Chemistry and Function of Pectins - American Chemical Society

7 Ionic Effects on the Conformation, Equilibrium, Properties, and Rheology of Pectate in Aqueous Solutions and Gels

Downloaded by UNIV OF NEW SOUTH WALES on August 14, 2015 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch007

S. Paoletti, A. Cesaro, F. Delben, and A . Ciana Dipartimento di Biochimica, Biofisica e Chimica delle Macromolecole, Università di Trieste, Piazzale Europa 1, I-34127 Trieste, Italy

The effect on the conformational-aggregational properties of pectate in aqueous solution brought by the addition of spec i f i c ions (H+, Ca +, Cu +) was studied by osmometric, microcalorimetric, d i l a t o metric, and rheological methods. Evidence is provided for the intramolecular nature of the pH induced conformational t r a n s i t ion. The addition of divalent ions brings about at the same time a conformational change of the chain of pectate and chain-chain association. 2

2

The study of the i n t e r a c t i o n of pectate (and i t s part i a l l y e s t e r i f i e d " d e r i v a t i v e s " , p e c t i n s ) with specific ions i n aqueous s o l u t i o n s i s of fundamental importance t o understand the p r o p e r t i e s of t h e i r s o l u t i o n s and g e l s a t the molecular l e v e l . Indeed, pectate aqueous s o l u t i o n s i n the presence of ions have been subjected to many inves t i g a t i o n s (1-5). However, only i n a few cases has the "course" of the i n t e r a c t i o n been followed over a wide range of the i o n t o polymer molar r a t i o , R. The s c r u t i n y of such a dependence f o r thermodynamic f u n c t i o n s l i k e Δ Η has allowed us t o d i s c l o s e an i n t r a m o l e c u l a r coopera t i v e conformational t r a n s i t i o n of pectate upon changing pH (5). Other p r o p e r t i e s i n v e s t i g a t e d were c o n s i s t e n t with the above i n t e r p r e t a t i o n . I t was a l s o suggested that the intramolecular conformational t r a n s i t i o n was a p r e r e q u i s i t e f o r the f u r t h e r aggregation of chain mo l e c u l e s o c c u r r i n g i n the g e l l i n g c o n d i t i o n s a t low pH. D i r e c t evidence from molecular weight measurements will be provided here i n favor of such a t r a n s i t i o n . Among the other parameters able t o induce s i m i l a r macroscopic e f f e c t s , the a d d i t i o n of d i v a l e n t c a t i o n s i s p a r t i c u l a r l y e f f e c t i v e . L i t e r a t u r e data are i n almost complete agreement with the view that an aggregation of chains induced by ions i s r e s p o n s i b l e f o r the g e l forma0097-6156/86/0310-0073$06.00/0 © 1986 American Chemical Society

In Chemistry and Function of Pectins; Fishman, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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t i o n (1,4). The c u r r e n t l y accepted model envisages ions b r i d g i n g p a r a l l e l chains to form a l i n e a r a r r a y which i s supposed to be the j u n c t i o n matrix f o r the g e l network ( 6 ) . T h i s l a r g e l y adopted model has eluded to d i r e c t thermodynamic confirmation because the formation of g e l has been a s e r i o u s l i m i t a t i o n to the r e p r o d u c i b i l i t y of measurements. On the other hand, an a n a l y s i s of the thermodynamic p r o p e r t i e s i s necessary i n order t o a s c e r t a i n whether the polymeric conformational " s t a t e " has been subjected to changes or not. The r e s u l t s presented here on pectate solutions c o n t a i n i n g calcium or copper ions b r i n g i n t e r e s t i n g evidence f o r an anomalous behavior of pectate p r o p e r t i e s i n the range of i o n to polymer r a t i o below that correspond i n g to the massive phase s e p a r a t i o n . The p o s s i b i l i t y that a conformational change proceeds along with the i n t e r a c t i o n with d i v a l e n t ions i s examined and d i s c u s s e d i n comparison with the case of the proton-induced conformational t r a n s i t i o n . In a d d i t i o n , the development of the g e l phase was followed by studying the r h e o l o g i c a l behavior of a complex calcium-pectate system. pH-induced Conformational

Transition

On the b a s i s of d i r e c t m i c r o c a l o r i m e t r i c r e s u l t s of enthalpy changes of d i s s o c i a t i o n and of d i l u t i o n , we have proposed that by changing pH from about 3 to about 7 the pectate chain undergoes an i n t r a m o l e c u l a r cooperat i v e conformational t r a n s i t i o n i n very dilute solution ( 5 ) . Although nothing could be s a i d about the a c t u a l geometry of the conformational s t a t e s i n v o l v e d , that f i n d i n g was s u b s t a n t i a t e d by a d d i t i o n a l v i s c o m e t r i c and ( c h i r o ) o p t i c a l evidence ( 5 ) . P e c t i c a c i d has a known tendency to give r i s e to chain-chain aggregation, especiall y so a t low pH values. Therefore i t was h i g h l y d e s i r a b l e to o b t a i n d i r e c t evidence from molecular weight measurements f o r the i n t r a m o l e c u l a r character of such a t r a n s i t i o n , by i d e n t i f y i n g a proper range of e x p e r i mental c o n d i t i o n s and using a s u i t a b l e technique. Membrane osmometry has been r e c e n t l y shown t o be u s e f u l i n studying the behavior of pectate in dilute s o l u t i o n (7). The r e s u l t s of osmotic pressure measurements obtained i n the g/L polymer c o n c e n t r a t i o n range are reported i n Figure 1. They have been obtained i n 0.1 M i o n i c s t r e n g t h a t 27°C at pH = 3 . 5 and pH = 6 . 5 , i.e. f o r pH values corresponding to the two conformations. No problem was found as t o the r e p r o d u c i b i l i t y of measurements on s o l u t i o n s p r e v i o u s l y e q u i l i b r a t e d by d i a l y s i s . Although the points obtained at low pH are a l i g n e d on a l i n e of negative slope ( p o s s i b l y i n d i c a t i n g a poor polymer-solvent i n t e r a c t i o n ) . s t i l l the e x t r a p o l a t e d value of the reduced osmotic pressure t o zero polymer concentrat i o n i s the same f o r both sets of data points. This i s a c l e a r - c u t evidence that the conformational transition



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Ionic Effects on Pectate

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that takes place i n d i l u t e s o l u t i o n i s an i n t r a m o l e c u l a r process; f u r t h e r a g g r e g a t i o n a l phenomena w i l l depend on increases i n polymer and/or H+ ion c o n c e n t r a t i o n , changes i n temperature and i o n i c s t r e n g t h . The second virial c o e f f i c i e n t of the reduced osmotic pressure p l o t of pectate changes from p o s i t i v e to negative on passing from n e u t r a l tô a c i d i c c o n d i t i o n s , i . e . from the charged to the uncharged form. T h i s f i n d i n g could be i n agreement with the marked decrease of the e m p i r i c a l S m i d s r 0 d ' s Β parameter of s t i f f n e s s upon decreasing the charge d e n s i ty of pectate, i n d i c a t i n g a notable stiffening of the chain ( 8 ) . Recent c a l c u l a t i o n s would i n d i c a t e that the value of the radius of g y r a t i o n of pectate i n 0.08 M phosphate b u f f e r passes from 60 A at pH 7.3 to 77 A at pH 3 . 7 , i n agreement with the p i c t u r e of a elongated conformation p r e v a i l i n g i n a c i d i c c o n d i t i o n s d e s p i t e the dramatic r e d u c t i o n of charge d e n s i t y ( 7 ) . E n e r g e t i c s (thermodynamics) of i n t e r a c t i o n with d i v a l e n t ions Pectate i s known to i n t e r a c t very s t r o n g l y with s e v e r a l d i v a l e n t i o n s , although the concept of "binding" may change among d i f f e r e n t authors, to i n c l u d e d i f f e r e n t modes of i n t e r a c t i o n . Nevertheless, our present investi g a t i o n seems to confirm such an e s t a b l i s h e d p i c t u r e . From e q u i l i b r i u m d i a l y s i s experiments (9), the amount of calcium "bound" to the polymer was taken equal to the d i f f e r e n c e between the c o n c e n t r a t i o n of i o n i n the polymeric phase and that i n the polymer-free solution. In the range 0 < R < 0 . 3 . an average value of 84 t 4 % of Ca + ions i s "bound" to the pectate chains i n aqueous 0 . 1 M NaC104.. In t u r n , from a previous investigation of t h i s . l a b o r a t o r y ( 1 0 , 1 1 ) , the percentage of copper ions bound under the same c o n d i t i o n s has been estimated to be l a r g e r than 99 %. T h i s l a t t e r value i n c l u d e s both the f r a c t i o n of ions s t r o n g l y i n t e r a c t i n g with the electro s t a t i c f i e l d generated by the macroion and the fraction ( i f any) of ions s p e c i f i c a l l y bound on g e o m e t r i c a l l y favorable s i t e s (12). A general r e s u l t of the e l e c t r o s t a t i c interactions between counterions and p o l y e l e c t r o l y t e s ( i n c l u d i n g site binding) i s the l a r g e p o s i t i v e volume change due to the r e l e a s e of e l e c t r o s t r i c t e d water molecules from the s o l vated groups i n t o the bulk of the s o l u t i o n ( d e s o l v a t i o n ) . In F i g u r e 2 i s reported the volume change of a pectate s o l u t i o n upon a d d i t i o n of d i v a l e n t c a t i o n s ( c o r r e c t e d f o r d i l u t i o n e f f e c t s ) . The e f f e c t i s l a r g e l y a s c r i b a b l e to changes of s o l v a t i o n of the carboxylate groups and of the i n t e r a c t i n g counterions. On a microscopic l e v e l the i n t e r a c t i o n process must involve the d e s o l v a t i o n of i o n i c species ( f r e e ions and polymeric carboxylate) i n order to approximate the groups to each other. The whole process i s , t h e r e f o r e , charac t e r i z e d by a l a r g e and positive entropy change which 2



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0

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Figure 1. Dependence of the reduced osmotic pressure on the polymer c o n c e n t r a t i o n , Cp , of sodium pectate i n 0.1 M i o n i c s t r e n g t h a t 27°C: ( O ) pH 6.5; ( · ) pH 3.5.

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Figure 2. Dependence of the c o r r e c t e d volume change of mixing sodium pectate with Cu + ( O ) and with Ca + (φ) i n 0.05 M aqueous NaClOq. at 25°C. R i s the i o n - t o polymer repeating u n i t molar r a t i o . 2

2


7. PAOLETTI ET AL.

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provides f o r the f a v o r a b l e d r i v i n g f o r c e . From the ent h a l p i c s i d e , such an i n t e r a c t i o n does not g i v e an exothermic enthalpy change because the strong endothermicity of the d e s o l v a t i o n step proper i s u s u a l l y not counterbalanced by other f a v o r a b l e c o n t r i b u t i o n s a r i s i n g from i o n i c and dipolar interactions. Such a behavior i s experimentally provided both by low molecular weight systems and by other p o l y c a r b o x y l a t e s i n t e r a c t i n g in a hicrhly s p e c i f i c mode with d i f f e r e n t d i v a l e n t ions ( 1518) . The enthalpy data f o r the mixing of a pectate solut i o n with d i v a l e n t c a t i o n s (Ca + or Cu +), i n aqueous s o l u t i o n at 25°C, are reported i n F i g u r e 3. It i s worthwhile to note the negative value of the enthalpy of i n t e r a c t i o n of the pectate with both i o n s , although the shape of the curves appears to be d i f f e r e n t . The data reported i n F i g u r e 3 represent, indeed, the excess enthalpy, i . e . they are c o r r e c t e d f o r the enthalpy changes due to the d i l u t i o n of both the polymer and the d i v a l e n t i o n . Therefore, they represent the true temperature c o e f f i c i e n t of the f r e e energy of the interaction process i n v o l v i n g the s o l v a t e d polymeric chain and the i o n s . In the same f i g u r e the enthalpy of interaction with protons and with Na+ ions i s a l s o reported, f o r comparison purposes. The general behavior of the enthalpy contribution i n a process i n v o l v i n g the i n t e r a c t i o n between charged species i s a positive"(endothermic) enthalpy change, as experimentally found f o r Na+ ions (Figure 3). T h i s experimental evidence i s s u b s t a n t i a t e d by theoretical approaches based on p o l y e l e c t r o l y t e t h e o r i e s (13)· * some cases strong c h e l a t i o n may result i n an enhanced endothermic enthalpy change, as d i s c u s s e d above about d e s o l v a t i o n . D e v i a t i o n s towards exothermicity have a l ways been reported and i n a l l cases a s c r i b e d to the presence of ion-induced conformational transitions, which may (e.g. Cs*-carrageenan) (14) or may not (see the curve of H+ of F i g u r e 3 corresponding to the pH-induced trans i t i o n of pectate) be accompanied by chain aggregation. In the case of mixing pectate with copper i o n s , the exothermic enthalpy of i n t e r a c t i o n i s a b s o l u t e l y unique i n t h i s c l a s s of compounds. In f a c t , a l l known polyuronates (15, 19), as w e l l the monomeric galacturonate molecul e (20), have a p o s i t i v e enthalpy of i n t e r a c t i o n , irres p e c t i v e of the stereochemistry of the sugar residue(s) and of the a b i l i t y of g e l formation. Most important, a positve enthalpy has been found a l s o f o r the system copper-polyguluronate (19), that i s the polymer which has a behavior very s i m i l a r to polygalacturonate and differs from the l a t t e r only f o r the stereochemistry of C(3). I t i s t h e r e f o r e mandatory to a s c r i b e the reported exothermic behavior of pectate with Ca + and Cu + ions to the very s p e c i f i c change of conformation of the polygalacturonate chain.


2

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n

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CHEMISTRY AND


0 0.2

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0.8

12

I

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FUNCTION OF PECTINS

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n *and Tr->n*transitions of the carboxylate neighbor to dissymmetric centers (a and b) and the onset of an extrinsic chiral charge-transfer band i n v o l v i n g the Cu + ion ( c ) , r e s p e c t i v e l y . The degree of l i n e a r i t y of the CD changes and the enthalpy changes f o r the Cu + t i t r a t i o n are p r e t t y s i m i l a r , likely indic a t i n g a common o r i g i n f o r the two phenomena. In the case of Ca +, a s l i g h t d i f f e r e n c e i n the dependence of the two f u n c t i o n s on R i s n o t i c e d . More information needs to be gained before a safe explanation can be i n f e r r e d therefrom; f o r example, one might propose that the p e r t u r b a t i o n occurs at local level only, or that progressive b i n d i n g of calcium occurs onto an already ordered conformation of pectate induced by very little amount of calcium ions. S p e c u l a t i o n on the nature of the ordered conformation induced by Ca + ions can hardly be made, not even i n r e l a t i o n to that p r e v a i l i n g at low pH. Conformational c a l c u l a t i o n s have been c a r r i e d out i n our laboratory on 1-^4 galacturonans (21), by using standard theoretical procedures already d e s c r i b e d i n the l i t e r a t u r e (22). From these c a l c u l a t i o n s the allowed conformational space i s l o c a t e d i n a s i n g l e region of the E ( p s i - p h i ) diagram. Even more i n t e r e s t i n g i s the f a c t that the p i t c h of the regular conformations generated by a given set of p s i phi values changes very s l i g h t l y w i t h i n the allowed range of conformations (from a minimum of 4.32 to a maximum of 4.54 A/monomer). Of course, the stability of each h e l i c a l conformation i s subjected to p o s s i b l e intramolec u l a r hydrogen bonds and i n t e r m o l e c u l a r s o l v a t i o n energ i e s , i n a d d i t i o n to the entropie d e s t a b i l i z a t i o n due to the existence of s e v e r a l allowed conformations (statistical fluctuation). 2

2


f

2

2

2

2

Aggregation

of

pectate

The aggregation of pectate i n the presence of copper and calcium ions has been repeatedly reported and is commonly used as a method f o r the q u a n t i t a t i v e p r e c i p i t a t i o n of the polymer. The a c t u a l d i s t r i b u t i o n of the p r e c i p i t a t e and the increase of the apparent molecular weight has been studied as f u n c t i o n of R by means of sedimentation and membrane osmometry, r e s p e c t i v e l y . The curves reported i n F i g u r e 5 show the dependence of the amount of p r e c i p i t a t e (as mass percentage) upon the a d d i t i o n of d i v a l e n t ions. The e f f e c t i v e n e s s of



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F i g u r e 4. Dependence on R of the s p e c i f i c change i n e l l i p t i c i t y of sodium pectate i n 0.05 M aqueous NaCIO* at 25°C: a, Cu + λ = 200 nm; b, C a + λ = 210 nm; c, Cu + λ = 235 nm. d: sample-case CD spectra of sodium pectate i n the absence (2) and i n the presence of Cu + ( 1 ) , R = 0.24, and of C a + ( 3 ) , R = 0.29, r e s p e c t i v e l y . 2

2

2

2

2



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PAOLETTI ET AL.


F i g u r e 5. Weight percentage from a 0.05 Maqueous NaClO^ a d d i t i o n of d i v a l e n t ions.

of pectate precipitated s o l u t i o n upon increasing


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FUNCTION OF PECTINS

copper ions i s more than three times l a r g e r than that of calcium i o n s , i n the same c o n d i t i o n s of temperature (22°C) and i o n i c strengh (0.05 M NaC104 ). Indeed, the a c t u a l amount of p r e c i p i t a t e i s almost zero up to a va lue of R = 0.15, f o r both calcium and copper, and sud denly increases with a sigmoidal dependence, very cha r a c t e r i s t i c of an a l l - o r - n o n e process, particularly in the case of copper. Osmometric experiments have been c a r r i e d out as f u n c t i o n of polymer concentration (Cp) and of R, i n the presence of 0.1 M NaCIO . T h i s ionic strength was re quested i n order to minimize the Donnan e f f e c t , which otherwise could seriously infirm the s i g n i f i c a n c e of the r e s u l t s . The molecular weight of the polymer (Figu re 6) increases by a f a c t o r two f o r copper and almost four f o r calcium, before any visible p r e c i p i t a t e could be detected. Apart from being the f i r s t reported data of a d i r e c t determination of the molecular weight of pecta te on i n c r e a s i n g the f r a c t i o n of bound ions per repea t i n g u n i t ( r ) , the r e s u l t s of Figure 6 show t h a t , f o r a given amount of bound i o n s , the r e l a t i v e increase of M i s l a r g e r f o r calcium than f o r copper. This could stem from a higher tendency of copper ions to give rise to long, l i n e a r s t r e t c h e s of bound species between two pec t a t e chains, while calcium ions might be able to form shorter but more evenly d i s p e r s e d j u n c t i o n s , with more e f f e c t i v e branching a b i l i t y . Provided a d e f i n i t e corre l a t i o n w i l l be proved between the extent of chain-chain a s s o c i a t i o n w i t h i n a j u n c t i o n and the strength of the r e s u l t i n g g e l (23), then some recent r h e o l o g i c a l r e s u l t s showing that Cu +/pectate g e l s are stronger than Ca +/pectate g e l s (24) could w e l l f i n d a molecular counterpart i n the present M data. _ 2

2

rt

It should be noted that the values of the Μη , re ported i n the Figure 6, are c a l c u l a t e d from the redu ced osmotic pressure e x t r a p o l a t e d to zero polymer con c e n t r a t i o n , whereas a l l the other results reported in t h i s work have been obtained at f i n i t e polymer concen t r a t i o n and t h e r e f o r e may i n c l u d e terms a r i s i n g from the c o n c e n t r a t i o n dependence of the i n v e s t i g a t e d property. Rheolocrv

2

of mixed H+/Ca + pectate g e l s

At the beginning of an i n v e s t i g a t i o n on the rheology of pectate g e l s , we were faced with the problem of finding s u i t a b l e measuring c o n d i t i o n s to determine viscoelastic parameters of such systems by using a r o t a t i n g Couette type rheometer. The most e f f e c t i v e way of preparing a g e l i n the measuring compartment turned out to be that developed by T o f t and d e s c r i b e d in details i n another Chapter of t h i s book (25). B r i e f l y , an amount of D-glucono-5-lactone i s added to a n e u t r a l s o l u t i o n c o n t a i n i n g pectate and CaEDTA. The number of moles of lactone i s four times as l a r g e as that of the repeating u n i t s of pectate. The slow h y d r o l y s i s of the lactone homogeneou-


n


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Figure 6. Dependence on the molar f r a c t i o n of hound d i valent ions, r , of the apparent number-average molecu l a r weight at r , M , r e l a t i v e to Rn determined i n the absence of d i v a l e n t ions. ( · ) C a + , ( O ) Cu + . Solvent 0.1 M aqueous NaC104, Τ = 25°C. n

z

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s l y lowers the pH of the system, thereby determining a uniform r e l e a s e of Ca + ions and the formation of a mi xed H+/Ca + g e l . I t was p o s s i b l e to f o l l o w the time course of the r e a c t i o n by using the rheometer i n the o s c i l l a t i n g mode. The measuring bob was allowed to o s c i l l a t e at f i x e d frequency (0.05 Hz) and with constant amplitude (20°) and the t s i g n a l was recorded as a fun c t i o n of time ( i . e . a n g l e ) . The g r a p h i c a l r e s u l t was a L i s s a i o u s f i g u r e , each of which was recorded i n a p e r i o d of time which i s one or two orders of magnitude shorter than the t o t a l time course of the r e a c t i o n (Figure 7, ac ) . The L i s s a i o u s f i g u r e s have been subjected to F o u r i e r a n a l y s i s , which showed that only the harmonics up to the t h i r d were s i g n i f i c a n t . Each harmonic (corresponding to a complex v i s c o s i t y was i n turn separated i n t o the viscous and i n t o the e l a s t i c components, r\' , i and i\" , i r e s p e c t i v e l y . The onset of g e l formation i s marked after about 35 min by a r a p i d increase of l \ * , l , and of both i t s components, although the viscous one i s dominant (Figure 7, r i g h t ) . The t h i r d harmonic, r\*,3 , which an a n a l y s i s reported elsewere (24) shows to be of l a r g e l y e l a s t i c nature, slowly increases and a f t e r about 70 min becomes l a r g e r than η." ,1 . T h i s could i n d i c a t e that the a l b e i t small o s c i l l a t i n g movement imposed on the g e l determines the formation of (at l e a s t ) two l e v e l s of s t r u c t u r e , the second one p o s s i b l y r e l a t e d t o a reshuf f l i n g of the chains to aquire a b e t t e r degree of chainchain i n t e r a c t i o n . Work i s i n progress to t e s t the e f f e c t of the many experimental v a r i a b l e s of the r h e o l o g i c a l behavior of mixed pectate g e l s , and i n p a r t i c u l a r to gain information on the separate e n t h a l p i c and entropie c o n t r i b u t i o n s to the e l a s t i c i t y from the a n a l y s i s of temperature dependence of the determined e l a s t i c moduli. 2


2

Experimental Sodium pectate was obtained by p u r i f i c a t i o n and succes s i v e n e u t r a l i z a t i o n with NaOH of a sample of p e c t i c a c i d purchased from Sigma Chemical Co. (Catalogue No. P-3889, s o l d as p o l y g a l a c t u r o n i c a c i d ) . P u r i f i c a t i o n and prepa r a t i o n of the s o l u t i o n s have been p r e v i o u s l y reported (5). A l l c a t i o n s were used i n the form of p e r c h l o r a t e s . Preparation and p u r i f i c a t i o n of copper p e r c h l o r a t e have been reported (10). Experimental methods and data e l a b o r a t i o n of the c a l o r i m e t r i c , v o l u m e t r i c , and s p e c t r o s c o p i c experiments have been reported (5, 15). In the phase e q u i l i b r i u m experiments ( p r e c i p i t a t i o n ) the determination of the polymer concentration was made by p o l a r i m e t r i c measure ment on the supernatant, after c e n t r i f u g a t i o n a t 3000 rpm f o r 30 min of the s o l u t i o n s which had been equili brated f o r 24 h. Osmometric measurements were c a r r i e d out using a Melabs Mod. CSM-2 membrane osmometer f o l l o w i n g standard procedures. D e t a i l s of the instrumentation and on the



F i g u r e 7. Rheocrram of pectate s o l u t i o n s c o n t a i n i n g g l u ~ cono-lactone and CaEDTA a t 25°C. L e f t , a-c: L i s s a i o u s f i g u r e s obtained a t d i f f e r e n t times d u r i n g the course of the h y d r o l y s i s r e a c t i o n . R i g h t : time course of the complex v i s c o s i t y components, r \ * , l - 3 , and of the v i scous. i \ ' , l , and e l a s t i c , M, components of i \ * , l .



86

procedure of data a n a l y s i s f o r the ments are given elsewhere (24).

rheoloçrical

measure-

Ac knowl edorment s


The Authors thank Mr. Roberto M i l e t t i f o r having p e r f o r med some osmotic pressure measurements. This work has been c a r r i e d out with the f i n a n c i a l support of the "Progetto F i n a l i z z a t o Chimica Fine e Secundaria", C.N.R., Roma, and that of the u n i v e r s i t y of Trieste. Lecrend of Symbols Ο

molar polymer c o n c e n t r a t i o n number average molecular weight R i o n t o polymer molar r a t i o r f r a c t i o n of bound ions per polymeric r e p e a t i n g u n i t Β Smidsrod's parameter of s t i f f n e s s p s i , p h i conformational angles a t g l y c o s i d i c oxygen i n oc -D-galacturonosyl-1,4-oC-D-gaiacturonic a c i d dimer E ( p s i - p h i ) t o t a l conformational energy of Q C -D-galactu ronosyl-1 , - D - g a l a c t u r o n i c a c i d dimer r\*,i complex v i s c o s i t y , i - t h harmonic i\',i viscous component of the complex viscosity, i - t h harmonic r\",i e l a s t i c component of the complex viscosity, i - t h harmonic

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Kohn, R. Pure Appl. Chem. 1975, 42, 371. Kohn, R.; Luknar, O. Collect. Czech. Chem. Commun. 1977, 42, 731. Rinaudo. M.; Ravanat. G.; Vincedon, M. Makromol. Chem. 1980, 181, 1059. Rees, D. A. Pure Appl. Chem. 1981, 53, 1. Cesàro, Α.; Ciana, Α.; Delben, F . ; Manzini, G.; Paoletti, S. Biopolvmers 1982, 21, 431. Grant, G. T . ; Morris, E. R.; Rees, D. Α.; Smith, P. J . C.; Thorn, D. FEBS Letters 1973, 32, 195. Fishman. M. L.; Pepper, L . ; Barford, R. Α.; J. Polymer Sci.,Polymer Phys. Ed. 1984, 22, 899. Smidsrød, O.; Haug, A. Biopolymers 1971, 10, 1213. Civitarese, G. Tesi di Laurea in Chimica, Università di Trieste, Trieste, Italy. Manzini, G.; Cesàro, Α.; Delben, F . ; Paoletti, S.; Reisenhofer, Ε. Bipelectrochemistry and Bioenergetics 1984, 12, 443. Reisenhofer, E . ; Cesàro, Α.; Delben. F . ; Manzini, G.; Paoletti, S. Bioelectrochemistry and Bioenergetics 1984, 12, 455. Reid, D. S. In "Developments in Ionic Polymers 1"; Wilson, A. D.; Prosser, H. J., Eds.; Applied Science Publishers: London, 1983; p. 269.


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Paoletti, S.; Cesàro, Α.; Delben, F . ; Crescenzi, V.; Rizzo, R. In "Microdomains in Polymer Solu tions"; Dubin, P., Ed.; Plenum Press: New York, (in press). Paoletti, S.; Delben, F . ; Cesàro, Α.; Grasdalen, H. Macromolecules 1985, 18, 0000. Paoletti, S.; Cesàro Α.; Ciana, Α.; Delben, F . ; Manzini, G.; Crescenzi, V. In "Solution Properties of Polysaccharides"; Brant, D. Α . , Ed.; ACS SYMPO SIUM SERIES No. 150, American Chemical Society: Washington, D.C., 981; pp. 379-386. Crescenzi, V.; Delben, F . ; Paoletti, S.; Skerjanc, J . J.Phys.Chem. 1974, 78, 607. Delben, F . ; Paoletti, S. J.Phys.Chem. 1974, 78, 1486. Paoletti, S.; Delben, F . ; Crescenzi, V. J.Phys. Chem. 1976, 80, 2564. Cesàro, Α.; Paoletti, S.; Delben, F . ; Crescenzi, V.; Rizzo, R.; Dentini, M. Gazz.Chim.Ital. 1982, 112, 115. Aruga, R. Bull.Chem.Soc.Jpn. 1981, 54, 1233. Di Grazia, L. Tesi di Laurea in Chimica, Università di Trieste, Trieste, Italy. Brant, D.A. Quart.Rev.Biophys. 1976, 9, 527. Andresen, I . L . ; Smidsrød, O. Cabohydr.Res. 1977, 58, 271. Lapasin, R.; Zanetti, F . ; Paoletti, S.; Cesàro, Α.; Delben, F. Manuscript in preparation. Toft, K.; Grasdalen, H.; Smidsrød, O. In this volume.

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Chemistry and Function of Pectins - American Chemical Society

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