The Synthesis of Five Heptapeptide Analogues by the Solid-phase Technique. Side Reactions of Tyrosyl and Glutamyl Residues.

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Abstract
Five heptapeptides with the general formula H-Tyr-X-X-X-Ala-Ala-Gly-OH (X = Tyr or Glu) were prepared by the solid-phase technique using an automated synthesizer. Coupling and .alpha.-amino deblocking yields were monitored by potentiometric perchloric acid titration, and cleavage of the peptides from the solid support was achieved by hydrogen bromide/trifluoroacetic acid-anisol (9:1, vol/vol). The formation of N-terminal pyroglutamyl during coupling could be followed by titration, and the corresponding truncated peptides isolated by ion exchange chromatography. Peptides containing a 3-benzyltyrosine residue, due to the O .fwdarw. C rearrangement of O-benzyltyrosyl during cleavage, were separated from the main product in yields of 21-36 mol%. Slow modes of titration of certain peptide sequences, i.e., H-Tyr(Bzl)-Glu(OBzl)-Glu(OBzl)-Tyr(Bzl)-Ala-Ala-Gly-O-resin, were observed and tentatively assigned to conformational properties of the protected and resin-bound peptide chains.