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The effects of extrinsic factors such as ionic strength, pH,
urea concentration,temperature etc. and intrinsic factor such as
molecular weight on conformation ofchitosan molecules in diluted
solution were studied. Chitin was extracted from redshrimp
(Solemocera prominenitis) process waste. Six different degree of
deacetylation(DD) chitosans were prepared by hot alkali
deacetylation from prepared chitin. Tendifferent molecular
weights with 83% DD chitosans were produced by
ultrasonificationtreatment on 83% DD chitosan acetic acid
solution for various times. The weightaverage molecular weight
(Mw) of these fifteen chitosans were measured by static
lightscattering. Those chitosans were then used as standard
molecular weight sample fordetermining weight average retention
volume (RVw) by high performance sizeexclusion chromatography
(HPSEC) or intrinsic viscosity ([h]) by capillary viscometry.
These relationships between Mw and RVw and intrinsic viscosity,
respectively weremade. Ten chitosans with the same 83% DD but
different in molecular weight (78 ℃).Intrinsic viscosity or
diffusion coefficient of these solutions were determined
bycapillary visocmetry or dynamic light scattering,
respectively. Double logarithmic plotsof intrinsic viscosities
or diffusion coefficients vs. Mws were made, and
regressionanalysis was performed to obtain Mark-Houwink
equations. The exponents a or eobtained were used as the
conformation indicators. The effects of solution conditions(
ionic strength, pH, and urea concentration), temperature, and
molecular weight onchitosan conformation were elucidated.The
results were divided into following 5 parts:1. The relationships
between weight average molecular weight and the weight
averageretention volume of high performance size exclusion
chromatography and Mark-Houwink viscometric constants for
chitosans: Relationships of RVw and Mw fordifferent Mw of 83% DD
chitosans are Log Mw = - 0.433 RVw + 11.660. However,the RVw of
other chitosans with DD other than 83% do not correlate well
with thisequation. It indicated DD of chitosan affect the
relationship of RVw and Mw ofchitosans studied. The
relationships between Mw and Mark-Houwink viscometricconstant
are that Mark-Houwink constant a decreased from 0.715 to 0.521,
as thesolution ionic strength increased from 0.01 M to 0.30 M,
while constant k increasedfrom 5.48×10-4 to 2.04×10-3 over the
same range of ionic strength solutions. Theequation established
and the constants calculated indicate that DD of
chitosaninfluences the relationships of Log Mw vs. RVw, and the
ionic strength of the solutionaffects the Mark-Houwink
constants. The established RVw and Mw equation and [h]and Mw
equation (Mark-Houwink equation) can be routinely used to
determine themolecular weight from RVw or from [h] of chitosan
by HPSEC or by capillaryviscometer respectively without the need
of an expensive instrumentation.2. Effect of molecular weight
and urea on the conformation of chitosan molecules indilute
solutions: The solutions with ionic strengths between 0.01 M and
0.30 M, therelative chain stiffness parameter B and the Mark-
Houwink exponent a of chitosanswhose molecular weights were
between 223 and 914 KDa fell between 0.143 and0.152 and from
0.404 to 0.497, respectively; whereas for chitosans whose
molecularweights were between 78 and 148 KDa these values fell
between 0.110 and 0.138 andfrom 0.653 to 1.009, respectively.
Both results indicate that the stiffness andconformations of
small molecular weight chitosans were more stiff and extended,
respectively, than higher molecular weight ones, and that
molecular weight-inducedconformational transition occurred.
Chitosans in solutions containing 4 M ureapossessed a rod-shaped
conformation in both molecular weight domains, and nomolecular
weight-induced conformational transition occurred.3. Effects of
ionic strength, pH on the diffusion coefficients and
conformation ofchitosan molecule in solution: The diffusion
coefficients increased with increasing ionicstrength or with
increasing pH or with decreasing Mw. Values of e and a
werebetween 0.503 to 0.571 and ranged from 0.543 to 0.632,
respectively. The resultsindicate chitosans conformation were in
random coil in solutions in the ranges of ionicstrength and pH
studied. The values of a*, e* and a**, e**, Mark-Houwink
exponentsof smaller and higher than 223 KDa chitosans,
respectively, were between 0.752 to0.988, 0.585 to 0.777 for
smaller Mw chitosan and between 0.406 to 0.428, 0.430 to0.518
for larger Mw chitosan, respectively. Molecular weight induced
conformationaltransition were occurred because smaller Mw
chitosans were more extended thanhigher Mw chitosans.4. Effect
of urea concentration on the conformation of chitosan and on the
shift ofbreak point of Mark-Houwink equation: The intrinsic
viscosities of chitosans increasedwith the increasing of urea
concentration. The [h] increase of higher Mw chitosanswere more
manifest than that of lower Mw ones and the break phenomena
occurred.The Mark-Houwink exponents (a) increased with
increasing concentration of urea.When solution contained 0, 2,
3, 4, and 6 M urea, the value of a increased from 0.715to 0.839,
0.894, 1.000, and 1.060, respectively. This indicated
conformationaltransition of chitosans happened. The changes of
conformation of higher Mw chitosanswere more pronounced than
smaller Mw ones. The conformation of smaller Mwchitosans were
all in rod shape in solutions contained with or without urea.
Whereasthe conformation of higher Mw chitosans changed to rod
shape in solutions contained4 or 6 M urea from a random coil in
solutions contained no urea. The break pointshifted from 223 KDa
in solution contained no urea to 280 KDa in 2 or 3 M
ureasolutions and to 362 KDa in 4 M urea solution and further to
481 KDa in 6 M ureasolution.5. Effect of temperature on the
intrinsic viscosity and the conformation of chitosans indilute
HCl solution: In 10 ℃, intrinsic viscosities of all ten
different molecular weightchitosans decreased linear with
measuring temperature. It indicated no temperatureinduced
conformational transition occurred. The d Ln [h]/d 1/T was
between 633 to1334 and increased with decreasing molecular
weight. This indicated that the highermolecular weight, the more
flexible the chitosan were. Between temperature of 10 ℃,Mark-
Houwink exponents a were between 0.635 ℃. Regression for larger
than andlower than 223 KDa chitosans were made to obetain the
respective Mark-Houwinkexponents. The values for exponent a**
were between 0.408 to 0.538 and for a* were0.958 to 1.067. The
Mark-Houwink exponents calculated indicated larger and
lowermolecular weight chitosans were in random coil and rod
shape, respectively.
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