29

PART-I

CHEMICAL INVESTIGATION OF THE

POLYSACCHARIDES OF SOME BROWN SEAWEED

SPECIES

30

Chapter-I.1

Chemical studies on the alginate of brown seaweed

species: Sargassum tenerrimum, Sargassum wightii,

Cystoseira indica and Padina tetrastromatica

31

I.1.1 INTRODUCTION

I.1.2 MATERIALS AND METHODS

I.1.2.1 Collection of seaweeds

I.1.2.2 Extraction of alginate from seaweeds

I.1.2.3 Characterization of alginate

I.1.2.3.1 Determination M/G ratio

I.1.2.3.2 Determination of M and G blocks

I.1.2.3.3 Viscosity measurement

I.1.2.3.4 Molecular weight determination (GPC)

I.1.2.3.5 FT-IR spectroscopy

I.1.2.3.6 13

C NMR spectroscopy

I.1.2.3.7 Optical rotation and Circular dichroism (CD) spectroscopy

I.1.2.3.8 Scanning electron microscope (SEM)

I.1.2.3.9 X-ray diffraction analysis

I.1.2.3.10 Rheological determinations

I.1.3 RESULTS AND DISCUSSION

I.1.3.1 Physicochemical results

I.1.3.2 Molecular weight determination (GPC)

I.1.3.3 FT-IR spectroscopy

I.1.3.4 13

C NMR spectroscopy

I.1.3.5 Optical rotation and Circular dichroism (CD) spectroscopy

I.1.3.6 Scanning electron microscope (SEM)

I.1.3.7 X-ray diffraction analysis

I.1.3.8 Rheological measurements

I.1.4 CONCLUSION

I.1.5 REFERENCES

Chemical studies on the alginate of brown seaweed species:

Sargassum tenerrimum, Sargassum wightii, Cystoseira indica

and Padina tetrastromatica 1

32

I.1.1 INTRODUCTION

Seaweeds are the only natural source of agar, carrageenan and alginates, the

industrially important polysaccharides. The former two phycocolloids are synthesized

by red seaweeds, and the latter being produced exclusively by their brown

counterparts except one recent report of alginate from a red calcareous alga Corallina

pilulifera Usov et al. (1995). It is noteworthy that alginates are also produced by

bacteria viz. Azotobacter vinelandii and several Pseudomonas species Skjak-Braek et

al. (1986). Alginates are hydrocolloids, water soluble biopolymers extracted from

brown seaweed. They were first investigated in the late 19th

century by a British

chemist E.C. Stanford. (www.fao.org). “Alginate” is a collective term for a family of

polysaccharides produced by brown algae (Painter 1983). It is a polysaccharide

consisting of linear co-polymer of β-1,4-D-mannuronic acid and α-1,4-L-guluronic

acid Haug et al. (1966) (Figure I.1.1). The average lengths of the blocks being about

20 units and the proportions of the two acids have been found to vary in the extracts

of different species and even in different parts of the same plant. Alginate is present

as salts of different metals, primarily Na+

and Ca+2

. It occurs in both the intercellular

regions and the cell walls and it is considered to be the principal skeletal material of

all brown seaweeds (Haug et al.1967; Ji et al. 1981; Cora et al. 1992; Nishide et al.

1996).

The main brown seaweeds that are processed commercially worldwide for the

alginate are Macrocystis pyrifera, Laminaria spp., Ascophyllum spp., Ecklonia spp.,

Lessonia spp. and Padina gymnospora. None of them occur in the Indian waters

except for some other Padina species. Extraction of alginic acid/alginate from brown

algae and characterization has been reported by many authors (Haug & Larsen 1962;

Haugh, Larsen & Smidsrod, 1974; Nishide et al. 1987; Andrade et al. 2004).

The worldwide annual industrial production of alginate is estimated to be 30,000

metric tons, which is probably less than 10% of the biosynthesized material in crops

of macroalgae. These figures allow us to consider such a polysaccharide as an

unlimited and renewable resource even for a steadily growing industry (Draget et al.

2005). In addition, alginate production by fermentation is also technically possible,

although it dose not meet the requirement of economic feasibility.

The Indian production of alginate is mainly from Sargassum spp. There exist a few

reports on alginate from Indian waters (Alankararao, 1988; Mody et al. 1992; Redekar

and Raje 2000 and Ganesan et al. 2001). There are, however, no reports on the

detailed systematic chemical investigation of alginates of Indian brown seaweeds.

33

Therefore, Sargassum tenerrimum, Sargassum wightii (Family: Sargassaceae),

Cystoseira indica (Family: Cystoseiraceae) and Padina tetrastromatica (Family:

Dictyotaceae) were selected for detailed chemical studies for their alginates.

The main industrial applications of alginate as a natural polymeric material are linked

to its stabilizing, viscosifying, gelling properties and its ability to retain water.

Alginate is largely used as a viscosifier in textile printing because of its shear-thinning

characteristics. Its use has also been reported to increase color yield and brightness. In

addition, various applications of alginate in the food industry are currently being

exploited. The ability of this polysaccharide to stabilize aqueous mixtures, dispersions

and emulsions together with its gel-forming and viscosifying properties represent key

features for the use of alginate in food applications. Several alginate-based

restructured products (pet food, reformed meet, onion rings, crab sticks, to name few)

are available on the market for large-scale distribution (Cotterll and Kovacs 1980;

Littlecott 1982; Sime 1990). In addition, alginate is widely used as an additive for the

production of low-sugar jam, jellies and fruit fillings (Toft et al. 1986). In these

applications, synergistic interactions with proteins and other polysaccharide are

exploited.

Alginates well meet all the requirements for their use in pharmaceutical and medical

applications. They have been largely used in wound dressings, dental impression and

formulations for preventing gastric reflux. However, the most advanced

biotechnological and biomedical applications of alginate resides in its use as a

hydrogel for cell immobilization for applications ranging from production of ethanol

from yeast cells and of antibiotics or steroids (Smidsrod and Skjak-Breaek 1990) to

transplantation and cell therapy (Lim and Sun 1980). In the latter case, alginate gel is

used as a selective immune barrier to protect the transplanted cells from the host

immune system.

There has been a notable increase in the number of alginate applications in recent

years and the possibility of using such a polysaccharide for advanced biomedical

therapies requires very detailed knowledge of its molecular characteristics. In fact, a

clear understanding of the structure-function relationships is crucial for successful

preparation of refined alginates for those applications where specific biochemical and

physicochemical features have to be met. In addition, engineering of alginate

molecules, by tailor-making their composition and properties or by introducing cell-

specific signals, represents an important step forward for future novel applications in

biotechnology field.

34

I.1.2 MATERIALS AND METHODS

I.1.2.1 Collection of seaweeds

The seaweeds of this investigation were collected from the Indian coasts and the

details are: Sargassum tenerrimum was collected during March 2008 to April 2008

from Veraval (20o 54.875’ N, 70

o 20.832

’ E), Diu (20

o 42.727’ N, 70

o 55.487

’ E), from

the inter-tidal zone on the west coast of India. Sargassum weightii used in this study

was collected during December 2007 to February 2008 from Hare island (09o 11.749’

N, 79o

03.31’ E) from the inter-tidal zone on the southeast coast. Cystoseira indica

used in this study was collected in January, 2008 from Diu (20o 42.727’ N, 70

o

55.487’ E), from the inter-tidal zone of west Coast of India. Padina tetrastromatica

used in this study was collected during January-February 2008 from Okha (22o

28.580’ N, 69o

04.254’ E) from the inter-tidal zone on the west coast and Valai island

(09o 10.445’ N, 78

o 55.55

’ E) from the inter-tidal zone on the southeast coast of India

(Oza and Zaidi, 2001; www.algaebase.org). Herbaria specimens of all the seaweed

species were submitted with the CSMCRI Herbarium.

All the seaweeds were washed with tap water to remove the solid impurities from the

plants and were dried in the shade and powdered in a rotating ball mill and stored in

separate plastic containers. Methanol (MeOH), formalin, sodium carbonate (Na2CO3),

sodium hydroxide (NaOH), hydrochloric acid (HCl), sulphuric acid (H2SO4) of LR

grade were used and were purchased from Ranbaxy Fine Chemicals Ltd., Mohali,

Punjab (India). Sigma (A2158-250G) alginate was used as a reference sample for

benchmarking.

I.1.2.2 Extraction of alginate from seaweeds

Pretreatment: Dried algal powder was defatted with repeated extraction with MeOH

(100ml X 4) in a percolator for four days at room temperature. Then 10g of each algal

material were treated with 90ml of 0.1% formalin overnight (Dea, 1986). The residual

formalin solution was drained off. Then the algal material was washed with water

thrice and dried at room temperature and then at 40oC.

Alginate was extracted by the method described by Nishide et al. (1996). A 10g

sample of pretreated dry algal fronds were first soaked in warm distilled water (60oC,

300ml) for 1h and then filtered through nylon cloth. After homogenization of the algal

material, it was extracted with 200ml of Na2CO3 solution (0.17N) under stirring

condition at 75oC for 4h. Distilled water (800ml) was then added and the solution was

35

separated from the residual algal powder by filtration through a bed of Celite-545 and

was acidified with 10% HCl to pH 1.00 to give a gelatinous precipitate. The gel was

allowed to stand at room temperature for 3h and was collected by centrifugation at

5000rpm for 20min. Then 200ml of 50% MeOH were added to the gel and the

mixture was neutralized with 10% NaOH with constant stirring. After standing

overnight, the mixture was filtered through a layer of cotton cloth to separate the

neutralized gel. The gel was washed successively with 60% MeOH, 95% MeOH and

finally with acetone and was dried at 30o for12h. The alginate obtained from the

treated algal material of S. tenerrimum, S. wightii, C. indica and P. tetrastromatica

were designated as STAL (yield 18%), SWAL (20%), CIAL (10%) and PTAL (14%),

respectively.

I.1.2.3 Characterization of alginate

I.1.2.3.1 Determination of M/G ratio

Mannuronic acid to guluronic acid ratios (M/G) of these alginates were determined by

a rapid one-pot method of hydrolysis of sodium alginate using microwave irradiation

(Chhatbar et al. 2009). Alginate (5g) was dissolved in 125ml of 0.15M oxalic acid or

0.25M H2SO4 and 25ml portions of the alginate solution were exposed to 100%

microwave power for five different exposure durations (1–5min). The experiment was

repeated with 0.05, 0.15, 0.25 and 0.5M oxalic acid/H2SO4. After microwave

irradiation Polymannuronic acid (PMA) and polyguluronic acid (PGA) were isolated

from the hydrolyzed samples using pH dependent method (Chandía et al. 2001;

Sakugawa et al. 2004) (Figure I.1.2). The mixture was neutralized with CaCO3 and

filtered and the solution was passed through Dowex 1-X8 anion exchange resin

column (100-200 mesh; 20 x 2cm) in acetate form and fractionated by gradient

elution with 0.5-2.0N acetic acid. Carbohydrate of each fraction was monitored by the

Phenol sulfuric acid method (Dubois et al. 1956). Two fractions were obtained,

former being guluronic acid (G) and the latter mannuronic acid (M). Both the pooled

M and G fractions were evaporated under vacuum to a small volume and passed

through a cation exchange resin column (IR-120) to eliminate the cations. The

effluent was again evaporated to a syrup and this treatment was repeated 2-3 times in

order to expel the acetic acid. Upon adding 30ml of water, a small amount of anion

exchange resin (OH- form) was added to remove the color and residual anions. After

filtration, the filtrate was treated with NaOH (pH 8) to sufficiently transform the

lactones to sodium uronate. The solution was allowed to pass through a cation

exchange resin column and the uronic acid liberated was titrated (after making up the

effluent to 50ml or 100ml) against 0.1N NaOH to confirm the exact concentration of

36

each uronic acid. The M/G ratios measured by this improved method (with 0.15M

oxalic acid/ 0.25M H2SO4 under 100% MW power for 4min) were comparable with

that obtained from conventionally hydrolyzed sodium alginate (with 80% H2SO4 at

20oC for 18h and then 2N H2SO4 at 100

oC for 6h), which was reported by Haug and

Larsen (1962) and Ji et al. (1981).

I.1.2.3.2 Determination of M and G blocks

The different uronic acid blocks of alginate were determined according to the method

of Haug et al. (1974). Partial hydrolysis was carried out by suspending 0.5g of

alginate in 100 parts of 0.3M HCl for 2h at 100oC. After 2h the suspension was cooled

and centrifuged to give the MG-block fraction in the supernatant. The amount of

material in the supernatant was determined by the method reported Knutson and Jeans

(1968) (vide section I.2.1.2.4). The residue was suspended in water and dissolved by

careful neutralization. The volume was adjusted to give an alginate concentration of

1%, NaCl was added to 0.1M concentration, and the solution was mixed with 25mM

HCl to pH 2.8-3.0. The precipitate (GG-block fraction) was collected by

centrifugation, suspended in water and solubilised by neutralization. The amounts of

uronic acids in the precipitate and in the supernatant (MM-block) were determined

(vide section I.2.1.2.4).

I.1.2.3.3 Viscosity measurement

Apparent viscosity of alginate samples e.g. STAL, SWAL, CIAL and PTAL (1.5% in

distilled water) were measured using a Brookfield Viscometer (DV-II +Pro) at 27oC.

Spindle SC4-18 was used for apparent viscosity measurement at 60 rpm.

I.1.2.3.4 Molecular weight determination (GPC)

For the determination of molecular weights (Mn, Mw and Mz) of alginate samples

STAL, SWAL, CIAL, PTAL and Sigma (A2158-250G) gel permeable chromatography

(GPC) was carried out on Waters alliance HPLC, with Waters 2695 separation

modules and 2414 refractive index detector. Two columns Ultra hydrogel 120 and

Ultra hydrogel 500 were used, column length and diameter was 300mm and 7.8mm,

respectively. The columns were eluted with 0.1M NaNO3 solution at a flow rate of

0.5ml/min. The oven temperature was maintained at 45oC. Dextran standards with Mp

value ranges 4.01 x 105; 1.96 x 10

5; 4.35 x 10

4; 4.4 x 10

3Da. (MW: 6.68 x 10

5; 2.73 x

105; 4.86 x 10

4; 5.2 x 10

3 Da, respectively) were used for preparation of calibration

curve (Figure I.1.3). The molecular weight of alginate samples STAL, SWAL, CIAL

37

and PTAL, poly dispersity and area measurements were calculated using standard

calibration curve by Empower2 software, USA provided with the instrument.

The polysaccharide sample (50mg) was dissolved in 100ml of HPLC grade water

using Tomy ES-315 autoclave (110oC, 30min.), filtered through nylon membrane

filter (Whatman 0.45µm) and 0.5ml of sample was injected in column. The molecular

weights of standards and samples were determined by GPC according to the method

described by Li et al. (2008).

I.1.2.3.5 FT-IR spectroscopy

Infrared spectra of the alginate samples STAL, SWAL, CIAL and PTAL were

recorded on a Perkin-Elmer Spectrum GX FT-IR system (USA), by taking 10.0mg of

sample in 600mg KBr. All spectra were average of two counts with 10 scans each and

a resolution of 5 cm-1

.

I.1.2.3.6 13

C NMR spectroscopy

Noise-decoupled 13

C NMR spectra were recorded on a Bruker Avance-II 500

(Ultrashield) Spectrometer, Switzerland, at 125 MHz. Alginate of SWAL and their

respective oligomers PMA and PGA were dissolved in NaOH+D2O mixture

(50mg/ml) and the spectra were recorded at 35oC with 5000–5200 accumulations,

pulse duration 5.9 μs, acquisition time 1.2059s and relaxation delay 6μs using DMSO

as internal standard (ca. δ 39.5).

I.1.2.3.7 Optical rotation and Circular dichroism (CD) spectroscopy

Optical rotations were measured for all alginate samples of SWAL, STAL, CIAL and

PTAL (0.250g/100ml at 27oC at 589nm) on a Rudolph Digi pol-781 Polarimeter

(Rudolph Instruments Inc, NJ, USA).

Circular dichroism (CD) spectra of alginates (Sigma and S. wightii), PMAs (Sigma

and S. wightii alginates) and PGAs (Sigma and S. wightii alginates) were recorded on

a Jasco model J-815 CD Spectrometer, using measurement range of 190–250nm and

at a concentration 0.8 mg/ml (800 ppm). Molar ellipticity values [θ] are reported in

mdeg. The ratio of peak height to trough depth was calculated using Eq. (1), described

by Morris et al. (1980).

Peak/trough ratio = (θtrough − θpeak/ θtrough) ---- (1)

38

I.1.2.3.8 Scanning electron microscopy (SEM)

The surface morphology of the alginate samples was analysed on a Carl-Zeiss Leo VP

1430 scanning electron microscope (SEM) applying an accelerating voltage of 10 or

20 kV and magnification 1 to 38 K respectively. Vacuum oven dried samples of

SWAL and their respective oligomeric samples were mounted on a sample holder and

coated with gold under vacuum prior to the studies.

I.1.2.3.9 X-ray diffraction analysis

Powder X-ray diffraction studies of SWAL and their respective oligomers (i.e. PGA

and PMA) were done on a Philips X’pert MPD X-ray powder diffractometer using 2θ

= 10o to 60

o.

I.1.2.3.10 Rheological determinations

The flow curves for STAL, SWAL, CIAL, PTAL and Sigma alginate in 0.1 M NaCl

aqueous solutions (1.0%) were obtained on a Anton Paar, Physica MCR 301

rheometer, cone-plate model C-CC27/T200/SS, at 25oC.

I.1.3 RESULTS AND DISCUSSION

I.1.3.1 Physicochemical results

Sulit & Juan, (1955) first studied the alginate contents of several Sargassum species.

There are many literature reports on the extraction of alginate, wherein different

extraction conditions and scales have been employed e.g. on laboratory and pilot plant

scales using various brown seaweed species of Indian waters e.g. Turbinaria ornata,

Hormophysa triquetra, Cystophyllum muricatum and Sargassum swartzii (Doshi et al.

1984; Sai Krishnamurthy 2000), Sargassum tenerrimum and Sargassum wightii were

reported by (Redekar and Raje et al. 2000) and (Ganeshan et al. 2001), respectively.

There exists, however, no report on the detailed systematic chemical investigation of

alginates of Indian brown seaweeds. The members belonging to order Fucales

(Cystoseira indica) and Dictyotales (Padina tetrastromatica) are little studied for their

polysaccharide contents. There exists no report of studies on alginate from Padina

tetrastromatica and Cystoseira indica of Indian waters. Therefore, Sargassum

tenerrimum, Sargassum wightii, Cystoseira indica and Padina tetrastromatica were

selected for detailed chemical studies for their alginates.

39

The yields alginates of S. tenerrimum, S. wightii, C. indica and P. tetrastromatica

were given as STAL (yield 18%), SWAL (20%), CIAL (10%) and PTAL (14%) in the

Table I.1.1. Yields were calculated on the basis of as received seaweeds. In the

present investigation the viscosities of the alginates obtained from S. tenerrimum

(STAL, 120 cps) was found to be higher than those of S. wightii, C. indica and P.

tetrastromatica (SWAL, 110 cps; CIAL, 80 cps and PTAl, 95 cps respectively).

Uronic acid sequence e.g M and G block was determined of the samples of alginate

and results were depicted in Table I.1.1

Table I.1.1 Analytical data of the alginates

Alginates of Sigma

alginate

(A2158-

250G)

Sargassum

tenerrimum

(STAL)

Sargassum

wightii

(SWAL)

Cystoseira

indica

(CIAL)

Padina

tetrastromatica

(PTAL)

Yielda (%) 18 20 10 14 -

Viscosity (cps)

(c 1.5 wt%) at

27oC

120 110 80 95 200

MG (%) 30 25 22 24 28

MM (%)

28 20 20 36 32

GG (%)

42 55 58 40 40

aYields were calculated on the basis of as received seaweeds

The M/G ratios and % weights of PGA and PMA obtained under optimized

microwave (0.15M Oxalic acid or 0.25M H2SO4, 4min) conditions in the present

method for the alginate of STAL, SWAL, CIAL, PTAL and Sigma were comparable

to the ones obtained by conventional method reported by Haug and Larsen (1962) and

Ji et al. (1981) (Table I.1.2). M/G ratios obtained with 0.15M Oxalic acid or 0.25M

H2SO4 and 0.5M Oxalic or 0.5M H2SO4 acids were also comparable (Table I.1.2).

This investigation demonstrated that sodium alginate can be hydrolyzed under

microwave irradiation, using mild hydrolytic conditions, as opposed the reported

methods using hydrolytic reaction with high acid concentration requiring longer

reaction time.

The M/G ratio of alginate obtained from C. indica (CIAL) were found to be lower

(0.32) then those of S. tenerrimum, S. wightii, P. tetrastromatica and Sigma alginate

(STAL, 0.61; SWAL, 0.39; PTAl, 0.53 and Sigma 0.78 respectively).

I.1.3.2 Molecular weight determination (GPC)

40

The standard dextran calibration curve and chromatograms of alginate samples of

Sigma, STAL, SWAL, CIAL and PTAL were shown in Figure I.1.4. The molecular

weights (Mn, Mw, Mp and MZ) and poly dispersity were depicted in Table I.1.3.

I.1.3.3 FT-IR Spectroscopy

The FT-IR spectrum of the alginates (STAL, SWAL, CIAL and PTAL) are depicted

in (Figure I.1.5). All the spectra appeared to be identical and the prominent bends

were in the range: νmax (KBr) (cm-1

) 3430-3440 (O-H str, br, s), 2928-2930 (C-H str,

w), 1604-1625 (asymmetric -C=O str, s), 1420-1462 (C-H bending, w), 1027-1031

(C-O-C str, s), the fingerprint, or anomeric, region (950–750 cm-1

) is the most

discussed in carbohydrates (cf. Tul’chinsky et al. 1976; Mathlouthi & Koenig 1986;

Silverstein et al. 1991; Leal et al. 2008) [br = broad, s = strong, m = medium, w =

weak, str = stretching]. The band at 948.5 cm-1

was assigned to the C–O stretching

vibration of uronic acid residues, and the one at 888.3 cm-1

to the C1–H deformation

vibration of β-mannuronic acid residues. The band at 820.0 cm-1

was presumably due

to mannuronic acid residues (Chandı´a et al. 2001 & 2004).

I.1.3.4 13

C NMR spectroscopy

The 13

C NMR spectra of the alginate of SWAL and their respective constituting

oligomers (e.g. PGA and PMA) have been presented in Figure I.1.6. The chemical

shifts are in good agreement with those reported in the literature (Tako et al. 2000;

Zhang et al. 2004; Sakugawa et al. 2004; Leroux et al. 2004) and are furnished in

Table I.1.4. The carbonyl carbons (C-6 position) of mannuronic (M) and guluronic

(G) acids appeared at 176.75 (G6) and 177.25 (M6) ppm (cf. Grasdalen et al. 1981),

and the corresponding anomeric carbons appeared at 101.45 (G1) and 100.97 (M1)

ppm. The remaining carbon resonances were attributed to as follows: 66.48 (G2),

71.50 (M2), 70.53 (G3), 72.90 (M3), 81.55 (G4), 79.35 (M4), 68.52 (G5), 77.30 (M5)

ppm. Very similar carbon resonances were obtained in the spectra of sodium alginate

derived from S. tenerrimum (STAL), C. indica (CIAL) and P. tetrastromatica (PTAL)

and their respective oligomers (e.g. PGA & PMA) (Figures and data for these are not

shown).

I.1.3.5 Optical rotation and circular dichroism (CD) spectroscopy

The specific rotation of the aqueous solution (0.25%) of the alginate samples SWAL,

STAL, CIAL and PTAL were measured and results were presented in the Table I.1.5.

All the alginate samples showed negative optical rotation values.

41

Table I.1.2 Comparison of M/G ratios of sodium alginate determined by conventional methoda with those of the present study using

microwave heating.

Reaction conditions STAL SWAL CIAL PTAL Alginate of Sigma

(A2158-250G)

PGAb

(%)

PMAb

(%)

M/G

ratio

PGAb

(%)

PMAb

(%)

M/G

ratio

PGAb

(%)

PMAb

(%)

M/G

ratio

PGAb

(%)

PMAb

(%)

M/G

ratio

PGAb

(%)

PMAb

(%)

M/G

ratio

Conventional

method (80%

H2SO4 (20oC, 18 h)

and then 2N H2SO4

(100oC, 6h)

38.5 23.1 0.60 55.20 21.40 0.387 58.4 19.3 0.33 45.6 24.2 0.53 46.7 35.5 0.76

MW (0.15 M Oxalic

acid, 4 min) c

44.3 27.0 0.61 61.8 24.1 0.39 61.2 19.6 0.32 48.2 25.5 0.53 52.3 40.8 0.78

MW (0.25 M

H2SO4, for 4 min) c

42.5 26.0 0.61 63.5 24.8 0.39 60.4 19.3 0.32 50.6 26.8 0.53 51.5 40.2 0.78

MW (0.5 M Oxalic

acid, 4 min) c

43.6 25.3 0.58 60.4 22.2 0.36 62.8 22.6 0.36 43.8 23.6 0.54 47.3 34.5 0.73

MW (0.5 M H2SO4,

for 4 min) c

40.0 22.8 0.57 57.9 21.5 0.37 59.4 20.8 0.35 44.0 22.0 0.50 45.8 34.3 0.75

a Haug and Larsen (1962) and Ji et al. (1981);

b % of PGA and PMA were calculated by UV-VIS spectroscopy as described by Haug

and Larsen (1962) and Ji et al. (1981); cMW=Microwave heating in the present study

42

Table I.1.3 Gel permeation chromatographic data of alginates

Table I.1.4 13

C NMR shifts, observed for sodium alginate, poly-guluronic acid (PGA) and

poly-mannuronic acid (PMA) recorded in D2O with DMSO as internal standard

Components δ ppm Assignment of carbon

Sodium alginate a

100.97 (100.97) C- 1 of MM

71.50 (70.83)

C-2 of MM

72.90 (72.29) C-3 of MM

79.35 (78.94) C- 4 of MM

77.30 (77.01) C- 5 of MM

176.25 (175.58) C-6 of MM

101.45 (101.44) C- 1 of GG

66.48 (66.04) C- 2 of GG

70.53 (70.01) C- 3 of GG

81.55 (80.71) C-4 of GG

68.52 (68.18) C- 5 of GG

176.68 (175.75)

C- 6 of GG

Poly-mannuronic acid b

100.11 (100.37) C-1 of MM

68.11 (68.78) C-2 of MM

71.43 (71.20) C-3 of MM

79.09 (78.16) C-4 of MM

76.83 (76.21) C-5 of MM

175.24 (174.3) C-6 of MM

Poly-guluronic acid b

101.03 (101.54) C-1 of GG

66.61 (66.04) C-2 of GG

70.27 (69.2) C-3 of GG

81.09 (80.40) C-4 of GG

67.00 (67.16) C-5 of GG

175.16 (173.85) C-6 of GG

aValues in the brackets (Tako, et al. 2000);

bValues in the brackets (Zhang, et al. 2004;

Sakugawa, et al. 2004; Leroux, et al. 2004)

Samples Retention

Time

(min)

Molecular weights (Da) Poly-

dispersity Mn Mw Mp Mz

Sigma

(A2158-250G)

13.8 285785 315668 72895 694348 1.10

STAL 13.0 182394 225066 158637 624652 1.23

SWAL 13.5 76722 119687 72212 297615 1.56

CIAL 13.8 42542 57007 38909 129614 1.34

PTAL 13.9 23053 57514 32435 123132 2.49

43

Circular dichroism (CD) spectra of alginate (Sigma and S. wightii), PMA (Sigma and S.

wightii alginates) and PGA (Sigma and S. wightii alginates) are shown in Figures I.1.7a-c.

The CD spectrum of poly-L-guluronate (PGA) showed entirely negative peak and poly-

D-mannuronate (PMA) had a strong positive peak indicating clear separation of PGA and

PMA after acid hydrolysis of the sodium alginate, while CD spectrum of alginate showed

intermediate behavior, with a peak at ca. 200 nm, and a trough at ca. 215 nm (Figure

I.1.7a). The relative amounts of D-mannuronate and L-guluronate residues were also

observed from the ratio of peak height to trough depth. The peak/trough ratios were less

than 1 for all the alginate samples, indicating high content of poly-guluronate residue in it

(Morris, et al. 1980).

If the peak/trough were < 1, the overall composition of any compound will show negative

CD spectrum. When the spectrum crosses the baseline, then the overall composition

shows entirely positive spectrum (i.e., peak/trough >1) (Morris et al. 1975; Morris et al.

1980; Dentini et al. 2006). The peak/trough ratios of alginate samples were calculated

and results were presented in the Table I.1.5. Very similar results were found with the CD

spectra of sodium alginate from S. tenerrimum (STAL), C. indica (CIAL) and P.

tetrastromatica (PTAL) (Figures are not shown).

Table I.1.5 Specific rotations and peak/trough ratios of alginate samples

Properties SWAL STAL CIAL PTAL Alginate of

Sigma

(A2158-250G)

Optical rotation [α]D

(c 0.25 in H2O, 27oC)

a

-25.67o -68.12

o -45.25

o -53.17

o -26.98

o

Peak/trough ratio 0.59 0.71 0.55 0.48 0.63

a[α]D (0.3% w/v) -0.098

o (10

oC); -0.110

o (at 60

oC) (Tako, et al. 2000); [α]D (1.0% w/v) -

113o

(Bi, et al. 2007) , used wavelength 589nm.

I.1.3.6 Scanning electron microscopy (SEM)

The SEM images of sodium alginate (SWAL), PGA and PMA have been given in Figure

I.1.8. The SEM image of the alginate showed mixture of PGA and PMA, while PGA and

PMA SEM images showed different morphologies indicating separation of PGA and

PMA (Figure I.1.8).

44

I.1.3.7 X-ray diffraction

X-ray diffraction patterns exhibited that the sodium alginate (SWAL) was crystalline in

nature, while PGA and PMA appeared in amorphous and crystalline forms, respectively

(Figure I.1.9). X-ray patterns of PGA and PMA confirmed that PMA in the alginate was

crystalline in nature, may be due to the ordered structure and regular geometric

arrangement of the molecules in the PMA of alginate (Figure I.1.9). Very similar X-ray

diffraction patterns were found with sodium alginates from S. tenerrimum (STAL), C.

indica (CIAL) and P. tetrastromatica (PTAL) and their representative oligomers (e.g.

PGA & PMA) (Figures are not shown).

I.1.3.8 Rheological measurements

Rheological behavior is an important parameter for the application of polysaccharides in

the food industry. Flow curves for sodium alginate samples STAL, SWAL, CIAL, PTAL

and sigma in aqueous solution were performed in the concentration 1.0% in 0.1M NaCl

(Figures I.1.10). In the Power-Law model, the solution behavior can be calculated using

Eq. (2) described by Torres et al. (2007).

mγn

----- (2)

Where m is the consistency index (Pa s), n the flow behavior index, the shear stress and

γ is the shear rate. The parameters m and n can be obtained by linear regression analysis

(Marcotte et al. 2001; Torres et al. 2007). The n values for all these samples were in the

range 0.8-0.95. For Newtonian fluids the flow behavior indices are closer to 1. No

variations in the viscosity of all alginate samples at different shear rate were obtained,

demonstrating that all these alginates behaved as Newtonian fluid in solution.

I.1.4 CONCLUSION

Alginates were extracted from the brown seaweeds Sargassum tenerrimum, Sargassum

wightii, Cystoseira indica and Padina tetrastromatica of Indian waters. The yields and

viscosity of the alginates of Sargassum tenerrimum and Cystoseira indica were the

highest and the lowest respectively in this series. A rapid new microwave assisted method

was developed wherein alginates were hydrolyzed to yield M- and G-acids. M/G ratios

determined by this method were in good agreement with those obtained by the

45

conventional heating method (Chhatbar et al. 2009). The M/G ratio of the alginate of

Sargassum tenerrimum was higher than that of Cystoseira indica alginate – this was in

line with the viscosity values obtained. This constitutes the first systematic chemical

studies on alginate of Indian brown seaweeds, which will be useful in the works on the

biodiversity of alginophytes.

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49

pH 2.85, by 0.1 N HCL

pH 1.0, by 0.1 N HCL

Precipitate (alginic acid) Filtrate

Precipitate dissolved in 0.1 M

Na2CO3

Sodium alginate

(Sargassum sp or Sigma)

Partial acid hydrolysis

(0.3 N Oxalic/ 0.5 N H2SO4 for 4 min, MW)

Precipitate

(Poly-guluronic acid,

PGA)

Filtrate

Precipitate

(Poly-mannuronic acid,

PMA)

Filtrate

Figure I.1.2 Procedure of polymanuuronic acid (PMA) and polyguluronic acid (PGA)

isolation from sodium alginate.

Figure I.1.1 Repeating units of MG block of alginate

L-Guluronic acid

D-Mannuronic acid

50

Figure I.1.3 GPC calibration curve of Dextran standards with different molecular weight

a

b

51

Figure I.1.4 Gel permeable chromatogram of Sigma alginate (a), STAL (b), SWAL (c),

CIAL (d) and PTAL (e).

c

d

e

52

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0 cm-1

%T

3436

2932

2340 2163

1610

1416

1299

1092 1034

947 888 818

779 717

621 492 419

3434

2928

2342

2144

1622

1421

1304

1099 1031

948 890

817 672 620

420

3439

2923 2854

2358

1626 1414

1093 1032

949 891

817

623

2934

2365 2146

1607

1413 1093 1034

950 893

816 620

419

a

b

c

d

Figure I.1.5 FT-IR spectra of sodium alginate (a) STAL (b) SWAL (c) CIAL and (d) PTAL

53

Figure I.1.6 13

C NMR of sodium alginate, PMA and PGA of S. wightii

13C NMR in D2O; DMSO

as internal standard

13C NMR in D2O;

DMSO as internal

standard

13C NMR in D2O;

DMSO as internal

standard

54

Figure I.1.7 Circular Dichroism (CD) of (a) Alginate (Sigma, using as reference and SWAL,

prepared in this study), (b & c) PGA and PMA of Sigma and Sargassum (SWAL) alginates.

190 200 210 220 230 240

-50

-40

-30

-20

-10

0

10

20

30

40

Sigma alginate

(Reference)

CD

[md

eg]

Wavelength(nm)

(SWAL) alginate

a

190 200 210 220 230 240 250

-20

0

20

PMA (Sigma alginate)

PGA (Sigma alginate)

CD

[md

eg]

wavelngth(nm)

c

190 200 210 220 230 240 250

-40

-30

-20

-10

0

10

20

30

40

50

poly-guluronic acid (PGA)

SWAL

poly-mannuronic acid (PMA)

SWAL

CD

[med

g]

Wavelength(nm)

b

55

PGA

PMA

Sodium alginate

Figure I.1.8 Scanning electron microscopy (SEM) images of sodium alginate as well as

PGA and PMA

56

(b)

(c)

Figure I.1.9 X-ray diffraction patterns of (a) sodium alginate (b) PMA and (c) PGA.

(a)

57

0 200 400 600 800 1000

20

30

40

50

60

70

80

Shear rate [1/s]

Vis

cosi

ty [

mP

a.s]

0

5000

10000

15000

20000

25000

30000

35000

40000

Sh

ear stress [mP

a]

0 200 400 600 800 1000

0

20

40

60

80

Shear rate [1/s]

Vis

co

sity

[m

Pa.s

]

0

5000

10000

15000

20000

25000

30000

35000

40000

Shear stre

ss [mP

a]

0 200 400 600 800 1000

0

5

10

15

20

25

30

35

40

Shear rate [1/s]

Vis

co

sity

[m

Pa.s

]

0

2000

4000

6000

8000

10000

Sh

ear stre

ss [mP

a]

100 200 300 400 500 600 700 800 900 1000 1100

10

15

20

25

30

35

40

45

50

Shear rate [1/s]

Vis

cosi

ty [

mP

a.s

]

0

5000

10000

15000

20000

25000

30000

Shear stre

ss [mP

a]

STAL SWAL

CIAL PTAL

Figure I.1.10 Effect of shear rate on the viscosity of STAL, SWAL, CIAL and PTAL

solutions in 0.1M NaCl at 25oC.