Conformational flexibility of L-alanine zwitterion determines shapes of Raman and Raman optical activity spectral bands

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.