Chemistry and Function of Pectins - American Chemical Society

in 0.05M and 0.1M NaCl for a series of pectins with varying degrees of ... Moreover, by membrane osmometry (0) and EGT we have shown that protonated a...
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3 A Critical Reexamination of Molecular Weight and Dimensions for Citrus Pectins Marshall L. Fishman, L. Pepper, W. C. Damert, J. G. Phillips, and R. A. Barford

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Agricultural Research Service, North Atlantic Area, Eastern Regional Research Center, U.S. Department of Agriculture, Wyndmoor, PA 19118

1

E-1000 and Ε-linear μ Bondagel high performance size exclu­ sion chromatography (HPSEC) columns were calibrated in root­ -mean square (RMS) radii of gyration (R ) by using a combina­ tion of pullulan and dextran standards. By assuming that universal calibration applies, to pectins the number-average R in 0.05M and 0.1M NaCl for a series of pectins with varying degrees of esterification in the protonated and sodium forms were calculated from chromatograms. This procedure yields well-defined R averages that can be used readily to cal­ culate molecular parameters for comparison with other methods. Such comparisons were not feasible when an earlier HPSEC method was used. Furthermore, by assuming rod-like structure for the pectins, number-average lengths (l ) and degree of polymerization (DP ) were calculated from R . Importantly, DP and l values from HPSEC fell between values from end group titration and membrane osmometry which is consistent with previous findings that pectin can undergo a concentration dependent disaggregation. g

g

g

n

n

n

gn

n

Over the l a s t ten years evidence has accumulated that pectin can undergo s e l f disaggregation

(1-4).

More recently, by size exclusion

chromatography (SEC) and determination of number-average degrees of

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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

Molecular Weight and Dimensions for Citrus Pectins

FISHMANETAL.

3.

23

polymerization (DP ) from end group t i t r a t i o n s (EGT) (5), we have n

demonstrated s e l f disaggregation over the pH range 3.7 - 7.3 f o r c i t r u s pectins with degree of methyl e s t e r f i c a t i o n (DM) between 37 and 73%.

Moreover, by membrane osmometry (0) and EGT we have shown

that protonated and neutralized pectins form metastable aggregates which can be dissociated by heat a c t i v a t i o n and that these a c t i ­ vated pectins undergo concentration dependent disaggregation (6,7). Furthermore, EGT gives the D P of p e c t i n monomer whereas osmometry q

gives the D P of aggregated pectins. Downloaded by UNIV OF ARIZONA on January 6, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch003

r

A l l but protonated pectins

with low DM (35 or 37%) exhibited disaggregation with a steep con­ centration dependence.

Thus, neutralized pectins and protonated

pectin with medium and high DM gave van't Hoff p l o t s which exhibited a minimum at about 0.1 g/dl. Furthermore, the osmotic data could be extrapolated to 7t/c values obtained from EGT (Figure 1). S i m i l a r behavior by proteins has been interpreted as that of a nonideal d i s ­ sociating system (8). In the case of the pectins, van't Hoff p l o t s appear l i n e a r and nonideal above 0.1 g/dl ( i . e . , they have a p o s i t i v e slope).

In the past, the l^m (τΐ/c) f o r the l i n e a r portion of the

curve has been used to obtain D P from membrane osmometry. r

obtain number-average (R (R

an(

), weight-average U*g ) * W

Here we

z-average

) RMS of r a d i i gyration from high performance s i z e exclusion

chromatography (HPSEC).

By assuming r o d - l i k e structure we obtain

DP

r

values from corresponding R^

DP

q

values from end group analysis and osmometry.

n

values.

These are compared with

Experimental

Materials.

Commercial c i t r u s pectins with degree of methyl e s t e r i -

f i c a t i o n (DM) 35, 58-60, and 70 were g i f t s from Bulmers Limited, Hereford, England.

Two other c i t r u s p e c t i n samples DM 37 and 72-73

were manufactured by Bulmers but were g i f t s from Drs. E. R. Morris and M. J . Gidley at Unilever.

The DM 57 c i t r u s p e c t i n was a g i f t

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

24

CHEMISTRY AND FUNCTION OF PECTINS

from Sunkist Growers, Corona, Ca. One p e c t i n sample was extracted from fresh grapefruit peels, according to standard procedure (9). I t had a degree of e s t e r i f i c a t i o n of 73 and was labeled 73G. Char­ a c t e r i z a t i o n and preparation of samples were as reported previously (5) with minor modification.

Samples to be neutralized with NaOH

were dissolved i n 0.01 M phosphate buffer (pH 6.1) containing 0.1 M EDTA, t i t r a t e d to pH 7 with 0.1 M NaOH, dialysed against four changes of water over 48 nr., centrifuged f o r 1 hr. at 30,000 χ g

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to remove insoluble matter and then l y o p h i l i z e d .

Protonated

samples were dissolved i n deionized water (Continental Water Systems) followed by d i a l y s i s , c e n t r i f u g a t i o n , and l y o p h i l l i z a t i o n weight cut o f f of 12,000.

Dextran standards were from Pharmacia

Chemical Co., Piscataway, N.J. The M^ values of the dextran 5

stand­

5

ards were as follows: T-500, 5.32 χ 10 ; T-250, 2.53 χ 10 ; T-110, 5

4

4

4

1.06 χ 10 ; T-70, 7 χ 10 ; T-40, 4.44 χ 10 ; T-20, 2.23 χ 10 ; T-10, 3

9.3 χ 10 . P u l l u l a n standards were from Polymer Laboratories, Inc., Amherst, MA. 5

5

The M values of the pullulans were 8.53 χ 10 , 3.80 χ w

5

5

4

4

4

10 , 1.86 χ 10 , 1.00 χ 10 , 4.8 χ 10 , 2.37 χ 10 , 1.22 χ 1 0 and 3

5.8 χ 10 . The pullulans had r a t i o s of M^/M^ of 1.14, 1.12, 1.13, 1.10, 1.09, 1.07, 1.06, and 1.07 respectively. HPSEC Apparatus, sample preparation and chromatographic conditions were as reported previously (5) with the following modifications.

High

performance s i z e exclusion chromatography was performed e i t h e r on a Waters E-1000 μ-Bondagel column (30 χ 0.39 cm I.D.) or an E - l i n e a r μ-Bondagel column (30 χ 0.39 cm I.D.).

Twenty μΐ of a 0.3 mg/ml

sample were i n j e c t e d . Mobile phase was e i t h e r 0.05 or 0.1M NaCl. The solvent i n the reservior was s t i r r e d with a magnetic s t i r r e r and the column was wrapped with a soft foam i n s u l a t o r . The chromatograph was kept i n a constant temperature room at 23±1°C. Flow rates were measured by an a i r bubble injected into a c a l i b r a t e d measuring pipette connected to the e x i t l i n e of the chromatograph (10). The pump was set at a nominal flow rate of 0.5 ml/min.

Long term flow

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

3.

25

Molecular Weight and Dimensions for Citrus Pectins

FISHMANETAL.

rates were measured to be w i t h i n ± 2% of the nominal value. any 8 hour period, flow rates were precise to ± 0.3%.

Over

Generally,

peak maxima f o r 3 consecutive runs agreed w i t h i n 2 seconds. Peaks emerging from the size exclusion column were detected by r e f r a c t i v e index and UV absorbance at 206 nm.

Analog signals

were d i g i t i z e d at a rate of 150 points per minute and i n a remote location by a modcomp 7861 minicomputer, equipped with an analog input subsystem.

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To obtain averaged root-mean-square r a d i i of gyration, p a r t i ­ t i o n c o e f f i c i e n t s ( K ) were transformed point by point to

values

ay

as appropriate integrals were summed. Integrations were by a trape­ z o i d a l algorithm.

Transformations were obtained from the following

c a l i b r a t i o n curves:

K->K ln(Y) = a + a- Κ 1 av ' 0 1 av v

(1)

n

2

Κ gn

(R

z-average,

), r a d i i of gyration. These are defined by equations

R Downloaded by UNIV OF ARIZONA on January 6, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch003

a n d

gw

4-6

= Σ C. / Σ (C./R ·) · ι . ι' g i

(*)

y

gn

Ί

R gw = Σ. C.ι R g i. / Σ. C.ι R

(5)

2

= Σ C. R . / Σ C. R ι gi ι gi

gz

±

(6)

i

Where R ^ i s the radius of gyration of species i and

i s i t s con­

centration. For each DM, R

< R < R , which i s the expected order, gn gw gz' I n t e r e s t i n g l y , f o r the radius of gyration at peak p o s i t i o n ( R ) r

gp

ό

(maximum concentration of p e c t i n ) , R

< R < R f o r Rgp < 135 A gn gP gw > 247 A. Typical chromatograms are 6 r

whereas R

< R

gw gp shown i n F i g . 3.

< R

gz

for R

Q

gp

y v

As indicated by Table I , pooling data without regard to column type, s a l t concentration i n the mobile phase, or p e c t i n form gave data with a standard deviation ranging from 3 - 15% of the mean f o r the number-, weight- or Z-average R^.

In case of the number average

radius of gyration, the standard deviation ranged from 3.5 - 10%. Since, i t was our i n t e n t i o n to compare molecular weight and size values from SEC with those from end group t i t r a t i o n s , and osmometry, (comparisons of log R

gn

reduced the heterogenity i n variance (16))

values were analyzed for variance at the (p^ 0.05) confidence l e v e l . Such analysis was used to determine i f R^ values were affected s i g n i ­ f i c a n t l y by: (1) the concentration of s a l t i n the mobile phase; (2) whether the form of the carboxylate ion was hydrogen or sodium; (3) the pore size d i s t r i b u t i o n (psd) of the HPSEC columns.

No global

trends were i d e n t i f i e d because of interactions between combinations

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

Molecular Weight and Dimensions for Citrus Pectins

Downloaded by UNIV OF ARIZONA on January 6, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch003

3. FISHMANETAL.

-1.0

-0.5 0 0.5 PARTITION COEFFICIENT

29

1.0

Figure 3. Typical chromotograms f o r pectins with 0, 37 and 70% methylation e s t e r f i c a t i o n .

ο

Λ

TABLE I RADIUS OF GYRATION (R , A) FOR PECTINS

1

\ R NUMBER-AVERAGE

WEIGHT-AVERAGE

Z-AVERAGE

PEAK P0S.

0

51.7 + 2.2

65.2 + 4.1

84.5 + 11

53.7 ± 4.2

35

99.6 + 3.8

144 + 3.0

212 + 7.8

37

70.5 + 2.2

105 + 2.9

165 + 12

81.5 ± 8.2

57

131 + 9.4

207 + 17

309 + 40

258 ± 28

58-60

124 + 4.3

194 + 4.5

288 + 11

252 ± 33

70

126 + 6.0

201 + 10

301 + 12

247 ± 35

123 + 12

206 + 11

314 + 12

292 ± 35

101 + 3.3

149 + 4.0

225 + 15

135 ± 31

72-73 73G

â

123 ± 11

1

DATA AVERAGED OVER MOBILE PHASE CONCENTRATION (0.05M & 0.1M NaCl), OVER FORM (ACID & NEUTRALIZED) AND OVER COLUMN (E-1000 AND E-LINEAR) 24 DETERMINATIONS.

2

STANDARD DEVIATION OF POOLED DATA AS IN (1).

3

DATA FOR E-1000 COLUMN ONLY (12 DETERMINATIONS).

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

2

30

CHEMISTRY AND FUNCTION OF PECTINS

of mobile phase concentration, carboxylate counter i o n and column psd.

In another approach, a t constant degree of methylation, R

g n

values were separated at the (P^ 0.05) confidence l e v e l through differences i n t h e i r logs by the Bonferroni LSD method (17). within any horizontal row i n

Thus

T a b l e I I , means not f o l l o w e d by the

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ν ο ο ο ν ο U Ό Ό «î «J • · . oo m m ΓΗ οο > ί ο ο σ ι θ Η θ Μ Η ^ C T i U O r H r H r H r H r H

u ΡΩ

vu

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PQ

^ ι η ι \o σι H H m

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