Hierarchical peptide aggregation processes

Thus the entropic phase separation barrier A /Jagg should survive in the thermodynamic limit, if the aggregation of semiflexible polymers is a first-order-like phase-separation process with coexistence of aggregates and Ifagments. 

Values for these quantities are listed in Table 12.1 for the three polymer systems considered here. We see that with increasing system size the surface entropies A surf decrease and thus the influence of surface effects is getting weaker. On the other hand, the latent heat per monomer increases, supporting the first-order character of the aggregation transition. 

Another very important example for a nucleation process that exhibits hierarchical subphase transitions is the aggregation of proteins. For this purpose, we return to our discussion of aggregation properties of the hydrophobic-polar peptide with the Fibonacci sequence FI AB2AB2ABAB2AB) in the previous chapter. 

Microcanonical temperature T(e) as a function of energy per monomer, e=E/N, where W is the total number of all monomers in the system. The horizontal Maxwell line marks the aggregation temperature Tagg. obtained by Gibbs construction. Vertical dashed lines separate the different phases and subphases, respectively. From [266]. 

The surface entropy curves As e) for various numbers of aggregating chains shown in Fig. 11.9 resemble the plots for the homopolymers in Fig. 12.3(c). Therefore, we expect that peptide aggregation processes also are hierarchical [266]. 

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Processing may have adverse as well as beneficial effects on the properties of food proteins, as a result of physical and chemical changes to one or more of the four hierarchical levels of protein structure. These can include changes in quaternary structure reflected by the state of aggregation or dissociation of protein molecules, in tertiary structure as indicated by increased exposure of hydrophobic groups previously buried in the core, in secondary structure as reflected by changes in the proportion of helical, sheet, turn and random coil or unordered structures, and finally, in primary structure as evidenced by hydrolytic cleavage of peptide bonds or by the production of modified side chains of amino acid residues. 

Figure 5 Schematic illustration of the structure formation processes in (a) polypeptides and (b) amphiphilic AB block copolymers. While the block copolymer forms micellar aggregates with a simple core-shell fine structure, the peptide organization leads to distinct nanostructures with a precise hierarchical inner structure ( primary structure the amino acid sequence of the peptide chains secondary structure locally defined substructures In a single protein molecule tertiary structure spatial arrangement of the secondary structures in a 3D structure of a single protein molecule and quaternary structure arrangement of tertiary structure subunit assembly). Reprinted with permission from Borner, H. G. Prog. Polym. Sci. 2009,34, 811-851. Copyright 2009, Elsevier.

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