Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 188

Table 7.2 Characteristic parameters of gelatin in aqueous solution at T = 50°C.

Parameter Symbol Numerical value

1 = 2

h R g i

Radius of gyration 35 ± 4 nm

(1.9 ± 0.1) × 10 5 g mol − 1 Average molecular mass M w

Polydispersity index M w /M n 2.3

(3 ± 1) × 10 - 4 mol ml g − 2 Second virial coefficient A 2

χ F

Flory parameter 0.48

Statistical segment length b 4 ± 0.5 nm

Cross-section of the chain R c 0.32 ± 0.1 nm

280 ± 80 g mol − 1 nm − 1 Mass per unit length M l

2×10 - 7 cm 2 s − 1 Self-diffusion coef cient D 0

Hydrodynamic radius R h 22 ± 2 nm

Overlap concentration c* 0.5 wt%

c = 1 wt%, ξ =7±1nm

Correlation length (semi-dilute solutions) ξ c = 2 wt% , ξ = 5 ± 0.5 nm

c = 5 wt%, ξ = 3.5 ± 0.3 nm

7.2.5.2 Solution properties

Gelatin is delivered as dry granules and ideally should be left for several hours in cold water
(4°C) so the grains swell without dissolving. The preliminary swelling is an important step
which allows the granules to dissolve fully with mild stirring at 40 - 50°C. Because M w is

quite low, and because the polypeptide chain itself is quite flexible and coil-like, gelatin

solutions are not very viscous and are Newtonian in the range of shear rates from 0.1 to
100 s − 1 . Gelatin in solution is a coil, so with increasing concentration the coils become
entangled, and viscosity increases (Herning et al., 1991 ; Pezron et al., 1991 ). The persis-
tence length of the gelatin coil is around 2 nm; since each tripeptide sequence has a length
of approximately 1 nm, this persistence length corresponds approximately to six residues.
The Flory parameter

χ F = 0.48 is close to 0.5, as is common for water-soluble polymers. The
overlap concentration c* is close to 0.5 wt% and, with increasing concentrations in the
semi-dilute range, the characteristic distance between coils (correlation length) decreases
according to statistical theories of entangled solutions, as was shown by dynamic light
scattering and neutron scattering experiments (Herning et al., 1991 ; Pezron et al., 1991 ).
Various geometrical parameters of typical gelatin coils are listed in Table 7.2 .

7.2.6 Gelation mechanism

When a gelatin solution of concentration 2% w/w is cooled below room temperature,

≳

the protein coils undergo a coil helix transition, and a polymer network begins to be

formed. The helices are reminiscent of the triple-helical native structure of tropocollagen.
The polypeptide chains only partly recover their native conformation, even when
samples are annealed for hours or days, and no real equilibrium is ever reached. When
the temperature is raised, the inverse helix

-

'
to give a liquid ( Figure 7.4 ). It is therefore very important to follow the conformational

- coil transition takes place and the gel ' melts