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 (C50 values of 2.25 and 1.60 μM vs. 10 μM), while the conjugates 1 and 3 with three HB units incorporated form less stable complexes (C50 values of 30 and 21 μM). 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.