Infra-red Spectrum and Molecular Structure of Diketopiperazine and Tetramethyldiketopiperazine

Abstract

SANBORN¹ has concluded from her work on the chemical properties of glycine anhydride (diketopiperazine), that the molecule exists only in the enolform (I). In view of the importance of the diketo-piperazine configuration for the knowledge of the structure of the proteins, it seemed not without interest to look for further means of investigating the molecular structure of this substance. A very suitable method is the analysis of the infra-red spectrum. The absorption bands of diketopiperazine and its tetramethyl derivative were therefore observed with a quartz spectrometer in the region 2·8–3·6 µ. The effective slit-width was 0·02 µ Each substance was deposited from a suspension in alcohol or ether, respectively, as a crystalline layer on the quartz window of an absorption cell. Following a suggestion of Dr. W. T. Astbury, the cell was then dried in vacuo over phosphorus pentoxide for 38 hours, so as to eliminate all traces of water which might have been adsorbed on the surface of the crystals. The cell was kept dry with phosphorus pentoxide throughout the course of the experiments.