Design and synthesis of aldol addition catalytic peptides

Abstract

The aldol reaction is regarded as one of the most prominent carbon-carbon bond forming organic reactions. For many decades enzymes have gained considerable attention as useful catalysts for this reaction, due to their fascinating properties such as their high selectivity and mild reactions conditions. Although enzymes are efficient catalysts, they still suffer from limitations such as narrow substrate tolerance, pH specificity and high cost; there is therefore a need for catalysts that mimic enzyme activity but have fewer drawbacks. Synthetic peptides, because of their structural similarity to enzymes, synthetic accessibility and diverse molecular structures; have emerged as the ideal candidate to realize an enzyme-like catalyst. The design of a peptide catalyst possessing secondary structural units (such as the alpha-helix and beta turn) can produce a novel, enzyme-like organocatalyst. The structural units of the peptides in this study incorporates the beta turn and catalytic amino acids taken from the active site of the fructose-1,6-bisphosphate aldolase (FBPA) enzyme. The peptide sequence also consists of cysteine residues that can form strong interactions with the gold surface via the thiol groups. The immobilization of catalyst onto gold nanoparticle (AuNPs) has been proven to restrict conformational flexibility of catalysts thereby improving their catalytic activity and reusability. The overall aim of this study is to design and synthesize peptides catalysts based on the structure and activity of the FBPA enzyme. The aldol reaction is regarded as one of the most prominent carbon-carbon bond forming organic reactions. For many decades enzymes have gained considerable attention as useful catalysts for this reaction, due to their fascinating properties such as their high selectivity and mild reactions conditions. Although enzymes are efficient catalysts, they still suffer from limitations such as narrow substrate tolerance, pH specificity and high cost; there is therefore a need for catalysts that mimic enzyme activity but have fewer drawbacks. Synthetic peptides, because of their structural similarity to enzymes, synthetic accessibility and diverse molecular structures; have emerged as the ideal candidate to realize an enzyme-like catalyst. The design of a peptide catalyst possessing secondary structural units (such as the alpha-helix and beta turn) can produce a novel, enzyme-like organocatalyst. The structural units of the peptides in this study incorporates the beta turn and catalytic amino acids taken from the active site of the fructose-1,6-bisphosphate aldolase (FBPA) enzyme. The peptide sequence also consists of cysteine residues that can form strong interactions with the gold surface via the thiol groups. The immobilization of catalyst onto gold nanoparticle (AuNPs) has been proven to restrict conformational flexibility of catalysts thereby improving their catalytic activity and reusability. The overall aim of this study is to design and synthesize peptides catalysts based on the structure and activity of the FBPA enzyme. The aldol reaction is regarded as one of the most prominent carbon-carbon bond forming organic reactions. For many decades enzymes have gained considerable attention as useful catalysts for this reaction, due to their fascinating properties such as their high selectivity and mild reactions conditions. Although enzymes are efficient catalysts, they still suffer from limitations such as narrow substrate tolerance, pH specificity and high cost; there is therefore a need for catalysts that mimic enzyme activity but have fewer drawbacks. Synthetic peptides, because of their structural similarity to enzymes, synthetic accessibility and diverse molecular structures; have emerged as the ideal candidate to realize an enzyme-like catalyst. The design of a peptide catalyst possessing secondary structural units (such as the alpha-helix and beta turn) can produce a novel, enzyme-like organocatalyst. The structural units of the peptides in this study incorporates the beta turn and catalytic amino acids taken from the active site of the fructose-1,6-bisphosphate aldolase (FBPA) enzyme. The peptide sequence also consists of cysteine residues that can form strong interactions with the gold surface via the thiol groups. The immobilization of catalyst onto gold nanoparticle (AuNPs) has been proven to restrict conformational flexibility of catalysts thereby improving their catalytic activity and reusability. The overall aim of this study is to design and synthesize peptides catalysts based on the structure and activity of the FBPA enzyme. The aldol reaction is regarded as one of the most prominent carbon-carbon bond forming organic reactions. For many decades enzymes have gained considerable attention as useful catalysts for this reaction, due to their fascinating properties such as their high selectivity and mild reactions conditions. Although enzymes are efficient catalysts, they still suffer from limitations such as narrow substrate tolerance, pH specificity and high cost; there is therefore a need for catalysts that mimic enzyme activity but have fewer drawbacks. Synthetic peptides, because of their structural similarity to enzymes, synthetic accessibility and diverse molecular structures; have emerged as the ideal candidate to realize an enzyme-like catalyst. The design of a peptide catalyst possessing secondary structural units (such as the alpha-helix and beta turn) can produce a novel, enzyme-like organocatalyst. The structural units of the peptides in this study incorporates the beta turn and catalytic amino acids taken from the active site of the fructose-1,6-bisphosphate aldolase (FBPA) enzyme. The peptide sequence also consists of cysteine residues that can form strong interactions with the gold surface via the thiol groups. The immobilization of catalyst onto gold nanoparticle (AuNPs) has been proven to restrict conformational flexibility of catalysts thereby improving their catalytic activity and reusability. The overall aim of this study is to design and synthesize peptides catalysts based on the structure and activity of the FBPA enzyme. Twelve mimetic peptides based on the catalytic active site of the FBPA enzyme [TP_Asp (1), TP_ADLys (2), TP_ADA (3), TP_AspC (4), TP_ADLysC (5), TP_ADAC (6), TP_Glu (7), TP_GDLys (8), TP_GDA (9), TP_GluC (10), TP_GDLysC (11) and TP_GDAC (12)] were designed and synthesized using the solid phase peptide synthesis strategy. NMR (nuclear magnetic resonance spectroscopy) and CD (circular dichroism) were used to determine possible folding patterns and secondary structural features. Both experiments provided confirmation that the peptides are not linear but possesses secondary structural elements. The screened selected peptides showed catalytic activity and the aldol products were obtained in low yields (up to 44%), excellent enantioselectivity (up to 94%) and moderate diastereoselectivity (65:35). The influence of water, substrate scope, catalyst loading and effect of solvents on the reactions were investigated. An increase in catalyst loading did not significantly improve the yields. The addition of water to the reaction medium to enhanced VII both the yields and selectivities. Substituting the organic solvent with a 100 mM phosphate (pH=8.0) buffer afforded improved catalytic activity (38 % yield) and enantioselectivity (up to 80% ee) because of the structural conformation that the peptide adopts in the solution. Aromatic aldehydes bearing a halogen group were shown to be inappropriate substrates for the peptide aldol catalyzed reaction while nitro substituted aromatic aldehydes are acceptable substrates for the peptide-promoted aldol reaction Different AuNPs particle sizes (16, 15 and 12 nm) were successfully conjugated to the peptide and characterized using surface plasmon resonance (SPR) and transmission electron microscopy (TEM). The AuNPs did not significantly improve the catalytic activity of the peptide. However, the use of peptide-capped AuNPs lead to a significant improvement of the enantioselectivity (up to 94%) and a moderate improvement of the diastereoselectivity of (62/48 anti/syn) for the reaction between p-nitrobenzaldehyde and cyclohexanone. This was attributed to the change in the peptide conformation caused by its interactions with the gold metal surface In conclusion, the buffer solution and the conjugation of the peptide onto AuNPs induced peptide structural conformations with improved selectivity.