J. C. Dobrowolsk et al.
130
It appeared that, in each case, one form accounted for at least 70 % abundance in
the mixture when the implicit IEF-PCm solvation model was used. Inclusion of
the explicit water molecules to improve the agreement between theory and experiment is under study. this is important because, indeed, it was shown that the stability of the zwitterionic over the neutral form of leucine, modelled at the CPCm/
B3LYP/6-311 + + g(d, p) level, increased from 1.6 kcal/mol to 4.8 kcal/mol for
zero and two water molecules, respectively [217]. the IR spectra at the three ph
levels were noticeably different because of the existence of different dominant species at each ph. the differences were easily demonstrable by the position of the
C = o stretching vibration band, the location of which varied by over 150 cm
−1
from 1723 cm
−1
(neutral), to 1611 cm
−1
(zwitterionic), to 1563 cm
−1
(deprotonated).
Interestingly, below 1800 cm
−1
, the absorption IR spectra of leucine in water can be
satisfactory reproduced by the sole PCm solvation model. this is, however, not the
case for the leucine vCd spectrum for which the PCm-solvated leucine-(water) n
cluster ( n = 4,5) was necessary for a fair reproduction of the experimental spectra (Fig. 5.16). the complex experimental spectral pattern in the 1500–1300 cm
−1
region was best captured by the leucine-(water) 5 /PCm cluster in which one water
molecule was locked between the Coo
−
and the Nh 3
+
groups. In conclusion, the
authors stated that to faithfully account for the solvent effects of leucine in water
and to reproduce the vCd spectra, the inclusion of the implicit solvent model combined with explicit water solvated clusters is critical.
Fig. 5.16 vCd spectra of L-leucine in neutral aqueous (d 2 o) solution: measured (a) and (b) predicted for different conformations and their superposition (in black). B3LYP/PCm/6-311 + + g(d,
p) calculations were applied (Reproduced from Ref. [216] with kind permission of American Institute of Physics)
130
It appeared that, in each case, one form accounted for at least 70 % abundance in
the mixture when the implicit IEF-PCm solvation model was used. Inclusion of
the explicit water molecules to improve the agreement between theory and experiment is under study. this is important because, indeed, it was shown that the stability of the zwitterionic over the neutral form of leucine, modelled at the CPCm/
B3LYP/6-311 + + g(d, p) level, increased from 1.6 kcal/mol to 4.8 kcal/mol for
zero and two water molecules, respectively [217]. the IR spectra at the three ph
levels were noticeably different because of the existence of different dominant species at each ph. the differences were easily demonstrable by the position of the
C = o stretching vibration band, the location of which varied by over 150 cm
−1
from 1723 cm
−1
(neutral), to 1611 cm
−1
(zwitterionic), to 1563 cm
−1
(deprotonated).
Interestingly, below 1800 cm
−1
, the absorption IR spectra of leucine in water can be
satisfactory reproduced by the sole PCm solvation model. this is, however, not the
case for the leucine vCd spectrum for which the PCm-solvated leucine-(water) n
cluster ( n = 4,5) was necessary for a fair reproduction of the experimental spectra (Fig. 5.16). the complex experimental spectral pattern in the 1500–1300 cm
−1
region was best captured by the leucine-(water) 5 /PCm cluster in which one water
molecule was locked between the Coo
−
and the Nh 3
+
groups. In conclusion, the
authors stated that to faithfully account for the solvent effects of leucine in water
and to reproduce the vCd spectra, the inclusion of the implicit solvent model combined with explicit water solvated clusters is critical.
Fig. 5.16 vCd spectra of L-leucine in neutral aqueous (d 2 o) solution: measured (a) and (b) predicted for different conformations and their superposition (in black). B3LYP/PCm/6-311 + + g(d,
p) calculations were applied (Reproduced from Ref. [216] with kind permission of American Institute of Physics)