Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 190

where χ at any time, concentration or temperature is given by

number of residues in helical conformation
total number of residues

χ ¼ ; ð 7 : 2 Þ

is the rotation angle, c is the solution concentration (g cm − 3 ) and l is the optical path.
The intrinsic optical rotation angle at a given wavelength

α

λ is

cl :

½ λ ¼ ð 7 : 3 Þ

collagen
λ

Since both ½ can be measured for native soluble tropocollagen rods (entirely

coil
λ

helical) and ½ for gelatin solutions entirely in the coil conformation (T=40 - 50°C),

χ can be calculated under varying thermal protocols. Experimentally observed OR values

ex
λ

α are of the order of degrees and can be easily detected and recorded with a computer-

controlled polarimeter, so the fraction of residues in helical conformation can be accurately
measured in gelatin solutions for concentrations as high as 20 wt%.

7.2.6.2 µDSC

An alternative measurement is provided by μDSC ( Chapter 2 ). Denaturation of collagen and
renaturation observed in gelatin solutions is accompanied by signi

flows, mainly
during heating and cooling ramps. An exothermic peak is observed during helix formation
when cooling at a

cant heat

fixed rate. By applying the same thermal history in polarimetry and
μDSC, a very good correlation is seen between the two techniques. A value of

H coil-helix
per gram of helix during cooling or heating ramps was found to be independent of gelatin
concentration and of the molecular mass of the extract, and close to the native collagen
denaturation enthalpy. An average absolute value is

Δ

Δ H coil-helix = 55 ± 2 J per gram of helix.

coil transition of gelatin is a reversible process, the local structure of a
gel at any time can evolve because of continuous molecular rearrangements.
Accordingly, the range of melting temperatures of the gel changes with concentration
or maturation time, probably the result of annealing (elimination of loops and short
sequences) and growth of helices, although the full collagen structure is never attained. It
was thought possible that the enthalpy could also depend on the degree of perfection of
the sequences, but this was not the case.
In mammalian gelatin gels, the melting temperature of the helices is below 36°C, while
for

Since the helix -

fish skin gelatins it is below the corresponding temperatures of denaturation of the
species

'

films (or very concentrated gels, c > 50 wt%), classical
DSC is an appropriate technique (Coppola et al., 2008 ), and even for these systems the
formation or melting enthalpy per gram of helix is constant and can be used in the
characterization of the state of aggregation in the gels. The structure of these very
concentrated gels is taken up later in this chapter.

collagen. For gelatin

7.2.6.3 Parameters affecting helix formation

Gelatin gels are never in true equilibrium, so that when temperature is lowered below the
coil - helix transition temperature, helices start to form but never approach the 100% of