Synthesis of Oligo(3‐hydroxybutanoate)(OHB)‐Containing Peptides with High Binding Affinity to a Class‐I‐MHC Protein

In the center of the immune system, there are major histocompatibility (MHC) protein/nonapeptide complexes which are recognized by T cell. The nonapeptides consist of three regions, an N‐terminal one containing three amino‐acid residues with a mandatory arginine in position 2, a C‐terminal one with a lysine or arginine in position 9, and a central, variable one of five residues ( cf. Fig. 1 ). We have now synthesized the first conjugates ( 1 – 4 ) of oligopeptides with oligo[( R )‐3‐hydroxybutanoates] (OHB) as analogs of MHC‐binding peptides. Of the approaches chosen ( Scheme 1 ), a fragment coupling of a hydroxy‐butanoyl‐amido ester ( 17 and 19 ) with an [(aminoalkanoyl)oxy]butanoyl chloride ( 27 ; Scheme 3 ), followed by two peptide‐coupling steps ( Scheme 4 ), turned out to be most efficient. The conjugates H‐Gln‐Arg‐Leu‐(HB) 3,4 ‐Lys‐OH ( 1 and 2 ) and H‐Ala‐Arg‐Leu‐(HB) 3,4 ‐Lys‐OH ( 3 and 4 ) were thus obtained in pure form. The conjugates 1 and 2 with N‐terminal glutamine have a tendency to undergo cyclization with formation of a pyroglutamate residue ( ef. Fig. 2 ). CD Measurements at different temperatures and so‐called epitope‐stabilization assays show that the complexes of the conjugates 2 and 4 , containing four HB units, with the HLA‐B27 class‐I‐MHC protein are more stable than those of a model nonapeptide ( C 50 values of 2.25 and 1.60 μ M vs. 10 μ M ), while the conjugates 1 and 3 with three HB units incorporated form less stable complexes ( C 50 values of 30 and 21 μ M ). The tetra(hydroxybutanoate)‐peptide conjugates 2 and 4 are the first nonapeptide analogs for which the modification of the central part leads to increased affinities for a class‐I‐MHC protein, as compared to a model nonapeptide.