Interaction of cationic cyanine dye with algal alginates: evidence for a polymer bound dye dimer
Sa’ib J. Khouri, Volker Buss
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DOI: 10.4236/jbpc.2011.24043   PDF    HTML   XML   4,903 Downloads   9,263 Views   Citations

Abstract

Under certain conditions algal alginates bind pinacyanol chloride in the form of dimers – this is the conclusion drawn from a uv/vis and circular dichroism (CD) spectroscopic study of aqueous solutions of the dye in a 10-fold molar excess of the polysaccharide from different sources. With its easily detected features the dimer holds promise as a diagnostic tool for alginate conformational analysis. Binding of the strongly blue-shifted (maximum wavelength = 485 nm) dimer is probably mediated by the charged groups and involves guluronate units only. By using the peakFit program, the two overlapping excitonic absorption bands together with the optically inactive band resulted from the interaction of pinacyanol with a specific alginate concentration were separated. The standard Gibbs energy of the interaction was calculated as –27.02 kJ.mol–1. The dimers are sensitive against acid and divalent cations.

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Khouri, S. and Buss, V. (2011) Interaction of cationic cyanine dye with algal alginates: evidence for a polymer bound dye dimer. Journal of Biophysical Chemistry, 2, 380-385. doi: 10.4236/jbpc.2011.24043.

1. INTRODUCTION

Algal alginates are structural polysaccharides found in high concentration in various types of brown seaweeds, which because of their gelling properties command considerable commercial interest. Chemically they are 1, 4-linked block polymers of β-D-mannuronate (M for short) and α-L-guluronate (G) or of alternating (MG) sequences with composition varying according to source and treatment. Due to the presence of carboxylate groups, which absorb at about 215 nm circular dichroism (CD)—the differential absorbance between left and right circularly polarized light—has played a major role in elucidating the structure of alginates and their cation mediated aggregation properties [1,2]. Interaction with dyes can shift the uvand the CD absorptions into the visible region and allow the study of biopolymers in a wavelength range, which is spectroscopically more easily accessible [3]. Seely and Hart have investigated the interaction between methylene blue and several types of sodium alginate by uv/vis and CD spectroscopy at high polymer to dye molar ratios [4], and Pal and Mandal have used several cationic dyes, among them pinacyanol chloride, to study the binding to potassium alginate [5]. Using this approach we have studied the interaction of pinacyanol with algal alginates from different sources in different concentrations. We have found what we believe is an alginate bound dye dimmer, which holds promise as an indicator for the various processes involving alginate chain conformations.

Peak analysis was used to analyse and separate the various new absorption bands that resulted by such interaction for the most optimal spectrum [6]. All measurements in this study are conducted mainly in 7.5% v/v ethanol/water solutions.

2. MATERIALS AND METHODS

2.1. Materials

Sodium alginate was purchased from Kelco, UK. According to manufacturer’s specifications, the alginate has a molecular mass of 90,000 g/mol and consists of approximately 450 monomers. The molar ratio of mannuronate to guluronate in the sample is 60 - 65 to 40 - 35, corresponding to an M/G ratio between 1.5 and 1.8 to 1. Mannuronate rich and guluronate rich alginates were prepared in our labs [7]; in the former, the M/G ratio was 5:1, in the latter it was 0.36:1. Pinacyanol chloride (1,1’- diethyl-2,2’-carbocyanine chloride) was obtained from Sigma and used as received. For the spectra, we used spectroscopy grade ethanol from Merck, and water was distilled three times.

2.2. Experimental Methods and Instruments Used

Standard pinacyanol and sodium alginate solutions were prepared in 25 ml volumetric flasks. Alginate-dye solutions (4.00 ml) were prepared in stoppered rolled rim glasses of 10 ml capacity. To prevent the dye from precipitating at the glass walls the alginate solutions were added first, followed by the required amount of the dye solution. The tendency of cyanine dyes to aggregate in aqueous solution is wellknown. In order to extend the concentration range we added a constant low concentration of 7.5% (v/v) ethanol in all our spectroscopic investigations.

UV/vis spectra were recorded with a Perkin-Elmer Lambda 5 spectrophotometer, and CD spectra were measured with an AVIV circular dichroism spectrometer, Model 62 DS. Both instruments were connected to a personal computer for data collection in ASCII-file format.

3. RESULTS AND DISCUSSION

3.1. Sodium Alginate; Characterization by CD

CD spectroscopy has been used to evaluate the alginate composition based on the carboxylate n®π* absorption at about 215 nm [2]. Figure 1 shows the CD spectrum of sodium alginate between 195 and 250 nm. The spectrum has been taken in aqueous solution, and the concentration of sodium alginate is 0.80 mg/ml, corresponding to 4.0 × 10–4 M (if monomeric sodium mannuronate or guluronate, C6H7O6Na, are taken as the molecular mass unit). The spectrum shows a positive peak at 200 nm, and a negative trough at 215. From the ratio of the peak to the trough amplitude the composition of sodium alginate, in terms of mannuronate and guluronate, can be calculated by using a specific equation [2]. The values obtained from such non-destructive method were found to be 63% and 37% for mannuronate and guluronate, respectively. Therefore, sodium alginate that was used in the following aggregation experiments contains an excess of mannuronate over guluronate in the ratio of 1.7:1.

3.2. UV/vis and CD Spectra at Different Alginate/Dye Ratios

Figure 2(a) shows how increasing the molar ratio of alginate to dye affects the UV/vis spectrum of an aqueous pinacyanol solution (7.5% v/v ethanol). At the pinacyanol concentration chosen (1.5 × 10–5 M) most of the dye molecules are present in the form of monomers (maximum wavelength = 600 nm) or dimers (546 nm). Addition of the alginate destroys the monomer/dimer band structure of the dye and shifts the absorbance to a new

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1] Rees, D.A. (1972) Shapely polysaccharides; The eighth colworth medal lecture. The Biochemical Journal, 126, 257-273.
[2] Morris, E.R., Rees, D.A. and Thom, D. (1980) Characterisation of alginate composition and block-structure by circular dichroism. Carbohydrate Research, 81, 305-314. doi;10.1016/S0008-6215(00)85661-X
[3] Hatano, M. (1986) Induced circular dichroism in biopolymer-dye systems. In: Kamura, S.O., Ed., Advances in Polymer Science (Vol. 77), Springer, Berlin.
[4] Seely, G.R. and Hart, R.L. (1979) Absorption and circular dichroism spectra of methylene blue bound to alginate. Biopolymers, 18, 2745-2768. doi;10.1002/bip.1979.360181108
[5] Pal, M.K. and Mandal, N. (1990) Binding of cationic dyes to potassium alginate: A spectrophotometric and dichroic probe. Biopolymers, 29, 1541-1548. doi;10.1002/bip.360291205
[6] Antonov, L. and Nedeltchera, D. (2000) Resolution of overlapping UV-Vis absorption bands and quantitative analysis. Chemical Society Reviews, 29, 217-227. doi;10.1039/a900007k
[7] Haug, A., Larsen, B. and Smidsr?d, O. (1974) Uronic acid sequence in alginate from different sources. Carbo- hydrate Research, 32, 217-225 (modified by Schürks, N., University of Duisburg-Essen). doi;10.1016/S0008-6215(00)82100-X
[8] Lightner, D.A. and Gurst, J.E. (2000) Organic conformational analysis and stereochemistry from circular dichroism spectroscopy. Wiley, New York.
[9] Benesi, H.A. and Hildebrand, J.H. (1949) A Spectrophotometric investigation of the interaction of iodine with aromatic hydrocarbons. Journal of the American Chemical Society, 71, 2703-2707. doi;10.1021/ja01176a030
[10] Draget, K.I., Smidsr?d, O. and Skaj?k-Br?k, G. (2005) Alginates from algae. In: Steinbüchel, A. and Rhee, S. K., Eds., Polysaccharides and Polyamides in the Food Industry. Properties, Production, and Patents, Wiley, Wein- heim.
[11] Haug, A., Myklestad, S., Larsen, B. and Smidsr?d, O. (1967) Correlation between chemical structure and physical properties of alginates. Acta Chemica Scandinavica, 21, 768-778. doi;10.3891/acta.chem.scand.21-0768
[12] Smidsr?d, O. and Haug, A. (1968) Dependence upon uronic acid composition of some ionexchange properties of alginates. Acta Chemica Scandinavic, 22, 1989-1997. doi;10.3891/acta.chem.scand.22-1989
[13] Grant, G.T., Morris, E.R., Rees, D.A., Smith, P.J.C. and Thom, D. (1973) Biological interactions between poly- saccharides and divalent cations: The egg-box model. Federation of European Biochemical Societies (FEBS) Letters, 32, 195-198.
[14] Scheibe, G. and Zanker, V. (1958) Physicochemical principles of metachromasia, Acta Histochemica Supplement- band, Suppl.1, 6-37.
[15] Pal, M.K. and Schubert, M. (1962) Measurement of the stability of metachromatic compounds. Journal of the American Chemical Society, 84, 4384-4393. doi;10.1021/ja00882a004
[16] Khouri, S.J. and Buss, V. (2010) UV/Vis spectral study of the self-aggregation of pinacyanol chloride in ethanol- water solutions. Journal of Solution Chemistry, 39, 121- 130. doi;10.1007/s10953-009-9476-2
[17] Harada, N. and Nakanishi, K. (1983) Circular dichroic spectroscopy. Exciton Coupling in Organic Stereochemistry. University Science Books, Mill Valley.
[18] Braccini, I., Grasso, R.P. and Pérez, S. (1999) Conformational and configurational features of acidic polysaccharides and their interactions with calcium ions: A molecular modeling investigation. Carbohydrate Research, 317, 119-130. doi;10.1016/S0008-6215(99)00062-2

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