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Cation effect on thermal transition ofι-carrageenan: a photon transmissionstudyÖnder Pekcan & Selim KaraPublished online: 02 Apr 2012.

To cite this article: Önder Pekcan & Selim Kara (2005) Cation effect on thermal transition ofι-carrageenan: a photon transmission study, Journal of Biomaterials Science, Polymer Edition,16:3, 317-333, DOI: 10.1163/1568562053654086

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Cation effect on thermal transition of ι-carrageenan:a photon transmission study

ÖNDER PEKCAN 1,∗ and SELIM KARA 2

1 Department of Physics, Istanbul Technical University, Maslak 80626 Istanbul, Turkey2 Department of Physics, Trakya University, 22030 Edirne, Turkey

Received 20 January 2004; accepted 28 May 2004

Abstract—Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of ι-carra-geenan in various cation (CaCl2) solutions upon heating and cooling were studied using the photontransmission technique. Photon transmission intensity, Itr, was monitored against temperature todetermine the (c-h) and (h-d) transition temperatures (Tch and Thd) and activation energies (�Echand �Ehd). An extra dimer-to-dimer (d-d) transition was observed during cooling at low temperatureregion. However, upon heating dimers directly decompose into double helices by making dimer-to-double helix (d-h) transition. Further heating results in double helix-to-coil (h-c) transition athigh temperature region. Tdh and Thc temperatures and �Edh and �Ehc activation energies weredetermined. It was observed that Thc and Tch temperatures and �Ech and �Ehc activation energieswere found to be strongly correlated to the CaCl2 content in the system.

Key words: Carrageenan; phase transition; activation energy; transition temperature.

INTRODUCTION

Carrageenan systems have been used in food, ceramic, anti-ice fluids, paper andtextile industries [1–5]. Carrageenan is unique in its ability to bind considerablequantities of material to form a gel network which, in meat applications, comple-ments rather than disrupts the protein network. The changed nature of carrageenanalso stabilizes water/fat emulsions during preparation, cooking and storage. Car-rageenan has been used as a carrier for ceramic coating during the manufacture ofautomotive spark plugs. It has been well known that carrageenan interacts stronglywith pigments such as calcium, silica and alumina. The mixing of silica precursorwith carrageenan in the aqueous solutions results in the formation of homogeneous

∗To whom correspondence should be addressed. Tel.: (90-212) 285-3213. Fax: (90-212) 285-6366.E-mail: pekcan@itu.edu.tr

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monolithic biomaterials which demonstrate a restriction on the conformational re-alignments of polysaccharide molecules owing to their mineralization [4]. Car-rageenan is fully soluble in hot water/glycol mixtures used for aviation machinery.It interacts with glycol to give a weak network with flow properties which makeit an ideal thickener for anti-ice fluids [5]. Quality paper production requires thecellulose fibers to be strengthened and the surface sized for specific applications.Carrageenans are excellent film formers which reinforce cellulose fibers to giveadded wear and tear strength and control ink bleed. Both screen and jet printingtechnologies require the flow properties of printing paste to be closely controlled inorder to achieve high definition and the required degree of penetration into fabricor carpet pile. The wide thixotropic flow behavior of carrageenan makes it ideallysuited for this purpose.

Other major applications of carrageenan are in cosmetics, pharmaceuticals andpersonal care industries [5]. It has been known that carrageenan interacts withcarotene to give soft skin and silky hair. In hand lotions and shampoos, carrageenannot only thickens the product but also promotes healthy skin and hair. Many phar-macologically active ingredients are destroyed in the stomach or gastrointestinaltract but can be safely absorbed through the buccal linings. There has been a con-siderable interest on the use of gums, carrageenan in particular, for entrapping theactive ingredients in a hydrocolloid matrix to give a delivery system which can re-lease the active ingredient in a controlled fashion [6]. If the material is in tabletform, then the technique can be used to administer drugs orally. The effects ofbinder toughness and plastic flow on tablet hardness, friability and capping havebeen studied [7]. A rate- and time-programming drug-release device was designedby using a hydrogel [8]. As a model system of pulsatile drug release, dibucainehydrochloride and carrageenan hydrogel were chosen as a drug and a device, re-spectively. The oscillatory changes was observed in the hydrogel weight in distilledwater which was considered to be caused by influx and efflux of water moleculesinto and from the surface and core of the hydrogel and by polymer liberation fromthe hydrogel. It has been known that carrageenan stabilizes toothpaste preparationsby a combination of viscosity, continuous phase gel formation and specific interac-tions with the flavor oil micelles [5].

In general, carrageenan possess a linear galactan backbone with alternatingdisaccharide repeating sequences of 1,4-linked α-D-galactose and 1,3-linked-β-D-galactose and a variable proportion of sulfate groups in different positions.Individual members of each series carry different amounts of sulfate groups, whichmainly allow carrageenan to be divided into three major types, namely, ι, κ andλ-carrageenan [9]. ι-Carrageenan is the most highly sulfated of the helix-formingpolysaccharides, a high-molecular-weight linear polymer consisting principallyof an alternating sequence of 3-linked β-D-galactose 4-sulfate and 4-linked 3,6-anhydro-β-D-galactose 2-sulfate. Thus, each monosaccharide unit in the idealpolysaccharide structure carries one sulfate group and, therefore, ι-carrageenanbehaves in aqueous solution as a highly charged polyanion with a linear charge

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Cation effect on thermal transition of ι-carrageenan 319

density of 1.4 in the extended conformation [10]. According to X-ray fiberdiffraction data, the polysaccharide has a double-helix conformation in the solidphase, while its calcium salt takes a 3-fold right-handed double helix with parallelstrands and a helix pitch of 2.66 nm [11, 12]. It has been known that, insolution, ι-carrageenan can reversibly transform from an ordered to a disorderedconformation. Naturally at high ionic strength and low temperature ι-carrageenanforms an ordered state. On heating, the helices dissolve and ι-carrageenan formsa random coil conformation [13]. Intermolecular double helix formation shouldresult in a doubling in the observed molecular weight of ι-carrageenan, which hasbeen observed in many groups [14, 15]. However, some authors have proposeda monomolecular single-helix formation [16]. It has been concluded that theformation of ι-carrageenan double-helix follows second-order reaction kineticswhile the back reaction is a first-order process.

A considerable amount of research has been performed on carrageenan systems toproduce specific properties for specific applications. The kinetics and equilibriumprocesses of the sol–gel and gel–sol transitions of agar or agarose gels as well asthe effect of gelation conditions on the microstructure of the gel and rheologicalproperties have been studied in last few years [17–20]. It was observed that gelationof agar molecules results in a large sigmoidal increase in the magnitude of the sol’sshear modulus [21, 22]. On reheating, the gel structure is destroyed and duringthe gel–sol transition the shear modulus follows another sigmoidal path back to itsinitial value, forming a hysteresis loop [23]. The observed values of the sol–gel andgel–sol temperatures found in this study are 36◦C and 78◦C, respectively. It wasunderstood that the sol–gel (gelation) and gel–sol (liquefaction) temperatures canbe affected by the agar concentration and the thermal history of the gel. Recently,cation effect on sol–gel and gel–sol phase transitions of a κ-carrageenan–watersystem has been investigated by us using the photon transmission technique [24].The same technique was employed to study the hysteresis phenomenon during sol–gel and gel–sol transition in similar systems [25].

In this work, phase transitions of ι-carrageenan in various concentrations ofCaCl2 were studied using the photon transmission technique. This technique haspreviously been used to monitor the formation of acrylamide gels in various cross-linker and water contents [26, 27] where, in both cases, transmitted photon intensity,Itr, decreased dramatically at a certain reaction time which is attributed to theformation of microgels in the system. More recently, the energy release duringgelation of acrylamide was observed using the same technique [28]. Here, inthis work it was observed that during gelation of ι-carrageenan molecules, the Itr

intensity first exhibited a decrease followed by a two-step increase upon furthercooling. On reheating, however, the gel structure was destroyed and during theliquefaction, the Itr intensity first decreased following a sigmoidal path back to itsminimal position by giving a hysteresis loop. Then the Itr increased by followingthe original path back to its original position. This low temperature hysteresis loopdisappeared at low CaCl2 concentrations at which the Itr presented a single path,

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both during cooling and heating processes. Phase-transition temperatures and rateswere determined and energy needs during these transitions were measured. Themechanisms of the conformational transitions are discussed.

MATERIALS AND METHODS

ι-Carrageenan (C-1138 Sigma) powder (4 wt%) was dissolved in various CaCl2(Merck)–distilled water solutions (0%, 0.1%, 0.2%, 0.3%, 0.4% and 0.5%). Thesesamples were marked as Ca00, Ca01, Ca02, Ca03, Ca04 and Ca05, respectively.During preparation, the heated carrageenan solutions were held at 90◦C and con-tinuously stirred by magnetic stirrer. The compositions of the studied carrageenansamples with various CaCl2 concentrations are listed in Table 1. Phase-transitionexperiments were performed in 3.5 × 1.5 × 1.0 cm glass cells equipped with a heatreservoir. Temperature was monitored and in situ photon transmission measure-ments were performed simultaneously using Perkin-Elmer Lambda2 UV-Visible(UVV) spectrometer. 450 nm was chosen for the photon transmission measure-ments. The carrageenan solution, at 90◦C, was transferred into the glass cell andleft to cool to room temperature before the UVV measurements were started. Thenthe ι-carrageenan sample was reheated up to 90◦C at the rate of 2 ± 0.1◦C/min toobserve the solid–liquid (gel–sol) transition. Cooling of the carrageenan sol from90◦C to room temperature was then performed at the same rate to detect liquid–solid(sol–gel) transition (the higher and lower cooling and heating rates such as 0.5, 1and 3◦C/min produced similar paths). Photon transmission intensities, Itr, weremonitored against temperature by using the time drive mode of the spectrometerduring both transitions.

RESULTS AND DISCUSSION

The behavior of Itr against temperature between 20 and 90◦C for all samples duringheating and cooling processes is presented in Figs 1 and 2, respectively. Both tran-sitions are compared for Ca05, Ca03, Ca02 and Ca01 samples in Fig. 3. It is seen

Table 1.The composition and transition temperatures of the studied ι-carrageenan in CaCl2 solutions

Ca01 Ca02 Ca03 Ca04 Ca05

CaCl2 (wt%) 0.1 0.2 0.3 0.4 0.5Tch (◦C) 66.0 72.2 75.7 79.9 83.0Thd (◦C) 57.3 61.3 63.3 66.4 67.6Tdd (◦C) 35.9 37.0 37.87 37.93 39.4Tdh (◦C) 55.5 56.0 58.0 58.5 60.3Thc (◦C) 69.6 72.8 77.0 81.2 83.9

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Cation effect on thermal transition of ι-carrageenan 321

Figure 1. Temperature variation of Itr during liquid–solid (heating) transitions for all samples.

Figure 2. Temperature variation of Itr during solid–liquid (cooling) transitions for all samples.

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Figure 3. Comparison of the temperature variation of Itr during liquid–solid and solid–liquidtransitions for (a) Ca05, (b) Ca03, (c) Ca02 and (d) Ca01.

that Itr first decreased upon heating the Ca05, Ca03 and Ca02 samples, indicatingthat low-temperature transition takes place. Ca01 does not show low-temperaturetransition upon heating, due to very low CaCl2 content. Further heating causes adramatic increase in Itr for all samples, predicting high-temperature transition. Onthe other hand, the Itr decreased dramatically upon cooling the carrageenan samplesin all cases presenting that the high-temperature back transition occurred. In sub-sequent cooling Ca05, Ca03 and Ca02 undergo a two-step low-temperature phasetransition. However, Ca01 does not go through this transition due to low CaCl2content. In order to interpret the results in Fig. 3 we have followed the model onaggregation of carrageenan helices which was proposed by Morris et al. [14]. Theexperimental evidence for aggregate-driven helix formation was also given by thesame group [29]. According to this model, the so-called ‘Domain Model’, there aretwo levels of ordering of ι-carrageenan in solutions and gels. This ordering of the

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Cation effect on thermal transition of ι-carrageenan 323

Figure 4. Graphic representation of the Domain Model.

polysaccharides can be in the form of double helix and clusters of the double heliceswhich may be called dimers. This model can be explained by the following scheme:

[H2]2 ⇔ 2H2 ⇔ 4C, (1)

where C is the random coil, H2 is the double helix and [H2]2 is the double helixdimer. The graphic presentation of this model is also given in Fig. 4. Accordingto this model high-temperature transition during cooling may correspond to coil-to-double helix (c-h) transition, where the system goes from transparent (high Itr) tothe turbid (low Itr) state, i.e. double helices form a heterogeneous network structureso that Itr decreases due to the light scattering [24–28]. In other words, duringc-h transition, double helix aggregates form a separate phase by excluding waterfrom their domains and as a result the ι-carrageenan–water system decomposesinto two phases with different network concentrations. Then the contrast betweencarrageenan and water phase can scatter light and dramatically reduce Itr. Inthe low-temperature region, further cooling causes the formation of double helixdimers, i.e. double helix to dimer (h-d) transition occurs during cooling of thecarrageenan–water system. Here Itr first increases slowly and then dramaticallyincreases by predicting two successive low-temperature transitions. Most probablydimer-to-dimer (d-d) transition follows the h-d transition upon cooling. As a resultdimers rearrange themselves to form a homogeneous network which does not scatterthe light as much as the network of double helices. Here we have to note thatdimers cannot be formed in the low CaCl2 content samples (Ca00 and Ca01). Whenthe network of dimers at low-temperature region is heated, the system undergoesdimer-to-double helix (d-h) transition in a single step, during which the Itr intensitydecreases slowly back to its minimum. Upon further heating double helices are

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Figure 5. Cartoon representation of liquid-to-solid and solid-to-liquid transition cycles of the ι-car-rageenan system.

decomposed into the coils and system goes into the double helix-to-coil (h-c)transition. During h-c transition Itr intensity dramatically increases by predicting theformation of homogeneous coil network where light scattering drops considerably(Fig. 5).

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Cation effect on thermal transition of ι-carrageenan 325

Figure 6. Determination of (a) Tch, (b) Thd and (c) Tdd from the peak positions of the first derivativesof the Itr curves in Fig. 3.

In order to quantify the data in Fig. 3, the first derivatives of the Itr curves areplotted in Fig. 6a, 6b and 6c for the c-h, h-d and d-d transitions, respectively. InFig. 6 the peak positions produce the coil-to-double helix (Tch), double helix-to-dimer (Thd) and dimer-to-dimer (Tdd) transition temperatures. Similarly dimer-to-

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Figure 7. Determination of (a) Thc and (b) Tdh from the peak positions of the first derivatives of theItr curves in Fig. 3.

helix (Tdh) and helix-to-coil (Thc) transition temperatures are determined from thefirst derivatives of the Itr curves in Fig. 7a and 7b, respectively. The measuredtransition temperatures are plotted versus CaCl2 content in Fig. 8 and listed inTable 1. Figure 8 shows that CaCl2 concentration has a significant impact onc-h and h-c transitions, i.e. Tch and Thc values change by varying CaCl2 content.On the other hand h-d, d-h and d-d transitions are not effected by the CaCl2content. These results are expected because double helices and dimers have astronger structure than coils and as a result cation content does not effect theh-d, d-h and d-d transitions. However, c-h and h-c transitions strongly dependon the cation content, i.e. increase in CaCl2 content causes an increase in theThc and Tch values. Careful analysis of low-temperature transitions for Ca05,Ca04, Ca03 and Ca02 shows hysteresis, i.e. phase transition during heating andcooling at low-temperature region follows different paths (see Fig. 3). In fact,dimer formation upon cooling follows two sequential paths with different transitiontemperatures; however, decomposition of dimer into double helices prefers a single

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Cation effect on thermal transition of ι-carrageenan 327

Figure 8. Variation of Thc, Tch, Thd, Tdh and Tdd with respect to CaCl2 content.

path with a single transition temperature. During cooling in the low-temperatureregion, phase-separated double-helix domains first form phase-separated dimerdomains, then dimers rearrange themselves into a homogeneous network. Duringheating, however, these dimers directly decompose into phase-separated double-helix domains (Fig. 5).

The maxima of the peak heights (dItr/dt)max of the curves in Figs 6 and 7 areplotted versus CaCl2 content in Figs 9 and 10, respectively. The curves in Figs 9and 10 present the rates of phase transitions during cooling and heating, respectively,where it is seen that c-h and h-c rates increase linearly by increasing CaCl2 content;however, h-d, d-h and d-d rates first increase, then tend to saturate due to the stronggeometrical structure of helices and dimers. In other words, high CaCl2 contentdoes not affect the h-d, d-h and d-d transition rates. On the other hand, c-h andh-c transition rates are highly sensitive to CaCl2 content, as were the transitiontemperatures. Itr versus T curves in Figs 1 and 2 can be analyzed using theArrhenius relation as written below:

Itr/I∞ = exp(−�Ei/kT ). (2)

Here it is assumed that Itr is proportional to the overall order parameter [30, 31]which is the quantitative measure of the alignment of the polysaccharides in thecarrageenan–water system. I∞ is the transmitted photon intensity at the highest

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Figure 9. Plots of the rate of (a) c-h, (b) h-d and (c) d-d transitions versus CaCl2 content.

temperature, where all polysaccharides are in coil form. After the c-h transition Itr

drops to its lowest value where double-helix domains are phase separated. Then, thesystem goes into h-d transition upon cooling, where phase-separated dimers form amore ordered state. Finally, after d-d transition, the highest order form of the system

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Cation effect on thermal transition of ι-carrageenan 329

Figure 10. Plots of the rate of (a) h-c and (b) d-h transitions versus CaCl2 content.

has been reached and Itr increases again. Heating the system back to the highesttemperature, d-h and h-c transitions occurred sequentially and Itr first decreasedand then increased back to its highest value by following the disordered to orderedform. In equation (2), �Ei presents the transition activation energies and k is theBoltzmann constant. The Arrhenius plots for Ca05, Ca03, Ca02 and Ca01 duringcooling are presented in Fig. 11a–d, respectively. The slopes of the curves in Fig. 11produce coil-to-double helix (�Ech), double helix-to-dimer (�Ehd) and dimer-to-dimer (�Edd) activation energies, which are plotted in Fig. 12 with respect to CaCl2content. It is seen in Fig. 12 that �Ech and �Edd energies are strongly dependent onthe CaCl2 content in the system; however, �Ehd energies have no impact on CaCl2content. It is understood that c-h transitions are exothermic, i.e. when the systemundergoes c-h transition it releases heat to its environment to obtain a network ofhelices. On the other hand, h-d transitions spend almost no energy compared toother transitions to make dimers from the helices, and this energy does not changeby varying the CaCl2 content. Here we have to point out that two different slopesduring h-d transitions produce almost similar activation energies and both are namedas �Ehd. Phase transition of Ca05, Ca03, Ca02 and Ca01 during heating is alsotreated using the above procedure. The slope of the Arrhenius curves in Fig. 13produces �Ehc and �Edh activation energies for the above samples. The plots ofthese activation energies versus CaCl2 content are shown in Fig. 14 and Table 2,where it is seen that d-h transition energy (Edh) does not change in great extentby varying cation (CaCl2) content. However, h-c transition energy was found tobe strongly correlated with the CaCl2 content. It is understood that during d-htransition the system needs little energy compared to h-c transition at which thesystem releases heat to the environment to accomplish the coil formation from thedouble helices.

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Figure 11. Plots of ln Itr versus T −1 for (a) Ca05, (b) Ca03, (c) Ca02 and (d) Ca01. The slopes ofthe curves produce �Ech, �Ehd and �Edd activation energies upon cooling.

CONCLUSIONS

This work has shown that CaCl2 strongly affects the thermal phase transition ofι-carrageenan. Especially during coil-to-helix (c-h) and helix-to-coil (h-c) transi-tions, activation energies and transition temperatures are dramatically influenced bythe inclusion of CaCl2. However, double helix-to-dimer (h-d), dimer-to-double he-lix (d-h) and dimer-to-dimer (d-d) transition hardly depend on CaCl2 content. Ithas to be also noted that thermal phase transition follows a different path when it iscooled than when it is heated at the low temperature region. The low temperaturephase transition during cooling can be explained by the transition from less ordereddimers to more ordered dimers. It is believed in that understanding of this hys-teresis behavior of ι-carrageenan is crucial for the relation between conformationaltransition and gelation. To our knowledge, the type of conformational transition,the relation between conformational transition and gelation, the sequence of stepsleading to the gelation, have been, and still are, matter of debate. From this point ofview, knowledge regarding the energetics of the variety of ordered conformationsfor carrageenans and of the interaction between them is essential to study and under-

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Figure 12. Variation of activation energies obtained from Fig. 11 versus CaCl2 content.

Figure 13. Plots of ln Itr versus T −1 for (a) Ca05, (b) Ca03, (c) Ca02 and (d) Ca01. The slopes ofthe curves produce �Ehc and �Edh activation energies during heating.

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Figure 14. Variation of activation energies obtained from Fig. 13 versus CaCl2 content.

Table 2.The composition and activation energies of the studied ι-carrageenan in CaCl2 solutions

Ca05 Ca04 Ca03 Ca02 Ca01

CaCl2 (wt%) 0.1 0.2 0.3 0.4 0.5�Ech (◦C) −1.5 −14.9 −24.0 −36.4 −44.8�Ehd (◦C) 0.4 2.7 4.5 5.6 6.8�Edd (◦C) 1.8 12.5 18.8 32.7 30.2�Edh (◦C) 0.33 3.5 5.6 6.4 7.9�Ehc (◦C) −2.4 −9.9 −19.8 −34.2 −43.1

stand. Here in this work we tried to enlighten these questions by studying the con-formational transition in the presence of a specific cation for the particular system.It is believed that we have learned the behavior of the ι-carrageenan–CaCl2 systemunder reversible thermal phase transition which may help to elaborate the partic-ular system for medical applications. For example, it is known that electrostaticinteractions between positively charged drugs and carrageenan polymer segmentsare crucial for constructing a pulsatile drug release system. Since ι-carrageenangel containing CaCl2 is much stronger than the salt-free gel, then one may use aι-carrageenan–CaCl2 combination to construct a pulsatile drug-release system. Onthe other hand, it is known that biopolymers containing silica have attracted atten-tion due to their biocompatibility with living matter. Also, ι-carrageenan producesa soft gelled solution compared to κ-carrageenan, which gives hard and brittle gels.Because of this ι-carrageenan can be chosen to form a hybrid silica material to pro-duce biocompatible nano-composites for living systems.

Of other applications of carrageenan in pharmaceuticals and personal care indus-tries, it is known that phase transition rates and temperatures are quite important,especially for drug-release processes in stomach and gastrointestinal tract. In thismatter this work has produced considerable amount of data for cation–carrageenaninteraction in pure water.

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