Photoelectron spectra and structures of three cyclic dipeptides: PhePhe, TyrPro, and HisGly

We have investigated the electronic structure of three cyclic dipeptides: cyclo(Histidyl-Glycyl) (cHisGly), cyclo(Tyrosyl-Prolyl) (cTyrPro), and cyclo(Phenylalanyl-Phenylalanyl) (cPhePhe) in the vapor phase, by means of photoemission spectroscopy and theoretical modeling. The last compound was evaporated from the solid linear dipeptide, but cyclised, losing water to form cPhePhe in the gas phase. The results are compared with our previous studies of three other cyclopeptides. Experimental valence and core level spectra have been interpreted in the light of calculations to identify the basic chemical properties associated with the central diketopiperazine ring, and with the additional functional groups. The valence spectra are generally characterized by a restricted set of outer valence orbitals separated by a gap from most other valence orbitals. The theoretically simulated core and valence spectra of all three cyclic dipeptides agree reasonably well with the experimental spectra. The central ring and the side chains act as independent chromophores whose spectra do not influence one another, except for prolyl dipeptides, where the pyrrole ring is fused with the central ring. In this case, significant changes in the valence and core level spectra were observed, and explained by stronger hybridization of the valence orbitals.