Cholesteric liquid crystalline phases given by three helical biological polymers : DNA, PBLG and xanthan. A comparative analysis of their textures

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Cholesteric liquid crystalline phases given by three helical biological polymers : DNA, PBLG and xanthan.

A comparative analysis of their textures

F. Livolant

To cite this version:

F. Livolant. Cholesteric liquid crystalline phases given by three helical biological polymers : DNA, PBLG and xanthan. A comparative analysis of their textures. Journal de Physique, 1986, 47 (9), pp.1605-1616. �10.1051/jphys:019860047090160500�. �jpa-00210359�

Cholesteric liquid crystalline phases given by ^{three helical} biological polymers : DNA, ^{PBLG and xanthan.}

A comparative analysis ^of their

textures

F. Livolant

Centre de Biologie Cellulaire (CNRS ^et EPHE),

67, ^{rue Maurice} Günsbourg, ⁹⁴²⁰⁰ Ivry-sur-Seine, ^France

(Reçu le 26 novembre 1985, accepté ^sous forme définitive ^{le 13 mai} 1986)

Résumé. ²⁰¹⁴ Des phases cristallines liquides cholestériques ^{ont été obtenues avec trois} polymères ^{d’intérêt biolo-} gique : ^un polypeptide (le PBLG), ^un polynucléotide (l’ADN) ^{et un} polysaccharide (le xanthane). Les défauts et textures des phases ^{obtenues avec ces trois molécules sont} analysés ^et comparés. ^La différence essentielle concerne

les coniques ^focales qui ^{sont absentes dans le} PBLG, ^rares dans l’ADN et très fréquentes dans le xanthane. Un dénombrement des dislocations de rotation et de translation, ^tenant compte ^des orientations moléculaires dans le

coeur des défauts, ^montre que les disinclinaisons - 03C0 sont exclusivement de type 03BB-, ^les disinclinaisons + 03C0 le

plus ^{souvent de} type 03BB+. ^Des différences apparaissent ^entre polymères ^{dans la} répartition ^des dislocations de translation mais dans les trois cas les défauts correspondant à l’addition d’une seule couche peuvent ^{être des deux} types possibles (03C4- ^{03BB+ ou 03BB-} 03C4+), ^ce qui différencie ces polymères ^des petites molécules étudiées jusqu’a présent.

Les principales ^{textures décrites} dans les cristaux liquides classiques ^{sont obtenues} également ^{avec ces} polymères (textures planaires, ^{textures riches en} disinclinaisons, ^textures polygonales). ^{Elles sont} cependant plus difficiles à

analyser, ^au moins dans l’ADN et le xanthane car le pas cholestérique peut être très faible (inférieur ^au pouvoir

de résolution du microscope).

Abstract ²⁰¹⁴ The cholesteric liquid crystalline phases ^{of three} polymers ^of biological interest have been investigated:

PBLG (a polypeptide), ^DNA (a polynucleotide) ^{and xanthan} (a polysaccharide). ^{The textures} (and the defects which they contain) of these three mesophases ^are analysed ^and compared ^The main difference concerns focal lines which apparently ^{do not occur in} PBLG, ^{are rare in DNA but occur} frequently in xanthan. The frequency ^of

occurrence of the different types of rotation and translation dislocations were measured : - 03C0 disclinations are

always ^{of the 03BB-} type ^{and + 03C0} disclinations are mainly ^03BB+. Among polymers, differences exist in the distribution of translation dislocations but in these three _cases, when only ^one layer ^is added, ^{the defect} may be either 03C4- 03BB+

or 03BB- 03C4+ which distinguishes polymer mesophases ^from those of small molecules previously studied The main textures found in classical (small molecule) cholesteric phases ^{were also obtained} (i.e. planar ^{textures, textures show-} ing ^numerous disclinations and polygonal textures). However, they ^{are more difficult to} analyse especially ^{for DNA}

and xanthan mesophases ^{where the} helicoidal pitches ^are small and may ^not be resolvable in an optical microscope.

Classification

Physics ^Abstracts

61.30 ^- 61.70

1. Introduction

There are many

examples

^of

liquid crystalline poly-

mers. Most are of

biological

interest and

belong

^to

different biochemical groups such ^as

polypeptides, polysaccharides

^and

polynucleotides (nucleic acids).

Among

these,

poly-y-benzyl-L-glutamate (PBLG)

has been the most

extensively

studied This is a

synthe-

tic

polypeptide

^which

gives

cholesteric

phases

ⁱⁿ

concentrated solution in various

organic

^solvents

(such

^as dioxane,

methylene

chloride,

chloroform) [1-3].

^The

liquid crystalline properties

of nucleic acids

were

reported by

^Robinson

[2]

and Lerman

[4]

^for

DNA

(desoxyribonucleic acid)

^and

by Spencer

^{et al.}

[5]

for r-RNA

(ribonucleic

acid). ^{In recent} years, ^nume-

rous

polysaccharides

have been found to

give

^choles-

teric

liquid crystalline phases

^{in aqueous solution :}

hydroxypropyl

^cellulose

[6-10], schizophyllan [11-13], scleroglucan [14]

^{and xanthan}

[1 S-17].

Although

^the

physicochemical properties

^{of these}

polymer

^{solutions have been}

extensively

studied, ^the

optical

^{textures of these}

mesophases

^{have never been}

thoroughly investigated

We decided therefore to carry ^out this

study

^{and to make a double} compara-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047090160500

tive

analysis, firstly

between the different

polymer

mesophases ^and

secondly

^between

polymer

^meso-

phases

^and small-molecule cholesteric

phases

^whose

textures were the only

examples previously

^consi-

dered

We have chosen three

polymers belonging

^to

different biochemical _{groups :} a

polypeptide (PBLG),

a

polysaccharide (xanthan)

^{and a} nucleic acid

(DNA)

and we have

improved

^the

experimental techniques

for

obtaining liquid crystalline phases

of each of these

polymers

^{in a}

reproducible

way. In ^a

previous

work, the structure of the cholesteric

droplets appearing

ⁱⁿ

the

isotropic phase

^was

analyzed

^and

compared

^with

the structure of

spherulites given by

^classical

liquid crystals

^{such as} twisted MBBA ^or PAA

[18].

^{In the}

present

study

^we consider the textures of

large-

molecule cholesteric

phases.

Differences revealed

by

this

comparative

analysis ^{raise new} fundamental _pro- blems in the

understanding

^of

liquid

crystalline geometry.

Moreover,

precise

^texture

analyses

of this kind appear ^to be _necessary if we want to understand the

morphologies

^of

living

systems made of such

poly-

mers.

2. Materials and methods.

2.1 MATERIALS. - Calf

thymus

^DNA

(Merck)

^was

solubilized in 15 mM Tris-Cl- buffer, pH 8,

by magnetically stirring

for 24 h. To obtain short DNA

fragments

the solution was sonicated with a Branson sonifier B 15 P

(20

kHz, ⁵⁰

W),

for times

ranging

from 5 to 35 _s,

by

successive

pulses

^{of 5 s}

interrupted by resting periods

of 15 s. The solution was

always kept

^at 0 OC. Each DNA fraction was

precipitated

ⁱⁿ ethanol, air dried and diluted in 10 mM Tris-Cl- buffer (pH 8) ^{with 1 mM} EDTA. These stock solutions

were stored at 4 °C.

In each fraction, ^the

length

of the DNA molecules

was estimated

by electrophoresis

^on agarose

gels by comparison

with DNA of

clearly

^defined

length (Lambda

^{DNA cut}

by

^Hind

III). They

^{are as follow :}

Three methods were

developed

^to obtain cholesteric

liquid crystalline phases

with sonicated DNA from fractions 4 to 7.

(i) According

^{to the first method,}

previously

repor- ted

by

^Lerman

[4], ýJ

^DNA

(Polymer

^{and Salt}

Induced

DNA) ^{was obtained}

by mixing

^DNA

with

a neutral

polymer

^{in the} presence of monovalent salts. A

phase

segragation occurred and DNA concentrated in one

phase

^and

precipitated

Good results were obtained with a wide _range of concentrations. However,

usually,

^a solution of 400

mg/ml

^of

polyethylene glycol

(PEG; ^{MM 8,000,}

Sigma)

and 2M KCl was

added drop

by

drop ^{to an} equal volume of the DNA solution (1

mg/ml

in 10 mM Tris-Cl- buffer, pH 8).

This

mixing

took about 30 min under continuous

magnetic stirring.

^DNA

precipitates

^were

deposited

between slide and

coverslip

^{and a} cholesteric _orga- nization

appeared

^{in a} few minutes at the

periphery.

(ii)

^In the second method, ^a

drop

^{of 10}

mg/ml

DNA was

slowly

mixed with an

equal

^{volume of a}

0.4 M KCI solution and

placed

between slide and

coverslip. Preparations

^{were stored at} 4 °C. As the solution became more concentrated the

sample

^divi-

ded into small domains surrounded

by

air. A choles- teric

organization appeared

^{in these}

regions

^after

several

days

and remained for a few weeks. This

procedure

is derived from that of Robinson

(1)

^{but we}

used KCI instead of NaCI ^as the monovalent salt

(iii)

^A

drop

^{of a}

highly

concentrated DNA solution

(50

mg/ml

^{in 10 mM Tris-Cl-} buffer,

pH

8) ^was

deposited

between slide and

coverslip.

^The

preparation

was

kept

^{in a} humid chamber either at 4 °C or 20 OC.

The cholesteric

organization appeared

^{one or a few}

days

^later.

’These methods may appear very different. However

only

^two conditions are

absolutely

necessary for the

production

of these cholesteric

phases : relatively

short chains of DNA (less ^{than 3} pm) ^{and a}

high

^DNA

concentration.

- Two kinds of

poly-y-benzyl-L-glutamate

^were

used : PBLG (MM 28,000,

Sigma)

^{and PBLG} (MM 20,000)

synthesized by

the Centre de

Biophy- sique

Mol6culaire

(Orl6ans, France).

^{A small amount}

of

powder

^was

placed

^{on a slide, a}

drop

^{of dioxan}

added and

immediately

covered with a

coverslip.

- Xanthan is a

polysaccharide

^secreted

by

^the

bacteria Xanthomonas campestris. ^{It was}

purified

^and

kindly provided by

^Drs. Rinaudo and Milas

(CERMAV,

CNRS, Grenoble, France). ^This

polymer (MM ,200,000) gives

cholesteric

phases

^{in aqueous}

solutions

(distilled

^{water, 60}

%

^{NaCI or KCI}

solutions).

Preparations

^{were made as}

previously

described with PBLG.

These three

polymers

^{are all} helicoidal and their chemical formulae ^are

given

ⁱⁿ

figure

^{1 :}

The

hydrated

form of DNA

(B

^form

DNA)

^{is a}

right-handed

^double helicoidal molecule whose dia- meter

equals

^{2 nm} and helical

pitch

^{3.4 nm. The two}

strands are

antiparallel

and coiled around each other.

They

^{are linked}

together by hydrogen

bonds which form between the bases of each

pair [10].

PBLG exists as an a helix in the cholesteric

phase.

The

cylindrical

molecule is about 1.8 nm in diameter.

Two successive residues which are

projected

^onto

the helical axis repeat ^at intervals of

approximately

0.15 nm [20].

Fig. 1. ^- Chemical formulae of the three biological polymers

used in this study.

The structure of xanthan has been the

subject

^of

considerable debate. It would be a fivefold

right-

handed helix whose

pitch equals

^{4.7 nm with side-}

chains

packed along

^the

polymer

^backbone

[21,22].

2.2 MICROSCOPICAL TECHNIQUES. ^- All prepara- tions were studied with a Leitz

(Orthoplan Pol)

^{or a}

Nikon

(Optiphot

^X

Pol) polarizing microscope,

^either

between crossed

polars

^or between circular

polarizers

of

opposite

^{sense. To} determine the molecular orien-

tations, ^a quartz first order retardation

plate

^was

inserted at 450 between

polarizer

^and

analyser.

^The

use of

circularly polarized light

^{is very convenient}

since under these conditions it is not necessary ^to orientate the

layers

^at 450 for the

banding

^{to be seen}

and the whole of a

fingerprint

pattern is therefore apparent ^{at the same time.}

In a cholesteric structure, the molecular orientation

rotates

continuously,

^each orientation

recoming

^after

a 1800 rotation. This

organization

^is

represented

ⁱⁿ

figure

2 with the helicoidal axis

parallel

^{to the}

plane

of the

drawing.

^{In this} situation, when the cholesteric axis is oriented at 450 between crossed

polars,

^the

intensity

of transmitted

light depends

^on the molecular orientation and follows a sine curve. The

intensity

^is

maximum when molecules lie in the

plane

^{of the}

preparation

and minimum when

they

^are

parallel

to the

microscope

^axis

(Fig

2b,

c).

^The cholesteric

organization

is revealed

by

^an alternation of

bright

and dark bands and the

period corresponds

^{to the}

half-helicoidal

pitch.

^This

pitch

^can be measured

directly

ⁱⁿ

regions

where the helicoidal axis is

exactly

horizontal;

larger

values would be obtained in

regions

where the cholesteric axis is

slightly oblique.

2.3 DRAWING CONVENTIONS. -

According

^{to the}

nail convention, ^{molecules are}

represented by

lines,

nails and dots when

they

^are

respectively parallel,

Fig. ^{2. -} Optical properties ^of the cholesteric organization.

According ^to the nail convention molecules are represented by lines, ^{nails and} points ^when they ^are respectively parallel, oblique ^{or normal to the} observation plane. ^When the helical axis of a cholesteric structure is in the plane of the prepara-

tion, ^{molecules are} alternatively parallel, oblique ^{and nor-}

mal to the observation plane (a). ^{When this structure is}

oriented at 450 between crossed polars (P, A), ^the intensity

of transmitted light depends ^on the molecular orientation and follows a sinusoidal curve. The intensity ^{is maximum}

when molecules are in the plane ^{of the} preparation ^{and mini-}

mum when they ^are parallel ^{to the} microscope ^axis (b). ^The

cholesteric organization ^is visualized by ^an alternation of

bright and dark bands (c), the distance separating ^{two bands} being ^the half-helical pitch pl2.

oblique

and normal to the

plane

^{of the}

drawing.

^The

head of the nail indicates the

extremity

of the molecule which is

pointed

towards the observer. In several _cases, nails have been omitted to

clarify

^the

drawing.

3. Results.

The

general

appearance of the

liquid crystalline phases

differ

significantly

^{from one}

polymer

^{to the other.}

In

particular

^the half-helicoidal

pitch (p/2)

^{can be as}

large

^as 1 J.1m in DNA, ²⁰ lim in xanthan and 100 _g in PBLG. The smallest values

ofp/2

^cannot be resolved

by

^the

light microscope. They

^{can reach 0.1 to 0.2} gm in DNA and xanthan and these two

polymers

^therefore

reflect

circularly polarized light

^{in the visible} range.

The other differences are described below in the

analysis

of defect lines and textures.

3.1 DEFECT LINES. ^- Defect lines are of three types : disclinations, dislocations and focal lines. It often

happens

that certain lines are absent : for

example

focal lines which were never observed in PBLG, ^are

rare in DNA but are

frequent

in xanthan. The different types of defects encountered with each

polymer

^are

shown in table I and their relative

frequencies

ⁱⁿ

table II and III.

Defect lines

(disclinations

^and

edge-dislocations)

may be studied more readily ^{in two main} situations,

which

correspond

^{to the two} main orientations of the cholesteric axis, ^either

parallel

^{or normal to the pre-} paration

plane.

^The

path

of the defect line can be

easily

followed in the second case and this kind of

analysis

has been used for

liquid crystals by

^Friedel

[23],

Cano [24] ^and

Bouligand [25].

Such lines were also followed in the different

polymers. They

^are

analysed

and

compared

^to those found in small-molecule

liquid crystals

in another _paper

(Livolant,

ⁱⁿ

preparation).

However, ⁱⁿ

polymers,

^this orientation does not allow

a

precise analysis

of molecular orientations in the

core of the defects. Therefore, ^{in this}

study

^{we have}

investigated

the alternative orientation, ^{where the}

defect line is normal to the

preparation plane.

Disclinations can be

regarded

^{as either}

triple points corresponding

^{to the}

junction

^{of three}

layers

(-

n)

or as

layers

^which

abruptly disappear

( + a). ^{In each}

case, the core of the defect _may be continuous ^or discontinuous

depending

^on whether the molecules lie

parallel (A)

^{or normal}

(r)

^to the defect line. These situations and their occurrence are summarized in table II.

In the three materials, - n disclinations

always

appear ^{as a} black core between crossed polars

(linear

or

circular)

^{and are therefore} of the A- type. ^{+ n} disclinations occur in all three

polymers

^{but the £+}

Table I. ^- Different types of defects encountered in

DNA, PBLG and xanthan.

Table II. ^- Relative

.frequencies

of disclinations observed in DNA, PBLG and xanthan.

type ^{is much more}

frequent

^thant the T + type

(which

^is

only rarely

encountered in DNA and

xanthan).

T + disclinations were never observed in PBLG.

In the two commonest situations, A- and

A+,

molecules lie

parallel

^to the defect line

along

^{its core}

and there is no

discontinuity

in the molecular orien- tations. K16man and Friedel

[19]

noted that this situation was the most

likely.

Table III. - Counting

of all

dislocations observed in well-resolved cholesteric regions of DNA, PBLG and xanthan.

For each value of ^the

Burgers’

^vector

(from p/2

^to

4 p)

^{the two}

possible

situations in the core of ^the defect ^were differentiated

Two disclinations

(one positive

^{and one}

negative)

may be associated ^to form either

elementary pinches

^or

edge-dislocations.

^{We shall consider}

only

^the

edge-

dislocations in this

study.

Translation dislocations are numerous in these

phases

^{but it is} often difficult to ascertain whether

they

are pure

edge-dislocations,

pure screw-dislocations or a

combination of both. Each dislocation

corresponds

to the addition ^or removal of one or more

layers

^{in the}

system. ^A

histogram

^{of the}

length

of the observed

Burgers’

^{vector is}

given

ⁱⁿ

figure

3, ^{for each}

polymer.

These counts were made in selected areas where the cholesteric stratification could be

easily

^followed

In DNA and xanthan, dislocations

corresponding

^to

the addition of one

layers (pl2)

^are

prevalent,

repre-

senting respectively

^{57 and 85}

%

^{of the} total, ^{the other} types ^of dislocation

becoming increasingly

^{rare as the}

length

^{of the} Burgers’ ^vector increases. On the other

hand, ⁱⁿ PBLG, ^the

length

^of Burgers’ ^vectors

is p

ⁱⁿ

general (or

^a

multiple of p)

^and

only

¹¹

%

^{of dislo-}

cations have

half-integral

^values.

We can also

distinguish

^{the two}

possible

^situations

in the core of the defect for each value of the

Burgers’

vector. These data are collected in table III and are

represented by

^different conventional

signs

^{in the}

histogram

^of

figure

^3.

When one

layer only

^{is .added}

(corresponding

^{to a}

Burgers’

^{vector of}

length p/2),

^{the two}

possible

^situa-

tions T - Å. + and A - T ’ both ^occur in each of the three

polymers.

However, the first

(T - A’)

is favoured

(62

%

ⁱⁿ DNA, ⁵⁵

%

in PBLG and 77

%

ⁱⁿ

xanthan).

It has been shown

previously

^{that in MBBA}

doped

with a chiral

twisting

agent, this situation was syste-

maticaly adopted

[25, 26].

When two

layers

^{are added}

(b

⁼

p),

the situation Å. - Å. + is the

only

^one observed in PBLG and xanthan.

It is

predominant

^{in DNA and the} alternative situation

(T - T ’)

represents

only 5 %.

For the

highest

^{values of} b, except ^{in DNA where} three

layers

^are

occasionally

^added

(b

^{= 3}

p/2

^{and the}

defect is of the type T-

Å. +),

^we

always

observe the addition of an even number of

layers (b

^{= 2} p, 3 _{p, 4 p} and so on) and the A- Å. + situation is the

only

^one

observed.

Fig. ^{3. -} Histogram ^{of the} length ^of the observed Burgers’

vectors associated to the edge-dislocations ^{which are}

encountered in the three polymers.

3.2 TExTum. - Three types ^of cholesteric textures are defined in terms of the type of defects which

they

contain :

-

planar

^textures with dislocations

only;

-

polygonal

^{textures with} dislocations and focal

conics ;

- fan textures with dislocations, disclinations and focal conics.

3.2.1 Planar textures. - The conditions

required

to obtain

planar

cholesteric textures vary from

polymer

^to

polymer, presumably

^{due to the}

differing anchoring

conditions of molecules to the

glass

^sur-

faces. Planar textures are

quite easily

^{obtained with}

DNA

in 0

conditions with _very thin

preparations

and with xanthan when the

polymer

^is diluted in distilled water instead of a saline solution.

Planar cholesteric structures have a strong rotatory

power, and reflect

circularly polarized light

^With

DNA and xanthan, ^we obtained this reflection in the visible spectrum. ^This

optical

^{effect is} very strong ⁱⁿ xanthan where a spectrum ^{can be seen as a}

large

^band

parallel

^{to the}

edge

^{of the}

coverslip.

^Localized

regions

with either smaller and

larger pitches

^{lie on either}

sides of the band. Such series of

parallel stripes,

^with

rainbow colours, ^were sometimes obtained in DNA cholesteric

preparations

^{when the solution} penetrates into a

clevage

^{in a KCI}

crystal.

These colours do not

depend

^{on the} orientation of the

polars

^{and are not}

therefore

birefringence

^{colours. We} verified that

they

are due to a reflection of

circularly polarized light

since

they

^{can be}

extinguished by

^a quater-wave

plate

À/4 ^followed

by

^a polar in conditions which are

detailed below.

The

wavelength (£o)

of reflected and transmitted

circularly polarized lights

^is

proportional

^{to the}

helicoidal

pitch

^{of the} structure. For small

pitches,

^a

Bragg

reflection occurs and the relation

Ao

= pn

applies

when the incident

light

^{is normal to} the strati- fication

(p being

the helicoidal

pitch

^{and n the} average refraction index of the structure)

[2].

The reflected colours reveal that in DNA and xanthan the helicoidal

pitch

may reach values

ranging

^{from 0.2 to} 0.4 J.1m if n is taken as about 1.5. Such colours were never

obtained with PBLG.

The handedness of the cholesteric structure may be determined

by

^the

analysis

^{of this}

circularly polarized light

Indeed, it is well known that for small helical

pitches,

^the

light illuminating

^a cholesteric

structure is

split

^{into two} components, ^one transmitted and the other reflected, ^both

being circularly polarized,

but in

opposite

^{senses. For a} left-handed cholesteric structure, the reflected

light

^{is left}

circularly polarized

and the transmitted

light

^is

right circularly polarized

The situation is reversed when the cholesteric

phase

is

right-handed

^{The nature of} transmitted and reflected

light

may be

easily

determined

by

^{the use of a}

À./4 plate

^followed

by

^a

polar.

The slow axis

(ns)

^{of the}

À./4 plate

^is

always

^{oriented at} 450 relative to the

analyser

direction but two situations are possible

(Fig.

4a, b). ^The first orientation of

À/4

(a)

extinguishes right-handed

circularly polarized

light

whereas the second one (b)

extinguishes

left-handed

circularly polarized light.

^{In the}

microscope,

the coloured spectrum of xanthan

disappears

in situation b which indicates that the transmitted

light

^is

circularly

pola-

rized and left-handed.

Conversely,

with reflected

light,

^{colours are}

extinguished

in situation a. It can

then be deduced that xanthan has a

right-handed

cholesteric

organization.

3.2.2 Textures

showing

^numerous disclinations. - There are several

possible

^textures

according

^{to the}

relative

disposition

of defects. Patterns found in cholesteric

phases

of small molecules also ^occur for the three

polymers

studied i.e. the

quadrilateral

patterns and the

zig-zag

lines first described in MBBA

doped

with cholesterol benzoate

[27].

The quadrilateral patterns (Pi. ^{II c,} e)

correspond

to the association of four disclinations

(two - n

^and

two + n).

The zig-zag ^lines (Pl. ^II b, d) ^{are due to a}

regular

alternation of + 1t and - n disclinations. The black

zig-zag

lines indicate the

places

where molecules are

parallel

^{to one of the two}

polars.

^{These lines} separate

two domains with two different orientations of the cholesteric axis. In the present ^case

they

^{were obtained}

in a

glass capillary

filled with the

polymer

^solution.

Fig. ^{4. -} Two orientations of a quater-wave plate (AI4)

oriented at 45° with respect ^to the polar A. The arrowed direction corresponds ^to the slow axis of the plate. Right-

handed circularly polarized light ^is extinguished ^{in situa-}

tion (a) while left-handed polarized light ^is extinguished ⁱⁿ

situation (b). (From Robinson, 1966.)

The

zig-zag

^{line runs}

parallel

^{to the}

edge

^{of the}

capillary

and separates ^the

peripheral

part ^{of the}

preparation

in which

layers

^are

aligned parallel

^{to the}

glass

^surface

and the inner part ^{in which}

layers

^{are rotated}

by

^900.

Regions showing ^{numerous double} spirals (Pl. ^II

g

may be of different kinds :

(i)

^{It can be a}

phase

^formed

by

fusion of cholesteric

spherulites

since cholesteric

globules frequently

^show

a double

spiral

pattern [28, 29].

Plate I. ^- Table summerizing disclinations and dislocations presented by ^DNA, PBLG and xanthan. The different defects

are schematically ^{drawn and} illustrated Observations between crossed polars.

- n disclinations may be eitherr ^or A but this latter situation is the only observed with the three polymers.

Edge-dislocations ^are separated according ^to the molecular orientations in the core of the defect and to the number of added

layers. ^{The rare} situation i - T ’ ( ⁺ p) in DNA has to be considered with care but we think that it is really ^that occurring ^here.

Plate II. ^- Textures with numerous disclinations in DNA

(e, t), ^PBLG (a, b, c) ^{and xanthan} (d). Observations between crossed linear polars (a-e) ^or between crossed circular polars (0.

a : large region showing ^a population ^of odd defects ( x 360).

b : region rich in disclinations drawing ^numerous zig-zag

lines and also a curved dark line because in this region ^the

helical pitch ^varies significantly ( x 360).

c : zig-zag ^{lines and} quadrilateral patterns ^{near the interface} with the isotropic phase ( x 265).

d : zig-zag ^line running parallel ^to the side of the glass capillary filled with the polymer ^solution ( x 360).

e : quadrilateral patterns ^{in a} concentrated DNA region

limited by ^air (x 965).

f : numerous spiralized patterns ⁱⁿ DNA ( x 385).

(ii) They

may also be

produced by

distortion of a

planar

cholesteric texture with formation of anticlinal domes and

synclinal

basins. This leads to

polygonal

textures in which double

spirals

may be observed in the

plane

of focus when the half-helicoidal

pitch

^{is suffi-}

ciently large.

(iii) Finally,

the cholesteric

layers

may be

simply

coiled and the

spiral

patterns

correspond

^{therefore to}

a transverse view of defects such as the ^«

tear-drop »

whose structure was

analysed by Bouligand

^[25]

and presented

again by Bouligand

and Livolant

[29].

Regions showing

^numerous

spirals

^{have a}

regular

array of disclinations : + 1t disclinations are localized

Plate III. ^- Polygonal ^{textures in DNA} (b) and xanthan

(a, c, d, e, f, g, h, i). Observations between crossed linear

polars.

a : large region ^of regular quadratic polygonal ^{fields in}

xanthan (NaCI) (x 240).

lb : a small domain with polygons ^{in DNA} ( x 320).

c : polygonal ^fields showing elongated draught-board pat-

terns in xanthan (NaCI) (x 550).

d, e, f : polygonal fields with star patterns observed in xanthan (NaCI). ^{The three} micrographs correspond ^{to the}

same region : d is focused on the coverslip plane, ^{f on the}

slide plane ^{and e} in between ( x 325).

g, h, i : distorted polygonal ^{fields obtained} with xanthan

(KCl). ^{The same} region is focused in the coverslip plane (g),

in the slide plane (i) and in between (h) ( x 480).

at the centre of the

spirals

whereas - n disclinations

are due

mainly

^{to the}

packing

^{of three}

spiral

^domains.

These double spirals ^are hither left-handed or

right-

handed in PBLG and DNA.

3.2.3

Polygonal

^{textures. -}

Polygonal

^{textures were}

never observed with PBLG and

they

^are

extremely

rare in DNA

(Pl.

^III

b). They

^are however very

frequent

in xanthan and show a

large diversity

of pattern. ^Some

examples

^{are shown in}

plate

^III. They ^{were obtained}

in various conditions

(distilled

water, NaCI ^or KCI

solutions).

^The

polygons

^are often square

(from

^{3 to}