Catalysis of transesterification reactions by a self-assembled nanosystem

Abstract

Histidine-containing peptides self-assemble on the surface of monolayer protected gold nanoparticles to form a catalytic system for transesterification reactions. Self-assembly is a prerequisite for catalysis, since the isolated peptides do not display catalytic activity by themselves. A series of catalytic peptides and substrates are studied in order to understand the structural parameters that are of relevance to the catalytic efficiency of the system. It is shown that the distance between the His-residue and the anionic tail does not affect the catalytic activity. On the other hand, the catalytic His-residue is sensitive to the chemical nature of the flanking amino acid residues. In particular, the presence of polar Ser-residues causes a significant increase in activity. Finally, kinetic studies of a series of substrates reveal that substrates with a hydrophobic component are very suitable for this catalytic system.

Figure 1

Schematic representation of the catalytic system and structures of catalysts and substrates.

Figure 2

(a) Tryptophan fluorescence intensity at 360 nm as a function of the concentration of peptides A–D in the presence of Au MPC 1 ([headgroup] = 60 ± 5 μM). Conditions: pH 7.0; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; T = 37 °C; (b) Changes in the absorbance at 400 nm upon the addition of substrate I to a solution of Au MPC 1 and either one of the peptides A–D. Conditions: Au MPC 1 [headgroup] = 60 ± 5 μM; [A] = 4.8 μM, [B] = 4.3 μM, [C] = 4.3 μM, [D] = 3.9 μM; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; pH 7.0; T = 37 °C.

Figure 3

(a) Tryptophan fluorescence intensity at 360 nm as a function of the concentration of peptides A–D in the presence of Au MPC 1 ([headgroup] = 60 ± 5 μM). Conditions: pH 7.0; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; T = 37 °C; (b) Changes in the absorbance at 400 nm upon the addition of I (10 μM) to a solution of Au MPC 1 and either one of the peptides E–H. Conditions: Au MPC 1 [headgroup] = 60 ± 5 μM; [E] = 4.85 μM, [F] = 5.30 μM, [G] = 4.25 μM, [H] = 7.95 μM; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; pH 7.0; T = 37 °C.

Figure 4

Amount of released p-nitrophenol as a function of time upon the addition of substrates II–VI to solutions of (a) background; (b) peptide A (4.8 μM); (c) Au MPC 1 ([headgroup] = 60 ± 5 μM and (d) Au MPC 1•A ([headgroup] = 60 ± 5 μM; [A] = 4.8 μM). Other conditions: [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; pH 7.0; T = 37 °C.

Figure 5

(a) Changes in the absorbance at 400 nm (blue) and the tryptophane fluorescence intensity at 360 nm (red) upon four consecutive additions of substrate II (4 × 10 μM) to a solution of Au MPC 1•A. Conditions: Au MPC 1 [headgroup] = 60 ± 5 μM; [A] = 4.8 μM; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; pH 7.0; T = 37 °C. λ_ex= 280 nm; slits 5/10. (b) Tryptophan fluorescence intensity at 360 nm as a function of the concentration of CBz-(L)-Trp-OH in the presence of Au MPC 1 ([headgroup] = 60 ± 5 μM). Conditions: pH 7.0; [HEPES] = 10 mM; H₂O/CH₃CN = 90:10; T = 37 °C.