Thermodynamic Study of Colorimetric Transitions in Polydiacetylene

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Thermodynamic Study of Colorimetric Transitions in Polydiacetylene Vesicles Induced by the Solvent Effect Ana Clarissa S. Pires,† Nilda de Fa´tima F. Soares,*,† Luis Henrique M. da Silva,‡ Maria C. Hespanhol da Silva,‡ Aparecida B. Mageste,‡ Reˆmili F. Soares,† ´ lvaro V. N. C. Teixeira,§ and Ne´lio J. Andrade† A Departamento de Tecnologia de Alimentos, Departamento de Quı´mica, and Departamento de Fı´sica, Centro de Cieˆncias Exatas e Tecnolo´gicas, UniVersidade Federal de Vic¸osa, AV. P.H. Rolfs, s/n, Vic¸osa, MG, 36570-000 Brazil ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: August 25, 2010

We report the synthesis of 10,12-pentacosadyinoic acid (PCDA) and PCDA + cholesterol (CHO) + sphingomyelin (SPH) vesicles dispersed in water and the determination of their colorimetric response induced by small amount of organic solvents. In the absence of solvent, PCDA and PCDA/CHO/SPH vesicles showed an intense blue color. The addition of CHCl3, CH2Cl2, and CCl4 caused a colorimetric transition (CT) in both structures with the following efficiency: CHCl3 > CH2Cl2 = CCl4. However, CH3OH did not cause a blueto-red transition. By microcalorimetric technique we also determined, for the first time, the enthalpy change associated with the CT process and the energy of interaction between solvent molecules and vesicle selfassembly. We observed that the chloride solvents induced a colorimetric transition, but the thermodynamic mechanism was different for each of them. CT induced by CHCl3 was enthalpically driven, while that caused by CH2Cl2 or CCl4 was entropically driven. 1. Introduction Polydiacetylenes (PDAs) are ene-yne alternated polymers that exhibit unique colorimetric properties.1 PDAs are formed by the 1,4 addition of diacetylenic monomers2 and can selfassemble into structures, such as monolayers,3-5 multilayers,6 nanostructured particles,7 and vesicles.8-11 These PDA vesicles can be produced in a simple and reproducible way and can also serve as a cell membrane mimic. The spectrophotometric transition of PDA vesicles has been exploited in many studies. The chromatic properties of PDAs have been intensively studied12,13 because they are able to change from blue to red depending on stimuli, such as temperature,10,11,14 pH,15,16 mechanical stress,17 and solvents.10,18,19 Some studies have revealed that biological processes also lead to structural perturbations in the PDA interface, which causes blue-red transitions in the assemblies.9,11,20-22 The mechanism involved in the color change of PDA vesicles is not fully understood. Some theories suggest a conversion between diacetylenic and butatrienic forms.3,23 However, UVphoton electron and X-ray absorption spectroscopies showed that the acetylenic structure is present in both forms (blue and red).24 Another hypothesis is that a colorimetric transition occurs via molecular conformation changes, such as side-chain packing, ordering, and orientation, which impart stresses to the polymer backbone and alter its conformation. Accordingly, these conformational changes could lead to alternations in electronic states and the corresponding optical absorption.25-28 PDA vesicles have been studied mainly by spectroscopy techniques, such as UV-vis, infrared, and X-ray. However, to the best of our knowledge, measurements of energy change * To whom correspondence should be addressed. Phone: +55 31 38991624. Fax: +55 31 38992208. E-mail: [email protected]. † Departamento de Tecnologia de Alimentos. ‡ Departamento de Quı´mica. § Departamento de Fı´sica.

associated with colorimetric transition and induced by different organic solvents have not been reported. Therefore, we determined for the first time the enthalpic energy involved in the blue-to-red transition of 10,12-pentacosadyinoic acid (PCDA) and PCDA + cholesterol (CHO) + sphingomielin (SPH) vesicles. These transitions were induced by the addition of chloroform, dichloromethane, methanol, or carbon tetrachloride, which were used as model molecules. 2. Experimental Methods 2.1. Vesicle Preparation. PCDA and PCDA/CHO/SPH vesicles were prepared as described previously.22,29,30 PCDA monomers, CHO, and SPH were dissolved in CHCl3, which was then removed by a stream of N2 gas. Deionized water was added to make the total lipid concentration 1 mM. The resulting suspension was sonicated (SoniTech, 300 W, 40 kHz) at 70 °C to obtain a clear solution, which was immediately filtered using a 0.45 µm PVDF filter (Milipore). The solution was then stored at 4 °C overnight to promote lipid crystallization. Photopolymerization of the lipid vesicles was carried out by exposure to UV radiation (254 nm) for 10 min and resulted in blue PDA vesicles. 2.2. Dynamic Light Scattering. Vesicle samples were diluted with deionized water, filtered through a 0.45 µm PVDF filter (Milipore) and stored in hermetic vials. Dynamic light scattering measurements were carried out using a 7 mW solid state laser (λ ) 670 nm) and an avalanche photodiode as a detector. The scattering angle was fixed at 90°. The correlation functions were calculated by a TurboCorr correlator board by Brookhaven Inst. Co. with 522 channels.31 DLS experiments yielded the temporal intensity correlation functions g(2)(t) ) 〈I(0)I(t)〉/〈I〉2, where I is the intensity of the scattered light. The experimental curves were fitted using cumulant analysis from which the hydrodynamic radius were obtained.

10.1021/jp105604t  2010 American Chemical Society Published on Web 09/30/2010

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To minimize concentration effects, we diluted the vesicle samples (about 20 times) until no changes in the intensity correlation functions were observed. The dilution also minimized artifacts from light attenuation by absorption, especially in the samples with blue color. 2.3. Colorimetric Response (CR %). To analyze the effects of different solvents on PCDA and PCDA/CHO/SPH vesicles, 42 µL of different solvents (CHCl3, CH2Cl2, CCl4, or CH3OH) were separately added to 3.0 mL of vesicle suspensions in 3.0 µL intervals. The spectra were obtained between 400 and 700 nm (Shimadzu UV-2550) at room temperature. To quantify the extent of blue-to-red color transitions within the vesicle, the CR (%) was calculated using the equation proposed by Charych et al.:20

CR )

(

(

Ablue Ablue + Ared

(

) ( b

Ablue Ablue + Ared

Ablue Ablue + Ared

)

b

)

a

)

Figure 1. Normalized correlation functions for the PCDA (O) and PCDA/CHO/SPH (2) vesicles. The continuous lines are the fits from cumulant analysis.

× 100

(1)

In eq 1, A is the absorbance of blue (∼650 nm) and red component (∼540 nm) obtained by UV-vis spectroscopy. The terms “blue” and “red” are related to material appearance, and the indices “b” and “a” represent the absorbances before and after the solvent stimuli exposition, respectively. 2.4. Isothermal Titration Calorimetry. The energetic analyses were performed on an isothermal titration microcalorimeter (ITµC) model CSC 4200 (Calorimeter Science Corporation), controlled by ITCRun software. The microreaction system was a titration mode with a 1.8 mL stainless steel vessel (sample and reference), which was maintained under constant stirring at 300 rpm. When thermal equilibrium between vessel and heat sink was reached, each solvent was titrated (1 µL) through a Hamilton microliter syringe at 60 min intervals. All calorimetric measurements were performed in triplicate, and the calculated relative standard deviation in the interaction enthalpy was on the order of (0.5%.32,33 3. Results and Discussion To determine the structural features of aggregated vesicles, the hydrodynamic radius (hd) was obtained by light scattering measurement. PCDA and PCDA/CHO/SPH vesicles showed the hd values of 87 ( 3 nm and 211 ( 3 nm, respectively (Figure 1). According to Okada et al.,2 the PCDA vesicle diameters vary between 50 and 300 nm, depending on the molecule incorporated. The incorporation of CHO and SPH inside the polydiacetylenic structures formed microdomains,34 which decreased molecular packaging and caused the vesicle size to increase. Figure 2 shows the electronic spectra of PCDA/CHO/SPH for both structural forms: blue (part a) and red (part c). Similar results were obtained for PCDA nanostructures. The colorimetric transitions were induced by different solvents, even in small quantities. The same figure also presents an electronic spectrum of 3 mL of vesicular sample after the addition of 15 µL of chloroform. This spectrum contained both colorimetric forms and the dispersion appeared as a purple color (part b). Such blue and red combination was also observed by Potisatityenyong et al.10 The PCDA and PCDA/CHO/SPH vesicles showed intense blue color with a maximum absorption band at 640 nm and a vibronic shoulder at 590 nm. The red vesicular suspensions

Figure 2. Absorption spectra of PCDA/CHO/SPH vesicular suspensions (3 mL): (a) without solvent added, (b) addition of 15 µL of chloroform, and (c) addition of 42 µL of chloroform.

revealed electronic and vibronic bands at 540 and 490 nm, respectively. As the solvent was added, a blue phase peak reduction and a red phase peak increase were observed. The presence of a single isosbestic point (IP) (560 nm) implied the presence of only two different chemical species, or alternatively, there were distinct molecular conformations that were responsible for the blue-to-red electronic transition.10 Therefore, there was no intermediate chemical species.35 Figures 3 and 4 illustrate the colorimetric responses, CR%, of PCDA and PCDA/CHO/SPH vesicles as a function of different amounts of solvents. The CR% represents the percentage of PCDA molecules that underwent blue-to-red transition. The presence of chloroform, dichloromethane, and carbon tetrachloride induced a colorimetric transition in both PCDA and PCDA/CHO/SPH bilayers. Methanol, however, was not capable of changing the vesicles’ color. Chloroform caused a higher percentage of molecule conversion (∼98% and ∼62% for PCDA and PCDA/CHO/SPH vesicles, respectively) compared to other chloride solvents (CH2Cl2 ∼52% and ∼42% and CCl4 ∼48% and ∼32%, for PCDA and PCDA/CHO/SPH, respectively). Prior work has shown PCDA solvatochromism induced by different solvents. Zhang et al.36 analyzed the spectrophotometric transition of modified polydiacetylene molecules in the presence of a good solvent (chloroform) and a bad solvent (methanol). The data from this work corroborated

Thermodynamic Study of Colorimetric Transitions

J. Phys. Chem. B, Vol. 114, No. 42, 2010 13367 TABLE 1: Water Solubility of Some Organic Solvents

a

Figure 3. Colorimetric response (CR%) of PCDA vesicles following addition of (O) chloroform, (9) dichloromethane, (2) carbon tetrachloride, and (×) methanol.

Figure 4. Colorimetric response (CR) of PCDA/CHO/SPH vesicles following addition of (O) chloroform, (9) dichloromethane, (2) carbon tetrachloride, and (×) methanol.

our findings because they also verified that the apolar solvent was able to result in a color change, while the polar one did not. Yoon et al.18 evaluated the color transition of PCDA caused by chloroform, tetrahydrofuran, ethyl acetate, and n-hexane. The authors observed that the former two solvents were able to induce a blue-to-red transition in PCDA molecules. However, ethyl acetate and n-hexane were not able to stimulate spectrophotometric change. Similar to our results, chloroform changed PCDA from the blue to red form. Interestingly, a water miscible compound (tetrahydrofuran) was able to induce a color change, which differed from our result for methanol. Lu et al.37 studied the solvatochromism of polydiacetylenic/ silica nanocomposite. In contrast to our results, this work found that many polar solvents, such as 2-propanol, acetone, ethanol, methanol, and dimethylformamide, induced color transitions from blue to red. In this case, polar solvents diffused through polar hydrophilic pendant chains. According to the authors, the accompanying solvatation stresses were transferred to the PCDA backbone, which reduced conjugation length and therefore induced blue-red transformations. Despite such examples, the solvatochromism of PCDA vesicles has not been explored in the literature, and the mechanism involved in this process is not fully understood. Nevertheless, the induction of a blue-to-red transition by organic solvents is interesting because PCDA and other polydiacetylenes

solvent

miscibility in water (g/100 mL)

CCl4 CHCl3 CH2Cl2 CH3OH

0.08 0.8 1.3 10.0

The source for these data was Weast and Astle.40

have been used as sensors and biosensors for many applications. Therefore, the colorimetric response might vary in an organic solvent-dependent manner.18 On the basis of our results, colorimetric transitions depend considerably on the molecular nature of the solvent and on the interactions between vesicle and stimulant molecules. Potisatityuenyong et al.10 suggested that interactions between ethanol and PCDA occur at this interface and that the process was initiated by the break of hydrogen, which was bound in the hydrophilic head of PCDA chain. Consequently, this process promotes the colorimetric transition. This model does not explain the transition induced by other solvents, such as CCl4, CH2Cl2, and CHCl3, which are hydrophobic molecules and therefore show a low interaction with hydrophilic functional groups at the vesicle/water interface. The colorimetric response caused by all solvents was higher for vesicles composed only of PCDA compared to PCDA/CHO/ SPH vesicles. This behavior could be due to cholesterol and sphingomyelin molecules, which promote different intermolecular interactions and can lead to a higher stability of selfassembled structures. Cholesterol and sphingomyelin have been demonstrated to form microdomains,38 which can stabilize nanoaggregates. According to Massey,39 sphingomyelin compared to other physiological phosphatidylcholines are more saturated and asymmetric due to the amine linkage of very long chain fatty acids. Sphingomyelin is also more prone to intermolecular hydrogen binding. Other phospholipids have been mixed with self-assembled polydiacetylene to increase their flexibility. Kim et al.11 incorporated a phospholipid (dimyristoylphosphatidylcholine, DMPC) into PCDA/antibody vesicles, and they observed that a higher DMPC concentration resulted in an enhanced colorimetric response to the bacteria stimuli. In such work, Fourier transform infrared (FTIR) spectra analysis suggested that DMPC incorporation decreased the strength of hydrogen binding between the amide and carboxylic acid groups of the polydiacetylene vesicles, which resulted in a less rigid structure of the PCDA backbone. Interestingly, though carbon tetrachloride induced colorimetric blue-to-red transition in a small percentage of polydiacetylene molecules compared to alterations caused by chloroform (Figures 3 and 4), such colorimetric change occurred initially with a smaller volume than was needed for CHCl3. Possibly, when some microliters of the solvents were titrated into vesicle suspensions, these solvents would be partitioned between two phases: one hydrophilic (water) and one hydrophobic (vesicle). When the water miscibility of the solvents was higher (Table 1), the partition of solvent into water was higher. Therefore, we could conclude that when a few microliters of CCl4 were added into PCDA and PCDA/CHO/SPH suspensions, the solvent molecules were immediately partitioned to the vesicle region because interactions between CCl4 and water are unfavorable. As seen in Table 1, the solubility of dichloromethane is slightly higher than that of chloroform. Independent of this small difference, similar colorimetric transition behaviors could be verified by comparing CHCl3 and CH2Cl2.

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Figure 5. Enthalpic variation (∆Hobs) of chloroform titration into (9) PCDA vesicles, (∆) PCDA/CHO/SPH vesicles, and (O) water.

Figure 6. Apparent molar interaction enthalpy (∆Hap-int) between chloroform and vesicles of (9) PCDA and (∆) PCDA/CHO/SPH.

In addition to the interactions between solvents and water or vesicles, we observed that three chloride solvents induced the blue-to-red transition, but the behavior between these solvents varied, especially for chloroform. When the solvent was partitioned into a self-assembled structure, it could be positioned into two regions: the inner hydrophobic vesicle or the hydrophilic interface. We believe that differences in the extension of colorimetric transition are related to where solvent molecules are placed. We verified that methanol was not able to cause spectrophotometric modification in either vesicles. As seen in Table 1, methanol is highly water-soluble. Therefore, its partition to water was more favorable than to vesicles. For this reason, methanol did not affect the PCDA backbone. On the basis of our results, we highlighted the essential role of interactions between solvent molecules and vesicles in the blue-to-red transition of PCDA. Nevertheless, we believe that emphasizing the job of solventwater interactions in such a mechanism is crucial. To obtain the energy involved in the solvent-vesicle and solvent-water interactions, we carried out microcalorimetric analysis related to the mixture of solvent and pure water or vesicular suspensions. Figure 5 shows the results of the successive addition of 1.0 µL of CHCl3 into 1.8 mL of PCDA or PCDA/CHO/SPH suspensions. This figure shows the variation observed for molar enthalpy (∆Hobs) versus micromoles of CHCl3 added to vesicles and the enthalpic variation referred to 1.0 µL of CHCl3 added into 1.8 mL of pure water. The enthalpy change of the mixture process of CHCl3 and water was exothermic, with initial values of -3.4 kJ mol-1. This liberated energy became less negative as the CHCl3 concentration in the system increased. After the addition of approximately 12.0 µL (149 µmols) of organic solvent, the mixture process became almost athermal. The miscibility of chloroform and water at 25 °C is 67 µmol CHCl3/mL of water.40 After the addition of approximately 121 µmol of chloroform into 1.8 mL of water, the process of phase separation began. On the basis of this behavior, the interaction between CHCl3 and H2O was enthalpically favorable (exothermic), though it occurred with a reduction in system entropy. This ∆Smix < 0 process was associated with chloroform solvatation, which had water molecules more ordered than water bulk molecules.41,42 To eliminate the entropic reduction, the system separated into two phases: one mainly rich in water and the other mainly rich in chloroform. The titration curves of CHCl3 into vesicular suspensions showed values of ∆Hobs completely distinct from those obtained

for water, which indicated a CHCl3-vesicle interaction that was strongly dependent on bilayer composition and structure. The addition of small volumes of such a solvent ( 0. This result characterized the contribution of hydrophobic interactions to colorimetric transition induced by CH2Cl2. As the quantity of CH2Cl2 molecules increased, the ∆Hap-int became less positive and reached negative values in the PCDA vesicles experiment. Interestingly, the enthalpic variation associated with colorimetric transition (∆Htr-col) was the same for both vesicles (∆Htr-col ) -1.5 kJ mol-1), demonstrating that, in this case, CH2Cl2 molecules interacted mainly with PCDA molecules. Additionally, CHO and SPH contributed little in the process of conformational change. As previously discussed, methanol was not capable of inducing colorimetric change in the PCDA and PCDA/CHO/ SPH vesicles. To establish a relationship between the interaction energy of the CH3OH-vesicle and the incapability to promote blue-to-red transition, microcalorimetric experiments were also repeated with this solvent. Figure 9 shows the enthalpic variation of the mixture process when small quantities were titrated in pure water. Unlike the other solvents, the mixture process between methanol and water was always exothermic and almost constant, which is a strong driver for high miscibility between these solvents. In the presence of vesicles, the mixture process became less exothermic, indicating that the CH3OH-water interaction was thermodynamically more favorable than the CH3OH-vesicle interaction. Most likely, this higher affinity between water and methanol molecules avoided the induction of colorimetric transition.

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Pires et al. The energy measurements associated with colorimetric transition support the hypothesis that such phenomena occurred due to conformational changes associated with the functional group rotation around the simple carbon-carbon bond present in PCDA chains. The organic solvents CHCl3, CH2Cl2, and CCl4 interact in different ways with PCDA and PCDA/CHO/SPH vesicles. The process of colorimetric transition for both vesicles was predominantly enthalpic in the presence of CHCl3, while in the presence of CH2Cl2 and CCl4, the event was entropically driven. The addition of small quantities of methanol did not induce the color change because these molecules did not interact with the hydrophobic portion of PCDA chain and did not cause the conformational alterations responsible for the blue-to-red transition.

Figure 9. Enthalpic variation (∆H) of methanol titration into (∆) PCDA vesicles, (9) PCDA/CHO/SPH vesicles, and (O) water.

Acknowledgment. We gratefully acknowledge Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq), Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais (FAPEMIG), and Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES) for financial support of this project. A.C.S.P. thanks CNPq, and A.B.M. thanks CAPES scholarships. References and Notes

Figure 10. Apparent molar interaction enthalpy (∆Hap-int) between methanol and vesicles of (9) PCDA and (∆) PCDA/CHO/SPH.

Figure 10 presents the apparent molar interaction enthalpy between methanol and PCDA and PCDA/CHO/SPH molecules. The transfer of CH3OH from water to the inner PCDA vesicle was enthalpically unfavorable, and there was an absorption of energy between +7.0 kJ mol-1 and +3.5 kJ mol-1. Due to the interaction between methanol, cholesterol, and sphingomyelin, the apparent molar interaction enthalpy of CH3OH-PCDA/ CHO/SPH was less positive, varying between +2.5 kJ mol-1 and +0.2 kJ mol-1. Although this was a smaller enthalpy, the methanol molecules do not induce colorimetric transition in the PCDA/CHO/SPH vesicles. The addition of carbon tetrachloride (CCl4) in water and in vesicular suspensions resulted in an enthalpy of zero, which means that the values obtained were smaller than the equipment limit (0.02 µW). This finding indicates that the interaction processes between CCl4 and vesicles that induce colorimetric transitions are mainly driven by entropic power. Conclusions PCDA and PCDA/CHO/SPH vesicles undergo colorimetric transition induced by different solvents, such as chloroform, dichloromethane, and carbon tetrachloride. This work presented the first results of energy involved in the solvatochromism process of PCDA and PCDA/CHO/SPH, which is essential to understand the mechanism of interaction between solvents and polydiacetylenic vesicles. The presence of cosolutes in the vesicle compositions strongly influences the blue-to-red mechanism.

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