Conformational Flexibility of l-Alanine Zwitterion Determines Shapes of Raman and Raman Optical Activity Spectral Bands

Abstract

Detailed analysis of Raman and Raman optical activity (ROA) of L-alanine zwitterion (ALAZW) revealed that shapes of the spectral bands are to a large extent determined by the rotation of the NH 3 + , CO 2 -, and CH 3 groups. Aqueous solution ALAZW spectra were measured down to 100 cm -1 and compared to complex simulations based on ab initio (B3LYP/CPCM/6-31++G**) computations of molecular energies and spectral parameters. The bands exhibit different sensitivities to the motion of the rotating group; typically, for more susceptible bands the Raman signal becomes broader and the ROA intensity decreases. When these dynamical factors are taken into account in Boltzmann averaging of conformer contributions, simulated spectra not only better agree with the experiment, but shapes of the rotational potentials can be estimated. Effects of the molecular flexibility could be also demonstrated on differences in Raman spectra of the solution, crystalline, and glass (gellike) solid states of ALAZW. Experimental Raman and ROA spectra of four model dipeptides of different rigidities (Ala-Pro, Pro-Ala, Pro-Gly, and Gly-Pro) indicate that the broadening of spectral lines can be used as a general site-specific indicator of molecular rigidity or flexibility.

Molecular dynamics with restrictions derived from optical spectra

“…9). This completes the earlier observations 14 where all the three motions were found important. The current model additionally suggests that not all geometry parameters can be separated, and that the consideration of the coupling provides a more accurate relation between the spectral shapes and molecular structure.…”

“…Previous studies indicate that the rotations are associated with relatively small energy differences and, consequently, the largeamplitude motion of these groups strongly influences the spectral shapes. 14,28 The continuous generation of the spectra during the MD run implemented in this work reveals further relations between the spectra and molecular structure. For example, the correlation coefficients between the spectral error and the angles,…”

“…7,11,12 By comparison of the computed and experimental Raman and ROA spectral shapes not only the equilibrium structure but also an important knowledge about molecular flexibility and multiconformational states could be acquired. [13][14][15] Even more objective information can be gained from a numerical spectra decomposition, 16 as a complement to the traditional visual comparison of the theoretical spectra with the experiment.…”

Comparison of Quantitative Conformer Analyses by Nuclear Magnetic Resonance and Raman Optical Activity Spectra for Model Dipeptides

“…14,20 The samples consisted of Gly-Pro (2 M), Pro-Gly (0.89 M), AlaPro (1.33 M), and Pro-Ala (0.23 M) aqueous solutions, and their raw Raman and ROA spectra and full experimental details can be found in the literature. 9 The IR spectra (about 10% (w/w) aqueous solutions, i.e. 0.53 M for Pro-Ala and Ala-Pro, and 0.58 M for Gly-Pro and Pro-Gly) were acquired on a Vectra 33 FTIR spectrometer (Bruker) using a single reflection diamond HATR accessory (Pike Technologies).…”

“…9 The presence of the proline ring additionally provides an opportunity to study conformation of a peptide side chain often undetectable by other spectroscopic techniques. The polarizable ring is invisible, for example, in VCD, 16 but provides a strong ROA signal.…”