Correlation of electronic structures of three cyclic dipeptides with their photoemission spectra

We have investigated the electronic structure of three cyclic dipeptides: cyclo(Glycyl-Glycyl) (cGG), cyclo(Leucyl-Prolyl) (cLP), and cyclo(Phenylalanyl-Prolyl) (cPP). These compounds are biologically active and cLP and cPP are derived from cGG (also known as diketopiperazine), by the addition of the respective functional groups of the amino acids, namely, phenyl, alkyl or a fused pyrrolidine ring (proline). Experimental valence and core level spectra have been interpreted in the light of theoretical calculations to identify the basic chemical properties associated with the central ring, and with the additional functional groups in cLP and cPP. The theoretically simulated spectra of all three cyclic dipeptides in both valence and core spaces agreed reasonably well with the experimental spectra. The three molecules displayed similarities in their core spectra, suggesting that the diketopiperazine structure plays an important role in determining the inner shell spectrum. The experimental C 1s spectra of cLP and cPP are analogous but differ from cGG due to the side chains attached to the diketopiperazine structure. Single spectral peaks in the N 1s (and O 1s) spectra of the dipeptides indicate that the chemical environment of the nitrogen atoms (and oxygen atoms) are very similar, although they show a small splitting in the simulated spectra of cPL and cPP, due to the reduction of their point group symmetry. Valence band spectra of the three dipeptides in the frontier orbital region of 9-11 eV exhibit similarities; however theoretical analysis shows that significant changes occur due to the involvement of the side chain in the frontier orbitals of cPP, while lesser changes are found for cLP.