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
% "