IR/Raman spectroscopy and DFT calculations of cyclic di-amino acid peptides. Part III:comparison of solid state and solution structures of cyclo(L-Ser-L-Ser)

摘要

B3-LYP/cc-pVDZ calculations of the gas-phase structure and vibrational spectra of the isolated molecule cyclo(L-Ser-L-Ser), acyclic di-amino acid peptide (CDAP), were carried out by assuming C_2symmetry. It is predicted that the minimum-energystructure is a boat conformation for the diketopiperazine (DKP) ring with both L-seryl side chains being folded slightly abovethe ring. An additional structure of higher energy (15.16 Id mol~(-1)) has been calculated for a DKP ring with planar geometry,although in this case two fundamental vibrations have been calculated with imaginary wavenumbers. The reported X-raycrystallographic structure of cyclo(L-Ser-L-Ser), shows that the DKP ring displays a near-planar conformation, with both thetwo L-seryl side chains being folded above the ring. It is hypothesized that the crystal packing forces constrain the DKP ring ina planar conformation and it is probable that the lower energy boat conformation may prevail in the aqueous environment.Raman scattering and Fourier-transform infrared (FT-IR) spectra of solid state and aqueous solution samples of cyclo(L-Ser-L-Ser)are reported and discussed. Vibrational band assignments have been made on the basis of comparisons with the calculatedvibrational spectra and band wavenumber shifts upon deuteration of labile protons. The experimental Raman and IR results forsolid-state samples show characteristic amide I vibrations which are split (Raman:1661 and 1687 cm~(-1),1R: 1666 and 1680 cm~(-1)),possibly due to interactions between molecules in a crystallographic unit cell. The cis amide I band is differentiated by itsdeuterium shift of ~30 cm~(-1), which is larger than that previously reported fortransamide I deuterium shifts. Acisamide IImode has been assigned to a Raman band located at 1520 cm~(-1). The occurrence of thiscisamide II mode at a wavenumberabove 1500 cm~(-1)concurs with results of previously examined CDAP molecules with low molecular weight substituents on the atoms, and is also indicative of a relatively unstrained DKP ring.