Synthetic Antigens with Sequential and Conformation‐Dependent Determinants Containing the Same l‐Tyrosyl‐l‐alanyl‐l‐glutamyl Sequence

Two synthetic copolymers have been used in order to investigate the role of conformation in immunogenicity and antigenic specificity. The first one is a high molecular weight ordered copolymer composed of the repeating sequencel‐tyrosyl‐l‐alanyl‐l‐glutamyl. This polymer exists as an α‐helix under physiological conditions. In the second polymer, the tripeptide l‐tyrosyl‐l‐alanyl‐l‐glutamic acid is attached to branched poly‐dl‐alanine. In this branched polymer the tripeptide exists as a random coil. These polymers elicit the formationof antibodies with distinct specificities, since there was no cross‐precipitation reaction between the two systems. Furthermore, antibodies against the branched polymer were efficiently inhibited by the tripeptide l‐tyrosyl‐l‐alanyl‐l‐glutamic acid, whereas antibodies against the helical polymer were inhibited neither by the tripeptide, nor by other small peptides of sequences related to the primary structure of the helical polymer. However, inhibition was obtained with oligopeptides possessing the general formula (Tyr‐Ala‐Glu)n (with n = 3, 4, 7, and 9), the inhibitory capacity increasing with the value of n. Peptides with n values of 7 and 9 were efficient inhibitors, whereas (Tyr‐Ala‐Glu)13 precipitated with the antibodies. The dependence of inhibitory capacity on a suggests that the peptides with n = 2–9 undergo transconformation (from random coil to α‐helix) upon reaction with the antibody combining site. The immunological studies showed that the antigenic determinats of the α‐helical polymer are conformation‐dependent, whereas the branched polymer contains sequential determinants.