Abstract:
Chiroptical, rheological and thermodynamic studies have been
undertaken to investigate temperature-induced changes in the ý
molecular organisation of gelatin. From the results obtained, a
unified model for gelation and melting has been developed, and
tested using Monte Carlo computer simulation.
The temperature at which gelatin gels are formed has a major
influence on the properties of the resulting network, with
higher curing temperatures conferring increased thermal
stability. In particular, gels formed by sequential curing at
two different temperatures show biphasic melting. This is
explained in terms of a temperature-dependence of helix length
within the junction zones of the gel, and quantified by
considering end-effects in the thermodynamics of helix stability.
Measurements of 'initial slope' kinetics, performed over a broad
concentration range, showed first-order kinetics at low gelatin
concentrations, while at higher concentrations a second-order
process was also evident. The results are interpreted as
triple-helix nucleation at metastable
'hairpin turns' in one
chain (bringing two chain segments into close proximity) together
with a third strand from either the same chain (first order) or a
different chain (second order). From simple geometric
considerations, the maximum length of intermolecular helices
( which contribute to the gel network) is greater than that of
twasted 9 intramolecular structures, giving a qualitative
explanation of the increased strength of gels formed by precuring
at higher temperatures (where only long helices are stable) over
those quenched
directly to low temperature.
Monte Carlo simulation incorporating an initial assumption that
helix propagation is rapid and proceeds to geometric limits gave
unrealistic helix lengths and simulated melting profiles, and
was replaced by the assumption that cis-trans isomerisation of
peptide bonds is the controlling factor in helix propagation.
Using the latter assumption, most aspects of the observed
behaviour were successfully reproduced using program variables
set within realistic ranges or, where possible, fixed at
experimentally-determined values. In particularg the
co-operativity of the simulated melting process was critically
dependent on the value of a parameter x (the number of triplet
units within each helix incapable of participating in bonding,
due to end-effects), with a value of x=1 giving the best fits
with experiment (consistent with accepted bonding patterns for
the collagen triple helix). Other key parameters were the
midpoint temperature for melting of the parent collagen, which
gave best agreement when set at 37-38"C, and t6e proportion of
cis peptide residues present in disordered gelatin chains, with
an optimum lower limit of 0.15.
Using these values, the simulation reproduced, with excellent
precision, the helix fraction and melting profile of gels formed
over a wide range of quench temperatures, and gave an acceptable
approximation to the form of reaction progress curves obtained
for helix formation. The biphasic melting of samples held at
intermediate temperature before final quenching was also modelled
realistically.