Service and the MRCBiomedical NMR Centre for the provision of NMR facilities,Dr. J. Slater (Birkbeck College) for use of his polarographicequipment, and Drs. I. M. Ismail, G. Klinkert, J. W.MacKinnon and J. Seager (Glaxo) for stimulating discussions.References1 C. F. Baxter, Chem. Br., 1992,28,445.2 D. R. Williams, J. Inorg. Nucl. Chem., 1977,39,711.3 W. A. Hermann, W. Herdtweck and L. Pajadla, Inorg. Chem., 1991,4 E. Asato, W. L. Driessen, R. A. G. de Graaff, F. B. Hulsbergen and5 W. A. Hermann, W. Herdtweck and L. Pajadla, Z. Kristallogr.,6 E. Asato, K. Katsura, M. Mikuriya, T. Fujii and J. Reedijk, Chem.7 E. Asato, K. Katsura, M. Mikuriya, T. Fujii and J. Reedijk, Inorg.30,2579.J. Reedijk, Znorg. 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