Structure, solubility and reactivity of peptides. A conformational study of two protected key intermediates from a large-scale synthesis of thymosin alpha 1.

A conformational study of two protected peptide segments, (1-10 and 11-28), spanning the entire sequence of thymosin alpha 1, in solvents of different polarity and capability of forming hydrogen bonds, is reported. By using infrared absorption and circular dichroism techniques the occurrence of the random coil conformation, the self-associated beta-structure, and the alpha-helix (the latter adopted only by the longer peptide) was established. The self-associated species of the two peptide segments were disrupted either by adding increasing amounts of hexamethylphosphoramide or by dilution. This structural transition was monitored by the disappearance of the amide-I C = O stretching band of strongly intermolecularly hydrogen-bonded molecules (near 1630 cm-1) in the infrared absorption spectra. The tendency of these peptides to aggregate is paralleled by a decrease in their solubility. The conformational findings are discussed in terms of the solvent-dependent product yields obtained in the reaction of segment (1-10) with the N alpha-deprotected (11-28) segment to give the fully protected thymosin alpha 1.