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9 Springer-Verlag 1984. Conformation. On the Existence of TWO Different Secondary. Structures for the Xanthan in Aqueous Solutions. M. Milas and M. Rinaudo.
Polymer Bulletin

Polymer Bulletin 12, 507-514 (1984)

9 Springer-Verlag1984

Conformation On the Existence of TWO Different Secondary Structures for the Xanthan in Aqueous Solutions M. Milas and M. Rinaudo Centre de Recherches sur les Macromolf~cules V6g~tales, C.N.R.S.*, B.P. 68, 38402 Saint Martin d'H~res cedex, France Summary

This

paper

conformations

gives

in

characteristics

experimental

solution

for the

evidence xanthan

are quite different

for

two

molecules.

different Their

ordered

hydrodynamic

and their existence is controlled by

experimental conditions.

Introduction

Different

recent

investigations

on xanthan

the existence of a single helical conformation

gum

in solution

(i, 2). Nevertheless

authors find results consistent with a double stranded helix

On the other hand, Sandford et al

support some

(3-7).

(8) show that there is an increase

of viscosity when the native polymer is heated, depending on the external salt concentration and the pyruvic acid content. To explain these results it is proposed by Seeger

(9) that there exist single-helical molecules in

addition to double helical threads, which are composed of several xanthan molecules. G. Southwick et al. suggest that aggregation of rod-like chains occurs

for xanthan

aggregation

in salt solution

is perhaps

(I0). These authors think that this

the explanation of the different results

found in

the literature.

In this paper indicate

that

two

a series of experiments different

conformations

is discussed really

which

exist

in

depending on the experimental conditions. * Laboratoire Propre du C.N.R.S. associ~ a I'Universit~ $cientifique et ME=dicale de Grenoble, France

seems

to

solution

508

Experimental

part

Polymer

samples

A native pyruvate (Ii))

xanthan

and

are

acetate

0.4

substituants through

broth

and

per

side

chain.

hours)

to eliminate

the

Diaflo

ultrafiltration to

avoid

was

cell

then

mixtures

ultrafiltered

solution

concentration

around

temperature,

These the

two

precipitation counterions radius from

diluted

treatments

form). were

of

differential

The viscosities

000

NaCI

g,

0.i

5.10 -3

N

(over

as The

(i00 g/l)

;

with

cases,

to

the

on

N

precipitate some

4

by ethanol

of NaCl

In

give

a

the NaCI

conformational

refractive

low shear v i s c o m e t e r

before

the

[~] was obtained

enabled

in the

5 000, at

increment at

low

the

angle

with

dn/dc

~ = 546

The

at A = 300 nm in a Spectropol

of

the

and

the

angles (6 ~ ) in

a Chromatix

is

equal

with to

limit by using higher

at the required

viscometer.

and

measured

nm

low shear rate

were heated

Mw,

scattering

LS 30 ; for temperatures

into

control

weights,

measured

index

in one of

ultrafiltration

chromatography

refractometer

were measured

solution

and

a FICA

also

of x a n t h a n

The

molecular

using

Mw were

and the viscometer

rotation

NaCI

proteins

average

C the solutions pouring

v/v).

of

filtration

(20

conformation.

the

at 80 ~ C

and by gel p e r m e a t i o n

a

a Contraves

9

suggested.

determined

(12). The

BRICE-PHOENIX

95

by

of the xanthan

washing

with

eliminated

150 ~ . Values

measurement

ordered

number

ultrafiltrated

with

lead to p r e c i p i t a t e s

Weight

KMX 6 p h o t o m e t e r

0.155.

to

spectroscopy

first

it was

of

TM, at 55 ~ C) before precipitation.

treatments (Na

50

degrees

average

in the presence by

conformations

of gyration

30 ~ to

static

(from

n.m.r,

as

XM i00)

the

10 -2 N and h e a t e d

different ordered

of

The

centrifugation

by p r e c i p i t a t i o n

removed

IH

purified

by

(Amicon

= 50 ~ v/v)

was

was

or

; secondly

collapse

entirely

by

expressed

broth ~m)

membranes

the

preheating.

(measured

debris

can be replaced

ethanol-water

change

The

(0.45

(final ratio e t h a n o l / w a t e r NaCI

without

respectively,

filters

solvent

used

substitutions 0.75

Millipore

ultrafiltration

was

than

35 ~

temperature

specific

optical

Ib from FICA.

509

Results

and d i s c u s s i o n

Different preparation

solutions

were

prepared

as

given

in

Table

all the solutions were at a c o n s t a n t p o l y m e r

TABLE I : P r e p a r a t i o n

Preparation

I.

After

concentration

of the solutions*

Conformational

Remarks

state

I : Native b r o t h filtered

filtered and ultra-

Ordered

in NaCI 0.I N ; then dilute

at 0 ~ C to NaCI

10 -2 N

II

after p r e c i p i t a t i o n

: Na-xanthan

is d i s s o l v e d

in w a t e r

conformation preserved

Conformation disordered

; then NaCl

is added to i0-2 N

before

in w a t e r

the addition of salt

III:

Na-xanthan

is d i s s o l v e d

after p r e c i p i t a t i o n

directly

in NaCI

Ordered conformation

10 -2 N

preserved

IV : N a - x a n t h a n p r e c i p i t a t e d 10 -2 N after p r e h e a t i n g dissolved

in w a t e r -2 added to IO N.

in NaCl

Disordered

80 ~ C is

conformation

before

precipitation

; then NaCI is

and

during dissolution in w a t e r

V : Na-xanthan precipitated 10 -2 N after p r e h e a t i n g dissolved directly

* The xanthan

final

80 ~ C is

in NaCl

concentrations

0.66 g/l.

Disordered

in NaCI

conformation

10 -2 N

of

the

before precipitation

solutions

are

all

: NaCI

10 -2

N

and

510

(0.66

g/l)

and

conditions, i).

The

change

at

the

at ambient

xanthan

in

NaCI

solutions

from the native

The relative

same

concentration,

temperature

an o r d e r e d

I and

III

weight

viscosity

0.5 x 106 . The

radius • 300

is

3 800

obtained

by using

the C h r o m a t i x

The

All structures

the

did

not

(scattering

viscosity

results

(A and B) (called

seem

strength

the m o l e c u l a r

to

the

variation

(figure

i). One starts

either with

II, IV, V)

; the results

which

conformational

of

from

gyration

30 ~ to

are

150 ~ ) and

to zero angle.

6

i00 ml/g

for

two

well

form,

the

be

after

carried

(9). But,

to be

secondary

conformation

than

can

defined

out

unlike

constant

of the

the

first

either the

; this

with

result

of

excludes

the

for

the

viscosity

during

the

two

conformations

monitored

the xanthan are compared transition

A

by

and may

optical

(samples

I and

be

rotation III)

or B

with m e a s u r e m e n t s

in i0 -I N

temperature

is

never

greatly

with

(TM)

in our experiments.

The

relative

temperature d).

The

relative (curve

that

conformation

(samples

and

is 2 940 + 150 A

~ + 0 is

relative

is given of

The

to 6.1 •

for the other solutions.

Seeger

the

NaCl

the

shear

I

threads.

2,

cycle

related

reached

of

cycle

like

radii

compact

is found

helical

figure

order-disorder-order

This

III

The

to extrapolate

indicate

more

weight

of double

the

in

6~

(II, IV, V). and equal

I and

angles

In the native

temperature

In

angle

seems cycle.

dissociation

(scattering

to

exists.

A)

or ionic

these (figure

a conformational

solutions

samples.

at a rate

order-disorder-order

Seeger,

undergo

to be constant

for samples

other

I and III and i0 500 ml/g

these

xanthan

for

the F I C A data

intrinsic

the solutions

is found

of g y r a t i o n A

point

; under exists

in the low shear rate limit of the solutions

of the xanthan

and

N

state before measurement.

and III is 35 • 2 b u t is 146 • 6 for the other molecular

10 -2

conformation

compared

All

of

to the A form

conformational

viscosity

c).

viscosity

for

these

the

the

B

form are

form

(compare

transition

variations

B

decrease

curves

at

TM

leads

(curve

b)

but

reversible

a and b with to a

small

except

curves

a decrease

when

of

increase we

start

c

the

for

A

from

511 I~1300nm Figure

I

:

Specific

optical

I"

/x

rotation versus

/b,c

-30

of

xanthan

temperature

solutions (o

C).

a

:

form B in NaCI i0 -I N ; b : form ~/

/~

B in NaCI 10 -2 N ; c : form A in -2 NaCI i0 N ; d : form A in NaCI -i I0 N (Polymer concentration

. ~ - ~ ' ~

/

a,d

-6O !

;~

I

I

~o

40

80

T9

0.66 g/l ; wavelength

300 nm).

nrel

I~X a,b

\

\

100

\

Figure

d I

O qrel

._ 9 .

9

.e

a,b

I

60 . ~ 4

.

.

80 .

2 ~ ..... K

/.

/

i00

N

/

50

_._._._.1"'.

(o C).

T

I

40 .

Figure

III

: Relative

20

! 40

viscosity

in the lower Newtonian region for

/

the different xanthan solutions (see Fig I) at T = 25 ~ C after

.

.

.

.

.

preheating

at

temperatures T . c 0

viscosity

(see Fig I ) as a fonction of temperature

0

: Relative

the different xanthan solutions

% "