摘要:

Novel, selective and co-operative assembly of cyclodextrins around[ 1,8-bis(pyridin=2-yl)-3,6-dithiaoctane] copper(@Sivagnanam Usha and Mallayan Palaniandavar *Depurtipicwt qf'Chemistry, Bharuthidasan University. Tiruchirupalli 620 024, IndiuThe redox chemistry of [CuL]' + [L = pdto = 1,8-bis(pyridin-2-y1)-3,6-dithiaoctane, bbdo = 1.8-bis(benziniidazol-2-yI)-3,6-dithiaoctane, pttn = 1,9-bis(pyridin-2-yl)-2,5,8-trithianonane or pttu = 1 , l l -bis( pyridin-2-yl)-3,6,9-trithiaundecane] in the presence of X - , P- and y-cyclodextrins (cd) in aqueous solutionhas been extensively investigated by cyclic and differential pulse voltammetric techniques. The addition ofcyclodextrins to the complexes results in a substantial decrease in peak currents rather than in peak potentials.The i,, rather than i, or AE, or E: is very sensitive to the variation in the cyclodextrin concentration.Thecouple Cu"-Cu' of [Cu(pdto)]'+ tends to become reversible, as shown by the decrease in AE, and that of i,,/i,,towards unity. Plots of i,,. i,,, E,, and AE, us. the number of moles of cyclodextrin show sharp inflections,interestingly at 5 , 4 and 3 mol of X - , P- and y-cd respectively. These limiting values do not correspond to theusual inclusion complex formation by cyclodextrins but to the formation of novel and regular arrays aroundthe complex. the number of molecules in the array being dictated by the size of the cyclodextrin. This alsoillustrates the prevention of adsorption of [Cu(pdto)] + on the glassy carbon electrode. For the other complexesthe changes in redox properties in the presence of cyclodextrins are not as regular and significant.Plots ofchanges in i,,, and i,, us. cyclodextrin concentration give Hill's coefficients greater than unity (1.3-2.1). Thevalues of K , , K , + for all the complexes and K, ( K 2 + ) for the complex formation of [Cu(pdto)]' + withcyclodextrins have been determined and discussed. Significant reduction or enhancement in E,,, values hasbeen observed both for the ligand-field and charge-transfer bands in the presence of all three cyclodextrins.Cyclodextrins are toroidally shaped polysaccharides made upof six ( x-ed), seven (P-cd) and eight (y-cd) D-glucose monomers,joined by Y( 1-4) bonds.' Owing to the hydrophobic nature oftheir inner cavities they act as hosts for many organicmolecules,'.3 inorganic ions and organometallic com-pounds7.* and form inclusion complexes when the guest andcavity are of appropriate size." They are the most importantand widclq studied examples of host molecules.The inclusionof bis( pyriclin-2-yImethanolato)copper(rr) by cyclodextrins hasbcen investigated by EPR and CD spectroscopy. Recentlythe effect of X - and P-cyclodextrins on the ligand-substitutionreactions of pentacyano( N-heterocycle)ferrate(lr) complexeshas been reported. " The electrochemical oxidation offerrocenecarboxylic acid in the presence of P-cd has beeninvestigated. ' ' . I ' The second-sphere co-ordination of plati-num and rhodium complexes bearing hydrophobic ligands,consisting of cyclodextrins. has been structurally characterizedby Alston ( ' I rrl.I s in the solid state using X-ray crystallo-graphy.A cyclodextrin is also a well constructed miniature of anenzyme in the sense that it has a hydrophobic cavity ofappropriate size, sites for introduction of catalytic groups atjuxtapositions, and satisfactory water solubility.' The rateand stereochemical pathway of organic reactions are signifi-cantly changed by the inclusion complexation of substrateswith cyclodextrins. Since the mechanisms of such reactionsarc very similar to those of enzymes or biological recep-tors, cyclodextrins have been regarded as good models forbiopolymers and have been studied extensively as models forenzyme active sites. 1 7 - '"As we have been concerned in constructing models for andelucidating the novel electrochemical behaviour, high Cu"-Cu'redox potentials and facile electron transfer, of blue copperproteins, the electrochemical behaviour of copper(r1) complexesdesigned a s models for these proteins in the presence ofcyclodextrins was of interest. Inclusion complex formation byHOOHn = 6 a-CDn = 7 p-CDR =8 ?I-CDHO'cyclodextrins, added in solution or bound to an electrode, hasbeen shown to enhance the electrochemical reversibility ofseveral organic guests.,' We have chosen the biomimeticcopper(1r) complexes [Cu(pdto)]' + [pdto = 1,8-bis(pyridin-2-y1)-3,6-dithiaoctane]," [Cu(bbdo)]" [bbdo = 1,8-bis(benz-imidazol-2-yl)-3,6-dithiaoctane] 2 2 [Cu(pttn)I2 ' [pttn = I ,9-bis(pyridin-2-yl)-2,5,8-trithianonane] 2 3 and [Cu(pttu)12 +[pttu = 1,ll -bis(pyridin-2-yl)-3,6.9-trithiaundecane] " for thepresent investigation.The advantages associated with thesecomplexes are their water solubility, known structures andwell characterized intense S(o)+Cu" charge-transfer (c.t.)band. Although there are a few reports on the inclusioncomplex formation of metal complexes with cyclodextrins, ' 2 3 2 4that described in the present study is novel.ExperimentalMaterialsThe cyclodextrins were generous gifts from American Maize-Products Co. 2-Vinylpyridine, ethane- 1,2-dithiol, bis(2-sulfan-ylethyl) sulfide and copper(r1) perchlorate hexahydrate werefrom Aldrich and used as such.J. Chem. SOC., Dalton Truns., 1996, Puges 26OY-26 15 260p d t o bbdop t t n p t t u[ CU (pd to) (OClO, )]'SynthesesThe compounds pdto, bbdo, pttn and pttu and their Cu(ClO,),complexes were prepared as described previously.' ' 2 3CAUTION : [I ,8-bis(pyridin-2-yl)dithiaoctane]copper(11) per-chlorate is a contact explosive.Brubaker" was deafened forseveral months by the explosion of about 1 g of the perchloratewhich detonated while being transferred from a sintered glassfunnel. No such incident occurred during this work. However,[Cu(pttu)][ClO,], exploded when transferred from a sinteredglass crucible.Physical measurementsThe reflectance and solution spectra were recorded on a HitachiU-3400 double-beam UV/VIS/NIR spectrophotometer, EPRspectra on a Varian E-I12 X-band spectrometer, the field beingcalibrated with diphenylpicrylhydrazyl (dpph).The values of g ,and A , were measured at ambient temperature and gll and A at77 K. Cyclic voltammetry and differential pulse voltammetryon a glassy carbon or platinum-sphere electrode wereperformed at 25.0 k 0.2 "C. A three-electrode cell configurationwas used, the reference electrode being a saturated calomelelectrode (SCE). The temperature of the electrochemical cellwas maintained by a cryocirculator (Haake D8 G). Thesolutions were deoxygenated by bubbling a stream of nitrogengas through them and an atmosphere of nitrogen wasmaintained over solutions during measurements. The instru-ments utilized included an EG&G PAR 273 potentiostat/gal-vanostat and an IBM PS-2 computer; EG&G M270 softwarewas employed to carry out the experiments and to acquirethe data.A HP plotter (DMP 40) was used to plot out thevoltammograms. The E : value observed under identicalconditions for (hydroxymethy1)ferrocene in aqueous 0.1 mmoldm-3 NaCIO, solution was 0.176 V us. SCE and AEpo (AEp atzero scan rate) was 73 mV.Results and DiscussionSeveral intermolecular interactions have been proposed anddiscussed as being responsible for the formation of cyclodextrininclusion complexes in aqueous solution. 6 , 1 8 , 2 3 , 2 6 They arehydrophobic, van der Waals, hydrogen bonding, the release ofhigh-energy water from a cyclodextrin-water adduct, togetherwith the formation of a hydrogen-bonding network aroundthe 0(2), O(3) (secondary hydroxyl groups) side of the torusof the cyclodextrin macrocycle, upon substrate inclusion.Recently the second-sphere co-ordination of transition-metalcomplexes bearing hydrophobic ligands by cyclodextrins hasbeen characterized structurally in the solid state by X-raycrystallography.' ' In these adducts the hydrophobic ligandof the complex penetrates the cavity of the cyclodextrin uiu thewider aperture of the receptor associated with the face bearingthe secondary hydroxy groups. Thus in the crystal structure l 4of the I : 1 adduct of trans-[Pt(PMe3)(NH,)C12] with P-cd thePMe, ligand is inserted into and bound to the narrow primaryhydroxy-group-bearing face of the P-cd torus. Further, aseverely disordered surface forms a lid on the wider secondaryhydroxy-group-bearing face of the 0-cd torus and the complexlies within this surface.In the present study we have success-fully employed electrochemical techniques to discern the selec-tive and co-operative assembly of cyclodextrins around[Cu(pdto)]'+ in aqueous solution.Electronic and EPR spectraIn aqueous solution all the present copper(I1) complexes exhibitone ( 16 700 cm ') or two (1 6 700, 1 1 200 cm-') intense ligand-field bands as well as a highly intense S(o)+Cu" charge-transferband (29 300-28 100 cm-') (Table 1, Fig. l).22.23*27 For almostall the complexes no change in the position of the bands isdiscernible on the addition of an excess (0.010 mol dm-3) of X - ,p- and y-cd in aqueous solution (Table 1 , Fig.1 ); however, thereis a small but significant reduction or enhancement in molarabsorption coefficient for both these bands. This is similar tothe observation made in acetonitrile solution, suggesting thatthe binding of cyclodextrins to the complex is not sufficientlystrong to perturb the primary co-ordination sphere. Thisinference is supported by the absence of any change in eithergll or A l l in the presence of an excess of cd (2% aqueoussolution).2610 J. Chem. Soc., Dalton Trans., 1996, Pages 2609-261200 50045.025.0-. % 5.0 --1 5.0h /nmFig. 1 Charge-transfer (200-500 nm) and ligand-field (400-850 nm)spectra of [Cu(pdto)]'+ (0.001 rnol dm-3) in aqueous ( a ) and aqueousa-cd (0.010 rnol dm 3 , (b), P-cd (0.010 mol dm 3 , (c) and y-cd (0.010 moldm ') ( r l ) solutions---Table 1presence of an excess' of cyclodextrinElectronic spectral data for copper(n) complexes" in theCmax/cm-I (~,,,/dm~ mol-' cm ')Complex[Cu(pdto)]'+[Cu(bbdo)]' '[Cu(pttn)]' +[Cu(pttu)]' +MediumWaterMeCNx-cdP-cdy-cdWaterMeCNx-cd0-cdy-cdWaterMeCNx-cdP-cdy-cdWaterMeCNa-cdP-cdy-cdLigand field1 6 800 (494)16 900 (743)16 800 (408)16 800 (61 9)16 800 (426)16 700 (168)11 400(189)16 900 (220)1 1 600 (210)16700(172)1 1 300(189)16 700 (173)11 300 (192)16 700 (187)1 1 400 (186)16 700 (41 1)11 200(179)I6 700 (450)11 200(190)16 700 (392)1 1 200(168)16 700 (375)1 I 200 (160)16 800 (384)1 I 200 (165)16 700 (296)16 300 (490)16 700 (196)16 600 (517)I6 800 (356)11 100 (100)Charge transfer28 600 (3210)28 000 (4660)28 600 (2900)28 600 (1 800)28 600 (2795)29 300 (3760)24 700 (1420)28 700 (2810)29 300 (1990)24 700 (680)29 300 (2070)24 700 (680)29 300 (2000)24 700 (640)28 100 (3590)27 700 (4760)28 300 (3100)28 300 (3420)28 300 (3400 )26 400 (1910:i26 100 (3280)26 400 ( 1 580)28 400 (20 1 5 )28 400 (2080)" Concentration of the complex 0.001-0.001 5 rnol dm-3.Concentrationof a-cd, 0.010; P-cd, 0.010; y-cd, 0.010 mol dm-3.Electrochemical behaviour of complexesThe cyclic voltammogram of [Cu(pdto)12 + in aqueous solutionat a glassy carbon electrode at 50 mV 5-l shows that the Cu"-I I I 1 I I1 -0.8 VI I I 1 I 1 I0.7 0.5 0.3 0.4E N vs.SC EFig. 2 Cyclic voltammograms of [Cu(pdt~)]~' (0.001 rnol dm 3 , inaqueous solution (0.1 rnol dm-3 NaC10,) in the absence (---) andpresence of 0.010 mol dm p-cd (- - ~- -) at 50 mV s scan rate2.402.00xg 1.60 \.Z1.200.800 0.20 0.40 0.60 0.80vi/vi s-iFig. 3 Plots of i,,ii,, us. v1 for [ C u ( p d t ~ ) ] ~ + in the absence (0) andpresence of a-cd (A), 0-cd (0) and y-cd (0) solutions-35.01 I I I I I0.8 0.7 0.6 0.5 0.4 0.3 0.2ENvs.SCEFig. 4 Cyclic voltammograms of [Cu(pttu)]" (0.001 rnol drn-') inaqueous solution (0.1 mol dm-3 NaC10,) in the absence (--) andpresence of 0.010 rnol dm-3 a-cd (. . .) and 0.01 mol dm y-cd (- - - -)at 50 mV s-l scan rateCu' couple is irreversible (Fig.2), as evident from the very highvalues of AE, and peak current ratio (ipa/iPc) (Table 2; AE,, = 59mV and i,,/i,, = 1 for a reversible one-electron transfer). Theweak adsorption of the product of reduction, [Cu(pdto)]+, isrevealed by the symmetrical anodic peak at high current andthe shape of the plot of i,,/i,,, us. vi (Fig. 3 ) . 2 8 The adsorp-tion of [Cu(phen),] + (phen = 1,lO-phenanthroline) containingaromatic rings on the surface of a glassy carbon electrode hasbeen previously noted.29 The redox cycle of [Cu(pdto)12+ isquasi-reversible (AE, I26 mV) at a platinum-sphere electrodeand the i,,/i,, value is near to unity suggesting no adsorptionJ. Chem. SOC., Dalton Trans., 1996, Pages 2609-261.5 261Table 2a glassy carbon electrode, scan rate 50 mV sRedox properties ofcopper(1r) complexes (0.001 niol dm 3 , in aqueous solution (0.1 mol dm NaCIO,) in the presence of cyclodextrins" at[Cu( pd to)]' + NonexPY[Cu(bbdo)]' + NonexPY[Cu(pttn)]' + NonexPYNone [Cu( pt tu)] 2 +xP40.304 0.430 40.00.296 0.418 34.30.318 0.424 33.90.286 0.442 33.80.428 0.552 23.80.394 0.551 13.70.412 0.530 18.60.400 0.520 14.70.288 0.372 54.60.280 0.364 50.60.276 0.362 49.80.272 0.358 43.80.434 0.512 38.90.428 0.512 36.00.436 0.522 29.90.458 0.542 38.6E+IV10"D )IComplex Cyclodextrin Epc/V E,,!V i,,ipA i,,lpA AE,imV CV DPVh ipd;ipc' cm' s ' K,:K,,79.4 126 0.367 0.369 2.3 2.8233.1 122 0.357 0.367 1.4 2.23 0.6829.8 106 0.371 0.371 1.2 3 9 7 0.5230.5 156 0.364 0.375 1.3 I .97 0.6358.2 124 0.490 0.457 2.8 1.2945.5 157 0.473 0.449 3.8 0.29 0.8659.9 I18 0.471 0.443 3.6 0.77 0.4839.9 120 0.460 0.433 3.1 0.40 0.3631 84 0.330 0.303 2.9 5.1292.6 84 0.322 0.294 2.3 4.79 0.8906 86 0.319 0.297 2.5 4.47 0.6583.8 86 0.315 0.295 2.3 3.24 0.5631.8 78 0.473 0.475 1 .1 1.2028.0 84 0.470 0.471 1 . 1 2.02 0.9624.0 86 0.479 0.477 1.1 1.40 1.2624.7 84 0.500 0.504 1 . 1 2.02 3.10Concentration of x - , P- and y-cd, 0.005 rnol dm-3. Scan rate for differentical pulse voltammetry 0.001 V s ' pulse height 50 mV. Calculated as[[(in& + 0.485(iqp)o]/i,,c~ + 0.086.of copper(1) species. The entirely different crystal structures 2 3of [Cu(pdto)(CIO,)] + (square pyramidal) and [Cu(pdto)] +(tetrahedral) species mean that significant configurationalchanges accompany the heterogeneous electron-transfer re-action.30 This leads to a large reorganizational energy whichis reflected in the decreased rate of electron transfer and hencethe higher AE, value observed for the Cu"-Cu' couple. Thebenzimidazole analogue of the pdto complex, [Cu(bbdo)]' +,also shows irreversible redox behaviour with high i,,ji,, andAE, values because of adsorption of copper(') species. Incontrast, [Cu(pttu)12 +, the homologue of [Cu(pdto)]' +,displays nearly reversible redox behaviour (Fig. 4) in aqueoussolution, as verified by the constancy of ip,iv4, low AE, valuesand an i,,/i,, value of unity (Table 2).Dithioether complexes in the presence of cycl&dextrins.Theincremental addition of X-, P- and y-cd to [Cu(pdto)12+ inaqueous solution diminishes the anodic peak currentconsiderably and there is a substantial cathodic shift in theanodic peak potential. However, the changes in current andpotential of the cathodic peak are comparatively small. Thus onthe addition of X- and P-cd the Cu"-Cu' redox cycle tends tobecome diffusion controlled and reversible, as inferred from thelinear plot of i,, us. v f passing through the origin (Fig. 5 ) , thedecrease in AE, and the approach of i,,/i,, values to unity(Table 2).When the cyclodextrin concentration is increased the valuesof both the anodic and cathodic peak currents decrease to acertain concentration and then remain constant, suggestingcompletion of adduct formation with the copper complex(Table 2).Plots of i,, [Fig. 6(a)-(c)], E,, and AE, us.cyclodextrin concentration show inflection points at approxi-mately 5 for K-, 4 for 0- and 3 mmol dm for y-cd, with thelimiting concentrations corresponding to adduct formation of[Cu(pdto)12' with M - , P- and y-cd in the mole ratios 1 : 5, 1 :4and I : 3 respectively. The observed stoichiometries depend,interestingly, upon the size of the cyclodextrin molecules andhence the number of u-glucose monomers. The observation ofsuch saturation points has often proved difficult or im-possible." However, Yokoi et aL6 used EPR spectroscopy toobserve the inclusion of one and two molecules of bis(pyridin-2-ylmethanolato)copper(ir) with M - and y-cd respectively.Haradaand Takahashi 24 found that ferrocene needs two molecules ofx-cd and one of y-cd to form inclusion complexes. However, nosuch regular array of cyclodoxtrins around even organici 140.00120.00100.00Q1 80.00Q60.0040.00I l l l l l l l l ~ l l l l " " l ~ l l l l " " ~ I " " ' I " ' 1 0 0.20 0.40 0.60 0.80v'/v'&Fig. 5 Plots of i,, us. v ' for [Cu(pdto)I2' (0.001 mol dm ') in theabsence (u) and presence of a-cd (0.005 mol dm-3) (h), 0-cd (0.004 rnoldm ') ( c ) and y-cd (0.003 rnol dm-3) ( d )substrates has been observed previously. It has been shown thatmodified or substituted P-cd forms an aggregate, specificallywith 4-nitrophenol and not with other substrates. l 2The peak-current ratio in the presence of X - , P- and y-cdincreases only slightly with increase in scan rate suggesting thatthe copper(r) species is very slightly adsorbed on the surface ofthe ele~trode,~' in contrast to the cd-free solutions in which it isweakly adsorbed. In the Cu"-Cu' redox potential range thecoverage of the electrode by cd molecules is greater 3 1 or theirorientation is adequate to form a more compact layer; this mayalso lead to the prevention of adsorption and explain thereduction in peak-current value and observed reversibility.However, the addition of u-glucose, even at higher con-centrations, did not confer reversibility. So it is clear thatonly adduct formation renders the redox behaviour diffusioncontrolled and reversible.Wc have shown that inclusioncomplex formation of methyl viologen dication 1,1 '-dimethyl-4,4'-bipyridinium by P-cd is an effective and selective methodto reduce adsorption and hence to confer reversibility on theelectroactive species.2o For [Cu(pdto)12 + a similar inclusion2612 J. Chem. SOC., Dalton Trans., 1996, Pages 2609-26180.00 + Cu2+ + e - -CU'70.0030.0060.00s .m 50.00.-a40.00- - = - -20.00 I I I I I I I I r n0.00 2.0-1 0.00 12.00l l l l l - ~ ~ r nla-cdl/mmol dm-380.0070.0060.0050.00\ sm-!? 40.0030.0020.00 1[,p-cd]/mmol dm-340.0030.0034.0030.0032.00 0.00[~-cd]/mmol dm-3Fig. 6 Plots of ( ( I ) observed anodic peak current us. concentration of3-cd added. ( h ) observed anodic peak current us. concentration of P-cdadded and ( c ) observed cathodic peak current us.concentration of y-cdadded, for [Cu(pdto)]'+ (0.001 mol dm 3 ,phenomenon is not obviously involved, as the sizes of the cdcavities arc much smaller than that of the complex.The interaction of the present complexes with cyclodextrinsmay involve expulsion of a few water molecules or the partialinclusion of the pyridine or benzimidazole ring into the primaryor secondary hydroxy-group-bearing face of the cd. Theexpulsion of the enthalpy-rich 3 2 water molecules enclosedwithin the uncomplexed cd cavity into bulk water uponScheme 1substrate inclusion results in a negative enthalpy change,together with a positive entropy change, leading to strongerinteraction. An X-ray crystallographic study of p-cd dodecahy-drate33 has shown that 6.5 water molecules within the cavityare disordered over eight sites and display extensive thermalmotion.Also there may be hydrogen-bonding interactions ofthe benzimidazole NH group of co-ordinated bbdo withhydroxyl groups of the cyclodextrins.From the slope of the plots of i,, us. v: plot the values of thediffusion coefficient (D) were calculated using Sevcik'se q ~ a t i o n . ~ " In the presence of X - , b- and y-cd the slope issuppressed and thus there is always a decrease in D (Table 2)suggesting that the copper(r1) complex diffuses slowly to theelectrode mainly in a cyclodextrin-bound form. The diffusioncoefficients of the cd-bound complexes (D,) calculated from thei,, values at the limiting cyclodextrin concentrations (Table 2),at which adduct formation is complete, are also typical33 ofone-electron transfer.The anodic and cathodic peak currents of [Cu(bbdo)]'+decrease and the peak-current ratio slightly increases when theconcentration of IX-, p- and y-cd is increased; however, noinflections are observed.The AEp value decreases slightly for allthe three cyclodextrins, suggesting that the copper(r1) species isnot stabilized in cd solutions.Trithioether complexes in the presence of cyclodextrins.Addition of the cyclodextrins in steps to aqueous [Cu(pttn)]'+and [Cu(pttu)12+ has little effect on the Cu"-Cu' redox process(Table 2, Fig. 4). Thus the values of AE, and E : remainunaffected; however, the peak currents decrease with o; withoutany change in i,,/i,, values, suggesting that the complexesinteract with the cyclodextrins appreciably. Addition of 0- andy-cd to [Cu(pttn)]'+ decreases E : as well as the peak-currentratio (Table 2) suggesting that adsorption of copper( I ) speciesformed on reduction is not completely prevented.For theaddition of p- and y-cd to [Cu(pttu)]'+ there is a slight increasein E, (see below). Thus, unlike [Cu(pdto)]". no interestingchanges in electrochemical properties were observed on addingthe cyclodextrins; this suggests that the interaction of[Cu(pdto)]' + with cyclodextrins is selective, probably becausethe complex is square planar. Further, there is no cleavage of ametal-ligand bond but a stereochemical rearrangement from asquare-planar to a tetrahedral geometry occurs on electrontransfer; on the other hand for the non-planar trithioethercomplexes cleavage of at least one M-L bond and hence breakup and reforming of the assembly of cyclodextrins around theredox-active species occurs.The presumably five-co-ordinate[Cu(pttu)I2+, like its homologue23 [Cu(pttn)]' would leadto the formation of four-co-ordinate [Cu(pttu)] ' in which oneof the Cu-N,, bonds is broken."Changes in redox potentialsFrom the shifts in redox potentials observed on the additionof cyclodextrins the ratio of the formation constants K + / K 2 +was calculated, assuming reversible electrode reaction as wellas reversible binding of copper-(11) and -(I) species to thecyclodextrins (Scheme 1) and using the general formula ' O (1)where Ebo and Efo are the formal potentials of the redox couple( + 2/ + 1) in the bound and free forms respectively and K , + andJ.Cliem. SOC., Dalton Trans., 1996, Pages 2609-2615 261Table 3 Hill coefficient (h) and K, * using i,, and i,, values for the interaction of [Cu(pdto)]’ + with cyclodextrinsx-cd P-cd y-cdParameterused h K,/dm3 mol-’ 11 K,/dm3 mol-’ h K,/dm3 moli, 2.09 3680 1.87 4730 1.44 900i,, 1.47 1450 1.25 2130 1.49 920* K, values are l/[intercept] and h values are the slopes of the plots of In [ Y/( 1 - Y)] us. In X(Fig. 7).0.00 72.00 :0.00 <6 4 -1 .oo-2.00 31 -2.00-3.00 I ~ ~ I ~ I I I I I I I I I I I I I I I I 1 1 1 1 1 1 1 l l l r ~ l i ~ ~ ~ ~ ~ ~ ~ ~ ~ i - l l l 1 1 1-1150 -1;OO -0150 0.60 0.50 1.00 1.50In XFig.7 Plots of In[ Y/( 1 - Y)] us. In X using the cathodic peak current(a) and the anodic peak current (b) of [Cu(pdto)]” in the presence of0-cdK , are the equilibrium constants for the binding of cd to thecopper-(11) and -(I) complexes, respectively. The formation anddissociation of the adducts are expected to be fast enough tomaintain an equilibrium on the time-scale of the cyclicvoltammetry experiment. The values of K+/K,+ (Table 2)illustrate that the copper-(n) rather than -(I) forms of almostall the complexes interact strongly with all the cyclodextrins;however, the values are greater than unity for [Cu(pttu)12 + inthe presence of P- and y-cd suggesting that the copper-(I) ratherthan -(II) form interacts strongly.Is the binding of cyclodextrins co-operative?Allosteric regulation of binding and catalysis is a commonfeature in the regulation of enzymes by molecular effectors.When the affinity of an enzyme for a substrate increases withincreasing effector concentration the allostery is termed positiveco-operativity, and the transition from the inactive to the activestate of the protein is the allosteric transition.Positive co-operativity in the binding of inorganic guests to synthetic hostshas been observed in dicoronands linked by a biphenyl,36 gableporphyrins, 37 porphyrin dimer~,~’ and crystalline haemmodels. 39 It has been reported also in micelle-catalysedreactions. 4 0 3 4 Ti trations of [Cu(pdto)]’ + with cyclodex trinsled to sigmoidal rather than hyperbolic curves.Replotting ofthe titration data according to the Hill equation 1 2 q 4 0 (2) where(2) In[ Y/(l - Y)] = h In X - In KdX = concentration of cyclodextrin added, Y = ratio of theobserved change in the value of i,, or ipc of the complex onadding the cd to the maximum change in the respectiveelectrochemical property, h = the Hill coefficient and Kd =overall dissociation constant, gave straight lines (Fig. 7) theslope of which is defined as the Hill coefficient (h), which isconstrued as an index of co-operativity. Non-co-operativesystems exhibit h 1.0, positively co-operative systems h >1.0 and negatively co-operative systems h < 1.0. As apoint of reference, haemoglobin has h = 2.8, while myoglobinhas h = I .O towards binding of oxygen.For the binding of [Cu(pdto)]’ + to cyclodextrins the h valuescalculated using i,, and i,, are collected in Table 3.For all threecyclodextrins the value of h is greater than unity (1.3-2. I). Hillcoefficients greater than unity are the experimental hallmark ofpositive co-operativity,’ in which initial binding events rendersubsequent binding events more favourable. Thus the presentstudy shows that the binding of cyclodextrins to [Cu(pdto)]* +is co-operative. The analysis of i, and i,, data gave values of theassociation constant K, (1 / K d ) which are of the same order ofmagnitude for all the cyclodextrins. These values suggest thatthe stability of [Cu(pdto)I2’ adducts varies in the order p-cd > a-cd > y-cd.For several organic substrates P-cd has beenshown to exhibit selective inclusion complex formation. For thepresent complex selectivity is not achieved obviously becauseinclusion is not involved; however, the stability is still thehighest. It appears that uncomplexed p-cd includes a number ofhigh-energy water molecules, a higher number than those ofuncomplexed X - C ~ . ~ ~ . ~ ’ So, if the major part of the bindingenergy for the present complex is derived from the release ofthese water molecules, P-cd should lead to a more negativeenthalpy of complexation and hence a higher associationconstant than would a-cd. In contrast, the values of AHo andA 9 for the formation of inclusion complexes of P-cd withp - n i t r ~ p h e n o l , ~ ~ p-nitrophenylglyc~sides,~~ and rn- and p -disubstituted benzenes 44 are considerably less negative thanthose for the corresponding a-cd complexes.The association constant K, for the interaction of an organichost molecule with n cyclodextrin molecules has been calculatedusing the diffusion coefficient of the free complex (D,) and theobserved diffusion coefficients ( D o h S ) at various cyclodextrinconcentrations using the relationship 43 (3).However, the2614 J. Chem. Soc., Dalton Trans., 1996, Pages 2609-261present adduct-formation data failed to fit this relationship forthe observed values of n = 5 , 4 and 3 for X - , p- and y-cdrespectively. This suggests that complete adduct formationoccurs only at the saturation points prior to which the co-operative adduct formation occurs.ConclusionThe present voltammetric study reveals that the Cu"-Cu' redoxcouple of [Cu(pdto)]* + becomes free from adsorption effectsand tend to enhance its reversibility in the presence of five, fourand three equivalents of a-, P- and y-cd respectively.Theselimiting concentrations decrease with increasing size of thecyclodextrin molecules and are consistent with the formationof arrays of cyclodextrins around the copper(r1) complex.However. for other copper(1r) complexes no significant changesin redox properties have been observed. Thus the binding ofcyclodextrins to [Cu(pdto)]' + tends to be selective and P-cdexhibits a stronger binding than a- and y-cd.Cyclodextrins have been reported to be effective hosts formany molecules by inclusion complex formation but the presentassembly of cyclodextrins around the redox-active species canbe regarded as a new and multifunctional model for thebiological systems involved in molecular recognition.3 2The self-assembly of cyclodextrin molecules to formfunctional aggregates like micelles illustrates the principle af co-operativity in enzymes. The positive co-operativity observed forthe binding of all the three cyclodextrins with [Cu(pdto)12+implies the stimulation of the interaction of additional substratemolecules upon interaction of the first molecule with theenzyme. *'AcknowledgementsS. U. acknowledges the University Grants Commission, NewDelhi for a Senior Research Fellowship. 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ISSN:1477-9226    

DOI:10.1039/DT9960002609

出版商:RSC    

年代:1996

数据来源: RSC