Exploiting Biocatalysis in the Synthesis of Supramolecular Polymers - Enzymatic Polymerisation - page 135

a

b

80

70
60
50
40
30
20
10
0 0.0

Fmoc-L 2

Fmoc-L 3

Fmoc L
Fmoc L 3
Fmoc L 4
Fmoc L 5
Fmoc L 6

Fmoc-L
+ L 2

Fmoc-L 4

Fmoc-L 5

Fmoc-L 6

0.5 1.0 750

2250 3000 3750 4500

5250 6000

1500

Time from Enzyme Addition (Hours)

Fig. 6 Dynamic combinatorial peptide library that exploits enzyme reactions to control self-
assembly processes under thermodynamic control. ( a ) Emergence of the potential peptide deriva-
tives of varying length in a library of interconverting molecules formed from the staring materials
of Fmoc L/L 2 system. Fmoc-L 5 is preferentially formed. Corresponding AFM images of the fibril-
lar structures formed at 5 min after the addition of enzyme, and the sheet-like structures observed
after 2000 h show that redistribution of the derivatives is accompanied by the remodelling from
fibres (Fmoc L 3 ) to sheet-like structures (Fmoc L 5 ). ( b ) HPLC analysis of the composition of the
system reveals the formation and the stabilisation of Fmoc-L 5 over time. Modified from [ 21 ]

5 Spatiotemporal Control of Nucleation and Growth

in Enzymatic Supramolecular Polymerisation

In supramolecular polymerisations, it is a major challenge to gain sufficient control
over the nucleation and the structure growth as these are generally bulk processes
that are controlled by overall changes in reaction conditions such as pH, ionic
strength, temperature, etc. Enzymatic supramolecular polymerisations offer unique
opportunities in this context. Depending on the relative rates of enzyme reaction
and diffusion of the building blocks near an active enzyme, it is possible to achieve
a local concentration above the critical aggregation that favours structure formation.
For example, microscopic analysis of the initial stages of the self-assembly pro-
cess has shown that the fibres propagate from spherical structures of approximately
30 nm in diameter (Fig. 7 a , b). Because enzymatic formation of the building blocks
is localised, it appears that self-assembly nucleates close to enzyme molecules. This
suggests that each sphere contains a small number of enzyme molecules from which
fibres propagate over time [ 21 ] .
Spatially confined self-assembly has been further confirmed by localizing ther-
molysin on certain areas of a PEGylated surface. Upon immersion of this modified
surface into a solution containing self-assembling precursors, nanostructures were
formed in the vicinity of the enzyme, as observed through congo-red staining
(Fig. 7 c ) [ 21 ] . Thus, enzyme-assisted self-assembly allows for construction of
supramolecular polymers with spatiotemporal control, i.e. where and when they are
required.