5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
107
and the onsager continuum model for bulk water) enabled the authors to find a
computationally stable L-alanine zwitterion due to the network of water molecules
stabilising and bridging the two charged groups. Without the explicit water molecules, one finds only the neutral conformers to be stable [156]. Also, for L-histidine
and L-tryptophan, four water molecules were necessary to stabilise their zwitterions
and to predict their vCd and RoA spectra. however, no other generalisations about
the vCd and RoA spectra amino acids other than those connected to the proper
reproduction of their zwitterionic forms were proposed. Now, the development of
computer technology and quantum chemical software abilities is so fast that, often,
already after a few years, the computational results become outdated. Nevertheless,
it seems that the conclusions of the Jalkanen et al. investigations [154] still remain
valid.
Recently, the vCd and IR spectra of the 20 coded L-amino acids were calculated at the B3LYP/cc-pvtZ level of theory [157]. despite the significant nonrigidity of amino acid molecules [2], the calculations were based on single structures available from a database [158]. A fair agreement between the calculated and
experimental spectra was declared; however, for the IR spectra, the generalising
conclusions were similar to those that could be drawn by inspection of the classical
correlation tables [159, 160]. For the vCd spectra, for which there is still a lack of
generalised rules about changes in vCd intensity upon changing different factors
(solvent, H-bonding, conformation, etc.), it was found that the ρ(OH) rocking and
ν(C = O) stretching modes in most amino acids exhibit the largest VCD rotational
strengths. It would be worth verifying this important conclusion at a sufficiently
high level of theory because the correct reproduction of the amino acid spectra
collected even in slightly interacting medium such as low-temperature matrices
requires a level of theory including the electron correlation effects and a large basis
set [161–164].
the ECd and vCd spectra of model compounds mimicking aromatic amino acid
residues in a protein (L-phenylalanine, L-tyrosine, L-histidine, and L-tryptophan
with the -Cooh and -CNh 2 groups replaced by -CoNhCh 3 and -CNhCoCh 3 )
were studied by tanaka et al. [165]. Quite a variety of conformations of each residue
were calculated at the td-dFt and dFt levels, and the spectra were superposed to
show how the conformations influence the spectral picture. the vCd spectra appeared to be more sensitive to side chain conformations than the ECd spectra, and
it was indicated that using dFt calculations, one can elucidate the main and side
chain conformations of the aromatic residues in proteins [165].
other studies on the vCd and RoA spectra of amino acids have been very unsystematic, so far. there are numerous thorough studies on L-alanine spectra, both
experimental and theoretical, and several papers have been devoted to L-proline and
some of its derivatives, while only a few papers describe the spectra of cysteine and
serine. the vCd and RoA spectra of the other amino acids have been analysed sporadically if at all. Below, we systematically discuss the experimental and theoretical
vCd and RoA studies on amino acids starting from spectroscopic investigations of
L-alanine and L-proline.
107
and the onsager continuum model for bulk water) enabled the authors to find a
computationally stable L-alanine zwitterion due to the network of water molecules
stabilising and bridging the two charged groups. Without the explicit water molecules, one finds only the neutral conformers to be stable [156]. Also, for L-histidine
and L-tryptophan, four water molecules were necessary to stabilise their zwitterions
and to predict their vCd and RoA spectra. however, no other generalisations about
the vCd and RoA spectra amino acids other than those connected to the proper
reproduction of their zwitterionic forms were proposed. Now, the development of
computer technology and quantum chemical software abilities is so fast that, often,
already after a few years, the computational results become outdated. Nevertheless,
it seems that the conclusions of the Jalkanen et al. investigations [154] still remain
valid.
Recently, the vCd and IR spectra of the 20 coded L-amino acids were calculated at the B3LYP/cc-pvtZ level of theory [157]. despite the significant nonrigidity of amino acid molecules [2], the calculations were based on single structures available from a database [158]. A fair agreement between the calculated and
experimental spectra was declared; however, for the IR spectra, the generalising
conclusions were similar to those that could be drawn by inspection of the classical
correlation tables [159, 160]. For the vCd spectra, for which there is still a lack of
generalised rules about changes in vCd intensity upon changing different factors
(solvent, H-bonding, conformation, etc.), it was found that the ρ(OH) rocking and
ν(C = O) stretching modes in most amino acids exhibit the largest VCD rotational
strengths. It would be worth verifying this important conclusion at a sufficiently
high level of theory because the correct reproduction of the amino acid spectra
collected even in slightly interacting medium such as low-temperature matrices
requires a level of theory including the electron correlation effects and a large basis
set [161–164].
the ECd and vCd spectra of model compounds mimicking aromatic amino acid
residues in a protein (L-phenylalanine, L-tyrosine, L-histidine, and L-tryptophan
with the -Cooh and -CNh 2 groups replaced by -CoNhCh 3 and -CNhCoCh 3 )
were studied by tanaka et al. [165]. Quite a variety of conformations of each residue
were calculated at the td-dFt and dFt levels, and the spectra were superposed to
show how the conformations influence the spectral picture. the vCd spectra appeared to be more sensitive to side chain conformations than the ECd spectra, and
it was indicated that using dFt calculations, one can elucidate the main and side
chain conformations of the aromatic residues in proteins [165].
other studies on the vCd and RoA spectra of amino acids have been very unsystematic, so far. there are numerous thorough studies on L-alanine spectra, both
experimental and theoretical, and several papers have been devoted to L-proline and
some of its derivatives, while only a few papers describe the spectra of cysteine and
serine. the vCd and RoA spectra of the other amino acids have been analysed sporadically if at all. Below, we systematically discuss the experimental and theoretical
vCd and RoA studies on amino acids starting from spectroscopic investigations of
L-alanine and L-proline.