Hydrolysis of a Ni-Schiff-Base Complex Using Conditions Suitable for Retention of Acid-labile Protecting Groups

Summary

Here, we present an efficient hydrolysis and subsequent Fmoc protection of an amino acid isolated from a Ni-Schiff-base complex. Hydrolysis conditions presented here are suitable for use when retention of acid-labile side-chain protecting groups is required. This technique may be adaptable to a variety of unnatural amino acid substrates.

Abstract

Unnatural amino acids, amino acids containing side-chain functionalities not commonly seen in nature, are increasingly found in synthetic peptide sequences. Synthesis of some unnatural amino acids often includes the use of a precursor consisting of a Schiff-base stabilized by a nickel cation. Unnatural side-chains can be installed on an amino acid backbone found in this Schiff-base complex. The resulting unnatural amino acid can then be isolated from this complex using hydrolysis of the Schiff-base, typically by employing reflux in strongly acidic solution. These highly acidic conditions may remove acid-labile side-chain protecting groups necessary for the unnatural amino acids to be used in microwave-assisted solid-phase peptide synthesis. In this work, we present an efficient hydrolysis and subsequent Fmoc protection of an amino acid isolated from a Ni-Schiff base complex. Hydrolysis conditions presented in this work are suitable for retention of acid-labile side-chain protecting groups and may be adaptable to a variety of unnatural amino acid substrates.

Introduction

Unnatural amino acids (UAA's) bearing side chains that vary from those of the twenty naturally occurring amino acids found in nature have found utility in a wide-range of applications. Synthesis of these UAA's, however, can be difficult depending on the structure of the side-chains and the stereochemistry of the amino acid backbone. C-H bond activation of glycine in the context of a nickel Schiff-base complex has been used to produce a variety of amino acid derivatives including α,β-diamino acids¹ and UAA's bearing fluorinated² or heterocyclic side-chains.³

After additi....

Protocol

1. Hydrolysis of Ni-Schiff-Base Complex

1. Dissolve 1 mmol of the Ni-PBP-Schiff-base complex in 40 mL of N,N-dimethylformamide (DMF) with stirring in a 250 mL round bottom flask at room temperature.
2. Add 60 mL of 0.2 M aqueous EDTA solution, pH 4.5.
3. Using a magnetic stir bar and stir plate, stir the combined solution overnight. As the Schiff-base complex is hydrolyzed, the color will shift from a deep red to white.
4. After completion of the reaction as indicated by the absence of any red coloring, transfer the reaction to a 250 mL separatory funnel.
5. Add 50 mL dichloromethane, cap the separatory funnel, an....

Results

We hypothesized that removal of the Ni²⁺ from the Ni-PBP-Gly complex could allow an efficient aqueous hydrolysis of the Schiff-base without the need for harsh pH conditions. As EDTA is an inexpensive and well-studied chelating agent,¹⁰ we hypothesized that addition of EDTA to a solution of Ni-PBP-Gly would facilitate chelation of Ni²⁺ ions, thereby promoting hydrolysis of the complex.

Discussion

The protocol described above is useful in its ability to facilitate the isolation of an amino acid backbone from a Ni-Schiff-base complex under mild pH conditions and subsequent Fmoc protection of this isolated amino acid through two critical steps. The first step involves stirring a DMF/water solution containing EDTA to facilitate release of the amino acid from the complex. Residual complex or organic byproducts can easily be removed with extraction. The second step of this protocol involves an Fmoc protection of the am.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ni-PBP-Gly			Synthesized from published protocol
DMF	Fisher	D119-4
EDTA	Fisher	S311-100
Dichloromethane	Acros	AC610050040
Sodium Bicarbonate	Fisher	S233-500
Fmoc-OSu	Chem-Impex	"00147"
Dioxane	Fisher	D111-500
Hydrochloric Acid	Fisher	A144-500
Ethyl Acetate	Acros	AC610060040
Magnesium Sulfate	Fisher	M65-500
ZEOPrep 60ECO Silica Gel	ZEOChem
Hexanes	Fisher	3200250.650.443
Chromatography Column
pH Test Strips
Rotary Evaporator
250 mL Separatory Funnel
250 mL Round Bottom Flask
Stir Bar
Stir Plate