Synthesis and NMR spectroscopy of peptides containing either phosphorylated or phosphonylated cis- or trans-4-hydroxy-L-proline

Abstract

Many proteins are regulated by reversible O-glycosylation and O-phosphorylation. Whereas O-glycosylation of hydroxy-L-proline is common and well investigated, phosphorylation has not been proved so far in vivo, but this post-translational modification is entirely possible. As a first step to identify this phosphoamino acid, we describe both the syntheses of peptides phosphorylated at 4-hydroxy-L-proline and the 1H and 31P NMR parameters of these phosphopeptides. The model peptides were synthesized on solid-phase using Fmoc-strategy. Both natural isomers of 4-hydroxy-L-proline (containing the hydroxyl group in either the cis or trans position) were introduced without side-chain protection. All peptides were globally phosphorylated with O,O'-tert-butyl-N,N-diethylphosphoramidite on the solid phase and cleaved with trifluoroacetic acid. Additionally, we synthesized two classes of phosphonopeptides that mimic phosphopeptides, namely H- and methylphosphonopeptides. The NMR data were based on the model peptide Gly-Gly-Hyp-Ala, which is regarded as a typical random-coil sequence. The NMR parameters showed a significant influence of the phosphate group on the cis-trans isomerization of the Gly-Hyp bond, which may reflect a possible regulation of proteins by changing their local conformations. The 1H and 31P NMR parameters differed for each isomer, and were distinct from the parameters of phosphorylated serine, threonine and tyrosine. These known shifts can be used to identify both cis- and trans-O-phospho-4-hydroxy-L-proline in vivo.