Investigation of the Gas-Phase Amino Acid Alanine by Synchrotron Radiation Photoelectron Spectroscopy

Abstract

Valence and C1s core level photoelectron spectra of gaseous alanine have been recorded with synchrotron radiation. Using ab initio Green's Function calculations of the vertical outer valence ionization energies and CMS-XR calculations of the orbital ionization cross-sections, it is possible to account well for the features of both the new hν ) 92 eV valence photoelectron spectrum and also its differences with an earlier hν ) 21.2 eV spectrum. Good agreement may be achieved by considering just the contribution of a single molecular conformation. This agrees with previous experimental findings, but conflicts with calculations which suggest that a range of molecular conformations would coexist in an equilibrium sample. A study of the valence photoelectron spectrum of the amino acid threonine complements that of alanine, but unlike the latter is limited by the effects of thermal decomposition of the sample. The C1s core level spectrum of alanine is reported and its peaks are assigned to ionization of the three C atoms in the molecule. A fourth minor peak that is observed is tentatively assigned to a peptide CONH 2 linkage which may be formed between alanine monomers.

Vertical Ionization Energies of α-L-Amino Acids as a Function of Their Conformation: an Ab Initio Study

“…Godfrey et al [64] emphasized that a rapid conversion from CF2 to CF1 was very probable due to large vibrational amplitudes of the low frequency modes. But more recently, Powis et al [27] showed that, both experimentally and theoretically, Ala could be observed in the two low-lying conformations. Moreover, the zero point energy and the thermal corrections on the energetic and entropic terms can also be responsible for an increased or a decreased separation between the conformers as can be seen from the results in Table 6.…”

“…Several conformations were already optimized and studied in the literature [27,48,[57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76] and are referenced in the Table. The species labeled S-Thr corresponds to a threonine where the chirality of the C β (which is R in the natural Thr) is reversed. By comparing Thr and S-Thr, it turns out that the CF1 conformations present the same hydrogen-bonding patterns while the CF2 ones are permuted: the H-bonding pattern of CF2 (1) in Thr corresponds to that of CF2 (2) in S-Thr and vice-versa.…”

“…the amino acids. Apparently, not all of them were already the object of an ionization energy determination [21][22][23][24][25][26][27] and when they were, the influence of the conformation was not addressed, except in the recent article by Powis et al [27] on alanine and threonine.…”

A reevaluation of computed proton affinities for the common α-amino acids

“…First, the G3MP2 global minima for neutral amino acids match structures inferred from microwave and other gas-phase spectrometric methods. The examples include glycine [3,5], alanine [11][12][13][14][15], isoleucine [16], leucine [17], serine [15,18], tryptophan [19], and valine [20,21]. However, there are some examples in the literature of global minima identified with the B3LYP functional that do not match experimental structures (although many do match).…”

Photoabsorption and photoionization studies of the amino acid proline in the VUV region

“…Powis et al [4] reported on the valence and C 1s core level photoelectron spectra and quantum calculations of gaseous alanine and threonine. Jochims et al [5] reported an extensive photoionization study in the 6-22 eV energy range, using quadrupole mass spectrometry, of fi ve amino acids: glycine, α-alanine, β-alanine, α-aminoisobutyric acid and α-valine.…”