260
Y. Yokoyama
15.2.3 Section Conclusion
Spiropyran 2 takes the merocyanine form 2mc in organic solvents due to the existence
of the sulfonic acid moiety in the molecule. Despite the zwitterioninc structure, 2mc
is yet soluble in MeCN because of the t-butyl group. Upon visible light irradiation at
450 nm, 2mc changes to the strong acid 2sp which transfers a proton to the stronger
base such as the phenanthroline in 3c and the dimethylamino group in 4o, giving rise
to dramatic changes in the principal properties of these photochromic compounds.
Namely, the absorption maximum wavelength of 3c shifted continuously [9], and the
firmly locked photochromic ability of 4o was unlocked by visible light irradiation
[16].
15.3 Light-Induced Chirality Transfer from a Peptide
Molecule Placed in a Quasi-artificial Medium
to Diarylethenes
15.3.1 Photochromic Molecules in Peptides
Photochromic molecules are gaining much more attention than ever because they can
be used in biological media as switching units for functions and imaging probes [19].
When such molecules are used in water, they should be water soluble, otherwise they
will form aggregates. When those molecules are attached or incorporated in a peptide
such as human serum albumin (HSA), they are water soluble, though the size of the
probe itself becomes large [20, 21]. In addition to water solubility, because peptides
are made of optically active amino acids, they offer chiral environments. When the
photochromic compound generates a racemic mixture upon photochromic reaction,
the ratio of enantiomers when photochromic reactions occur in the peptide would not
be 50/50 due to the chiral environment. The photochromic molecules incorporated in
the peptide will then possess chirality-related information such as CD signals, optical
rotation and circularly polarized luminescence in addition to their usual properties
[22].
We have been interested in the stereoselective photochromism of fulgides and
diarylethenes, and in the early stages, we have reported on diastereoselective
photochromism in many different ways of chirality induction [23–27]. Subsequently,
enantioselective photochromism was carried out in HSA [28, 29]. Here, we present
our results showing that the fine-tuning of the reaction media surrounding HSA plays
an important role in attaining highly enantioselective photochromic ring closure of
diarylethenes [29].
Y. Yokoyama
15.2.3 Section Conclusion
Spiropyran 2 takes the merocyanine form 2mc in organic solvents due to the existence
of the sulfonic acid moiety in the molecule. Despite the zwitterioninc structure, 2mc
is yet soluble in MeCN because of the t-butyl group. Upon visible light irradiation at
450 nm, 2mc changes to the strong acid 2sp which transfers a proton to the stronger
base such as the phenanthroline in 3c and the dimethylamino group in 4o, giving rise
to dramatic changes in the principal properties of these photochromic compounds.
Namely, the absorption maximum wavelength of 3c shifted continuously [9], and the
firmly locked photochromic ability of 4o was unlocked by visible light irradiation
[16].
15.3 Light-Induced Chirality Transfer from a Peptide
Molecule Placed in a Quasi-artificial Medium
to Diarylethenes
15.3.1 Photochromic Molecules in Peptides
Photochromic molecules are gaining much more attention than ever because they can
be used in biological media as switching units for functions and imaging probes [19].
When such molecules are used in water, they should be water soluble, otherwise they
will form aggregates. When those molecules are attached or incorporated in a peptide
such as human serum albumin (HSA), they are water soluble, though the size of the
probe itself becomes large [20, 21]. In addition to water solubility, because peptides
are made of optically active amino acids, they offer chiral environments. When the
photochromic compound generates a racemic mixture upon photochromic reaction,
the ratio of enantiomers when photochromic reactions occur in the peptide would not
be 50/50 due to the chiral environment. The photochromic molecules incorporated in
the peptide will then possess chirality-related information such as CD signals, optical
rotation and circularly polarized luminescence in addition to their usual properties
[22].
We have been interested in the stereoselective photochromism of fulgides and
diarylethenes, and in the early stages, we have reported on diastereoselective
photochromism in many different ways of chirality induction [23–27]. Subsequently,
enantioselective photochromism was carried out in HSA [28, 29]. Here, we present
our results showing that the fine-tuning of the reaction media surrounding HSA plays
an important role in attaining highly enantioselective photochromic ring closure of
diarylethenes [29].