Computer-Assisted Modeling of Gelatin Properties: Effects of Peptide Size and Alpha-Chain Sequence

Abstract

The broad molecular-weight distributions ofcommercial gelatins are well established, but the extent of chemical heterogeneity has not been assessed. Compositional heterogeneity must occur to some extent in gelatins-the result of coupling (i) the nonuniform distributions of most amino acids along the collagen alpha-I and alpha-2 chains and (ii) many different cleavage positions. This paper explores the possible extent of chemical heterogeneities and calculates the influence on certain gelatin properties. The alpha-chain sequence was used in a microcomputer program that defined all possible peptides of any length, N. For each N, the number of possible peptides N_t is given by N_t= 1052 - N + 1.

Systematic examination of thousands of these hypothetical peptides demonstrated numerous and substantial deviationsfrom the average composition-for the same as well as different values of N.

Compositions of peptides with N = 100-1052 residues/chain were used to calculate isoelectric pH, net charge, and percentage of certain amino acids. The pi values for peptides in a single gelatin type were distributed over several pH units-in agreement with published isoelectric focussing results. Thus, the peptide net charges in an alkali-ossein gelatin may consist of both positive and negative values at a pH of photographic interest. Effects of heterogeneity on electrophoretic migration and other properties are also discussed.