The optical properties of alanine and proline diketopiperazines.

Abstract

AbstractThe optical properties of the diketopiperazine chromophore of the cyclic dipeptides have been investigated as a function of molecular conformation. The rotatory strengths of L‐alanyl–L‐alanine diketopiperazine and L‐prolyl–L‐proline diketopiperazine have been calculated as a function of the angle of fold of the diketopiperazine ring. The results of these theoretical calculations have been compared with experimental circular dichroism and optical rotatory dispersion data. It is shown that the observed optical properties of these molecules can be explained only if their diketopiperazine rings are folded in opposite directions. The direction of fold is established for each molecule. In solution, the diketopiperazine ring of L‐alanyl‐L‐alanine diketopiperazine is folded in the direction opposite to that found by X‐ray diffraction analysis of crystals. It has been observed that the degree of conservatism of the π → π* couplet of L‐propyl–L‐proline diketopiperazine depends markedly upon the nature of the solvent that is used. In addition, a shoulder has been discovered in the CD spectrum of L‐alanyl–L‐alanine diketopiperazine, which may not be directly attributable to the n → π* and π → π* transitions of the peptide chromophores.