ln&ghts on the Amino Acid Skbchah In

The effect of single amino acid substitutions at positions 18 and 20 on the T-cell determinant (TD) character of peptide ~12-26 from lambda repressor protein and on its recognition by a mono- clonal antibody was studied by means of 40 synthetic peptides of a length of 15 amino acids. ELISA competition experiments showed that the identity of amino acid at position 20 is very important for anti- body recognition, whereas that of amino acid at position 18 is much less important. In contrast, both Leu 18 and Ala 20 are important residues in defining the TD character of peptide ~12-26. The most tolerated replacements, ordered in increasing disrupting power are: Ala 20 by Cys, Ser or Gly and Leu 18 by Ile or Val. Any other amino acid replacement completely abolishes the TD capacity of peptide ~12-26. The peptides used in this study were synthesized using a multiple solid-phase peptide synthesizer newly designed. Their purity was very high as shown by amino acid sequence experiments.