A density-functional theory investigation of the radiation products of L-α-alanine

Density-functional theory is used to investigate the radiation products (radicals R1, (CH)-C-.(CH3)COOH; R2, (H3N+C.)(CH3)COO-; and R3, H2NC.(CH3)COOH) of L-alpha-alanine at 295 K. Four conformers were found for RI and R3. A planar structure of R2 in a zwitterionic form was obtained with the Onsager model. The relative energies of each of the four conformers of R1 and R3 show that structures with intramolecular hydrogen bonding are more stable. The computed hyperfine couplings are shown to be in good agreement with the accurate results obtained from the electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and EIE (ENDOR-induced EPR) experiments performed by Sagstuen et al. [Sagstuen, E.; Hole, E. O.; Haugedal, S. R.; Nelson, W. H. J. Phys. Chem. A 1997, 101, 9763]. The effects of rotation about the NC2 bond on the HFCCs of the amino protons in R2 support the previous suggestion that the amino protons are fixed by intermolecular hydrogen bonding in L-alpha-alanine crystals. Moreover, a good correlation between the structure of the amino group and HFCCs in the four conformers of R3 was found.