5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
105
in a study by Jalkanen et al. on the vibrational spectra of the alanine dipeptide
[144] and in the investigations of mukhopadhyay et al. of RoA spectra in aqueous solution using monte Carlo simulations for water surroundings [145, 146]. As
concluded by Jalkanen et al. [144], the hybrid model is the proper approach for
further studies of vCd and RoA spectra of amino acids in water. however, very
recently, the most promising, yet time-consuming, multiscale approach has been to
merge dynamic methods with supermolecular methods and with the PCm model
[137–141]. the algorithm contains a few steps: (1) a quantum-chemical exploration
of the conformation landscape of the solute molecule (a scan of the potential energy surface, PES) with the PCm model; (2) reoptimisation of the geometry of the
complex with a few solvent (water) molecules by quantum methods, or, by a more
rigorous approach, finding the geometry of the complexes using the md or CP-md
methods; (3) calculation of the spectra for the individual complexes and averaging
the final spectrum based on the Boltzmann populations of the complexes, or, by a
more rigorous approach, calculation of the spectra for hundreds/thousands of md
complexes extracted from snapshots. the final spectra represents the average data.
the above mentioned multiscale approach opens up a new generation of methods
that can be used for simulations of vCd and RoA spectra, and show better agreement between calculations and experiment than the previously used hybrid methods. the increased computational power of modern parallel computers will make
this technique more and more popular.
5.5 Vibrational Chiroptical Spectra of α-Amino Acids
in Solution
the number of studies devoted to vibrational circular dichroism or Raman optical
activity spectra of amino acids has remained limited and the studies have not been
synthesised for all amino acids in a coherent manner. however, a few studies have
been dedicated to the spectra of more than two amino acids at once. We begin this
section first by referring to the findings of these summarising papers. therefore, this
subchapter is divided into two parts: the first referring to studies on several amino
acids and the second to investigations focusing on one or two amino acids.
5.5.1 Vibrational Chiroptical Studies on Common Amino Acid
Characteristics
the experimental vCd studies comparing results obtained for several amino acids
started in 1983 by the observation that the C*h vCd band of L-amino acids in aqueous solution exhibits a strong positive bias [147]. In that study, all natural α-amino
acids (except aspartic and glutamic acids, tyrosine, and tryptophan, but additionally
allo-threonine, 4-hydroxy proline, and allo-4-hydroxy proline) and penicillamine
105
in a study by Jalkanen et al. on the vibrational spectra of the alanine dipeptide
[144] and in the investigations of mukhopadhyay et al. of RoA spectra in aqueous solution using monte Carlo simulations for water surroundings [145, 146]. As
concluded by Jalkanen et al. [144], the hybrid model is the proper approach for
further studies of vCd and RoA spectra of amino acids in water. however, very
recently, the most promising, yet time-consuming, multiscale approach has been to
merge dynamic methods with supermolecular methods and with the PCm model
[137–141]. the algorithm contains a few steps: (1) a quantum-chemical exploration
of the conformation landscape of the solute molecule (a scan of the potential energy surface, PES) with the PCm model; (2) reoptimisation of the geometry of the
complex with a few solvent (water) molecules by quantum methods, or, by a more
rigorous approach, finding the geometry of the complexes using the md or CP-md
methods; (3) calculation of the spectra for the individual complexes and averaging
the final spectrum based on the Boltzmann populations of the complexes, or, by a
more rigorous approach, calculation of the spectra for hundreds/thousands of md
complexes extracted from snapshots. the final spectra represents the average data.
the above mentioned multiscale approach opens up a new generation of methods
that can be used for simulations of vCd and RoA spectra, and show better agreement between calculations and experiment than the previously used hybrid methods. the increased computational power of modern parallel computers will make
this technique more and more popular.
5.5 Vibrational Chiroptical Spectra of α-Amino Acids
in Solution
the number of studies devoted to vibrational circular dichroism or Raman optical
activity spectra of amino acids has remained limited and the studies have not been
synthesised for all amino acids in a coherent manner. however, a few studies have
been dedicated to the spectra of more than two amino acids at once. We begin this
section first by referring to the findings of these summarising papers. therefore, this
subchapter is divided into two parts: the first referring to studies on several amino
acids and the second to investigations focusing on one or two amino acids.
5.5.1 Vibrational Chiroptical Studies on Common Amino Acid
Characteristics
the experimental vCd studies comparing results obtained for several amino acids
started in 1983 by the observation that the C*h vCd band of L-amino acids in aqueous solution exhibits a strong positive bias [147]. In that study, all natural α-amino
acids (except aspartic and glutamic acids, tyrosine, and tryptophan, but additionally
allo-threonine, 4-hydroxy proline, and allo-4-hydroxy proline) and penicillamine