Chiral recognition and supramolecular photoreaction of 1,1'-binaphthol with bovine and human serum albumins

Article abstract of DOI:10.1007/s11164-012-0655-1

In their pioneering study in 1991, Levi-Minzi and Zandomeneghi discovered that photoirradiation of racemic 1,1'-binaphthol (rac-1) in the presence of bovine serum albumin (BSA) in distilled water gave (R)-1 in 99 % enantiomeric excess (ee) after 77 % of the starting material had been consumed. No similar attempt was made with human serum albumin (HSA). In this study of the effects of phosphate buffer solution on the ground-state affinity and excited-state photobehavior of 1 with serum albumin we found that both BSA and HSA preferentially bind the (S) enantiomer of 1 and that photoreaction of rac-1 mediated by BSA and HSA affords (R)-1 in 98 % ee with 99 % conversion and in 46 % ee with 65 % conversion, respectively.

Full text of DOI:10.1007/s11164-012-0655-1

Res Chem Intermed (2013) 39:371–383
DOI 10.1007/s11164-012-0655-1
Chiral recognition and supramolecular photoreaction
of 1,10-binaphthol with bovine and human serum
albumins
•
Masaki Nishijima Jae-Won Chang Cheng Yang
•
•
•
Gaku Fukuhara Tadashi Mori Yoshihisa Inoue
•
Received: 14 November 2011 / Accepted: 31 December 2011 / Published online: 29 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract In their pioneering study in 1991, Levi-Minzi and Zandomeneghi dis-
covered that photoirradiation of racemic 1,10-binaphthol (rac-1) in the presence of
bovine serum albumin (BSA) in distilled water gave (R)-1 in 99 % enantiomeric
excess (ee) after 77 % of the starting material had been consumed. No similar
attempt was made with human serum albumin (HSA). In this study of the effects of
phosphate buffer solution on the ground-state affinity and excited-state photobe-
havior of 1 with serum albumin we found that both BSA and HSA preferentially
bind the (S) enantiomer of 1 and that photoreaction of rac-1 mediated by BSA and
HSA affords (R)-1 in 98 % ee with 99 % conversion and in 46 % ee with 65 %
conversion, respectively.
Keywords Chiral photochemistry ꢀ Serum albumin ꢀ Binaphthol ꢀ Supramolecular
photochirogenesis
Introduction
Chiral photochemistry, or photochirogenesis, has attracted much attention as an
ecologically benign alternative to conventional thermal asymmetric synthesis
mediated by chiral transition metal catalysts [1–3]. Recently, chiral photochemistry
has been combined with supramolecular chemistry to create the interdisciplinary
field of supramolecular photochirogenesis in which a wide variety of optically
This paper is dedicated to Professor Kazuhiko Mizuno on the occasion of his retirement.
M. Nishijima
Office for University-Industry Collaboration, Osaka University, Suita 565-0871, Japan
J.-W. Chang ꢀ C. Yang ꢀ G. Fukuhara ꢀ T. Mori ꢀ Y. Inoue (&)
Department of Applied Chemistry, Osaka University, Suita 565-0871, Japan
e-mail: inoue@chem.eng.osaka-u.ac.jp
123

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M. Nishijima et al.
active supramolecular hosts have been proposed as chiral environments for
efficiently achieving chirality in the excited state. The chiral supramolecular
systems used have included cyclodextrins [4–8], modified zeolites [9–11],
hydrogen-bonding templates [12–15], nanoporous materials [16], saccharide
scaffolds [17, 18], DNA [19], and proteins [20–30]. Among these inorganic,
organic, and biomolecular hosts, proteins are of particular interest, because of their
inherent chirality and well-defined three-dimensional structures.
Serum albumin (SA) is the most abundant blood protein. It binds and transports a
variety of endogenous and exogenous molecules, for example fatty acids, hormones,
bilirubin, and drugs [31]. In their pioneering work on supramolecular photochiro-
genesis with a protein host approximately 20 years ago, Levi-Minzi and Zando-
meneghi demonstrated that bovine serum albumin (BSA) preferentially binds the
(S) enantiomer of 1,10-binaphthol (1) and mediates photochemical kinetic resolution
of racemic 1,10-binaphthol (rac-1) in unbuffered water to achieve an enantiomeric
excess (ee) of 99 % after 77 % consumption of the starting material [20–22]. This
highly efficient kinetic resolution of rac-1 with BSA was explained mostly on the
basis of photodecomposition of anionic (S)-1 naphtholate produced upon complex-
ation with BSA; this was, however, accompanied by photoinversion to (R)-1 as a
minor path which made a contribution of 9 % to the photodecomposition [22].
More recently, we have shown that BSA and human serum albumin (HSA) can
mediate the enantiodifferentiating [4 ? 4] photocyclodimerization of 2-anthracene-
carboxylate (AC) to yield a mixture of four stereoisomeric cyclodimers, two of which
are chiral [26–30]. Interestingly, the photocyclodimerization mediated by BSA
affords the chiral syn-head-to-tail and anti-head-to-head cyclodimers in 29 and 41 %
ee, respectively; these are substantially enhanced to 82 and 90 % ee, respectively,
when mediated by HSA. These results indicate that although BSA and HSA have
76 % homologous amino acid sequences [32–34], they differ substantially in both
binding and photochirogenic behavior, and this prompted us to (re)examine the
photochemical kinetic resolution of rac-1 not only by BSA but also by HSA.
In this study, we first investigated the binding and photochemical kinetic
resolution behavior of 1 mediated by BSA in phosphate buffer solution (PBS) at
pH 7. The results obtained were compared with those obtained in an unbuffered
aqueous solution to elucidate whether, and to what extent, the reaction media can
affect the ground and excited-state behavior [1–3]. We further performed binding
and photochemical kinetic resolution studies using HSA as host mediator to expand
the scope of SA-mediated photochirogenesis (Scheme 1).
hν
OH HO
OH HO
OH HO
OH HO
BSA / HSA
in pH 7 phosphate buffer
(R) - 1
(S) - 1
Racemic mixture
Scheme 1 Enantioselective photoreaction of rac-1 mediated by serum albumins
123

Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
373
Experimental
Materials
BSA (fatty acid-free, lot # 40K0883) and HSA (lyophilized powder, fatty acid-free,
globulin-free, lot # 109K7550) were purchased from Sigma, and rac-1, (R)-1, (S)-1,
and trifluoroacetic acid (TFA) from TCI; all were used as received. Disodium
hydrogen phosphate, potassium dihydrogen phosphate, and sodium hydroxide for
preparing PBS were obtained from Wako. Hexane and 2-propanol for HPLC (from
TCI, Kanto, and/or Wako) were used after distillation.
Instruments
UV–visible and circular dichroism (CD) spectra were recorded at 25 °C on a Jasco
V550 or V560 spectrophotometer and a J720-WI spectropolarimeter, respectively;
both were equipped with an ETC505T temperature controller.
Chiral HPLC analysis was performed at 25 °C on a Jasco ChromNaV system
equipped with a Daicel Chiralcel AD column; the mobile phase was an 8:2 (v/v) mixture
of hexane and 2-propanol containing 0.1 % TFA, at a flow rate of 0.7 mL/min.
UV–visible and CD spectral titration
Aqueous buffer solutions of BSA and HSA (0.03 mM) were prepared by dissolving
20 mg SA in PBS (pH 7) in a 10-mL volumetric flask. Each of (R), (S), and rac-1
was dissolved in 10 mM aqueous NaOH solution to prepare stock solutions of
2.3 mM concentration. UV–visible and CD spectral titrations were performed by
gradually adding the stock solution of (R), (S), or rac-1 to the SA solution in a
quartz cell maintained at 25 °C. The titration was continued until the 1-to-SA ratio
reached 4 or 5. The binding constants of (R), (S), and rac-1 with BSA and HSA were
calculated from the titration data by use of the nonlinear least-squares fitting
procedure in KaleidaGraph.
UV–visible and CD spectral job plots
In a 50 mL volumetric flask, 199 mg SA was dissolved in PBS to prepare a
0.06 mM solution. The 2.3 mM stock solution of 1 prepared above was diluted with
PBS to 0.06 mM. Each 0.06 mM solution of 1 and SA was mixed to prepare a series
of solutions in which the fraction (x) of 1 varied stepwise from 0 to 1 (x = 0, 0.1,
0.3, 0.5, 0.6, 0.67, 0.8, 0.9, and 1.0) while keeping the total concentration constant at
0.06 mM. The sample solutions thus prepared were subjected to UV–visible and CD
spectrometry.
Photoreaction of rac-1 in the presence of BSA or HSA in PBS
A 0.15 mM solution of SA was prepared by dissolving 498 mg SA in 50 mL PBS,
and a rac-1 solution at the same concentration was prepared by diluting the stock
123

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M. Nishijima et al.
solution with PBS. The samples for photoreaction were then prepared by mixing
appropriate volumes of the SA and rac-1 solutions in a volumetric flask. The sample
(4 mL) was then placed in a quartz cell, purged with nitrogen gas for 30 min in an
ice bath, and then irradiated at 25 °C with a 300-W high-pressure mercury lamp
fitted with an uranium filter (cut-off wavelength: 320 nm; effective wavelength:
365 nm). After irradiation, an equal amount of ether was added to the irradiated
solution, the resulting mixture was stirred for 30 min, and the ether layer was
separated. Substrate consumption (conversion) and the ee of remaining 1 were
determined by normal-phase chiral HPLC on a Daicel Chiralcel AD column.
Conversion was calculated by use of the equation:
ꢀ
ꢁ
Sn=Rn
S0=R0
% conv: ¼ 1 ꢁ
ꢂ 100
where Rn and Sn denote the amounts of (R) and (S)-1 remaining after irradiation for
n min, and R0 and S0 denote the initial amounts before photoirradiation.
Results and discussion
Complexation of binaphthol with BSA and HSA
We first investigated the binding behavior of enantiopure and racemic binaphthol,
i.e., (R), (S), and rac-1, with BSA and HSA in PBS by UV–visible and CD spectral
titration. Figure 1 illustrates the original CD spectra of (R), (S), and rac-1 in pH 7
PBS in the absence of SA; these are assignable to neutral (non-deprotonated)
binaphthol, because its pKa value is 10.9 [35].
Titration with BSA
Stepwise addition of 0–4 equivalents of (R) or (S)-1 dissolved in a 10 mM aqueous
NaOH solution to an aqueous buffer solution of 0.03 mM BSA caused significant
Fig. 1 CD spectra of
0.05 mM (R)-1 (solid line), (S)-1
(dashed line), and rac-1 (chain)
in PBS at pH 7, measured in a
1 cm cell; the inset shows the
enlarged spectra
123

Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
375
UV–visible and CD spectral changes, as shown in Fig. 2. BSA has inherent UV
absorption centered at 280 nm; modest negative Cotton effects are observed up to
320 nm, which partially overlap with the UV and CD bands of binaphthol, in
particular at wavelengths \300 nm.
Nevertheless, addition of (R)-1 led to smooth CD spectral changes, with an
isodichroic point at 275 nm, which were accompanied by the development of
intense positive Cotton effect peaks at 290 and 320 nm and a new UV and positive
CD band at 365 nm. According to the Zandomeneghi’s assignment, the former
bands originate from neutral naphthol included in binding site II of BSA whereas
the latter arise from the naphtholate anion accommodated in basic binding site I
[20–22].
On addition of antipodal (S)-1, a new band appeared similarly at 365 nm, the
absorbance and ellipticity of which were larger by a factor of ca 3 than those
observed for (R)-1. In contrast, the absorbance and ellipticity of the bands at 290 and
320 nm became smaller, which is compatible with a competitive binding
mechanism with the two binding sites I and II. The much larger ellipticity induced
at 365 nm indicates preferential binding of (S)-1 to the basic binding site I of BSA.
Interestingly, no obvious isodichroic point was observed upon titration with (S)-1
(a)
(b)
2.5
2.5
0.2
0.15
0.1
0.2
0.15
0.1
0.05
0.05
2
1.5
1
2
1.5
1
0
340
0
340
360
380
400
360
380
400
Wavelength / nm
Wavelength / nm
(R)-1 / BSA
(S)-1 / BSA
4.0 mol equiv.
4.0 mol equiv.
0 mol equiv.
0 mol equiv.
0.5
0
0.5
0
2
1.5
1
20
40
30
20
10
0
0.5
0
340
θ
10
0
360
380
400
Wavelength / nm
(R)-1 / BSA
(S)-1 / BSA
4.0 mol equiv.
-10
-20
-30
-40
-50
0 mol equiv.
θ
θ
0 mol equiv.
4.0 mol equiv.
0
-1
-2
-3
-10
-20
-4
θ
-5
340
360
380
400
Wavelength / nm
250
300
350
400
450
500
250
300
350
400
450
500
Wavelength / nm
Wavelength / nm
Fig. 2 UV–visible and CD spectral titration of: a (R)-1 (0, 0.0075, 0.015, 0.023, 0.030, 0.038, 0.053,
0.060, 0.068, 0.083, 0.090, and 0.12 mM) and b (S)-1 (0, 0.0075, 0.015, 0.023, 0.030, 0.038, 0.053, 0.060,
0.068, 0.083, 0.090, and 0.12 mM) with 0.03 mM BSA in PBS at pH 7; the insets show the new bands
emerging at the absorption edge
123

376
M. Nishijima et al.
(Fig. 2b, bottom), suggesting significant conformational changes of BSA on
ionizing binding of (S)-1.
Titration with HSA
As shown in Fig. 3, UV–visible and CD spectral titrations of HSA with enantiomeric
1 of up to five equivalents afforded qualitatively very similar results, with intense
negative or positive Cotton effects for (R) or (S)-1 at 290 and 320 nm, and a weaker
one of the same sign at 365 nm, with an accompanying isodichroic point at 275 nm,
for (R)-1 only. However, closer examinations revealed that the changing profile of
the ellipticity induced at 365 nm significantly differed between BSA and HSA, with
relatively slow ellipticity growth upon addition of (R)-1 to BSA and to HSA but very
rapid saturation in the early stages of titration upon addition of (S)-1 to HSA—
compare Fig. 3b (bottom) with Figs. 2a (bottom) and 3a (bottom). This suggests that,
despite the much larger ellipticity change caused by BSA (Fig. 2b (bottom)) than by
HSA (Fig. 3b (bottom)) and the comparable ellipticity changes upon addition of
(R) and (S)-1 to HSA (Figs. 3a (bottom), b (bottom)), HSA’s affinity for (S)-1 and
preference for (S) over (R)-1 are higher than any others (vide infra).
(a)
(b)
2.5
2.5
0.2
0.15
0.1
0.2
0.15
0.1
0.05
0
0.05
0
2
1.5
1
2
1.5
1
340
360
380
400
340
360
380
400
Wavelength / nm
Wavelength / nm
(R)-1 / HSA
5.0 mol equiv.
(S)-1 / HSA
5.0 mol equiv.
0 mol equiv.
0 mol equiv.
0.5
0
0.5
0
2
1.5
1
0.5
0
20
40
30
20
10
0
10
0
θ
340
360
380
400
Wavelength / nm
(S)-1 / HSA
(R)-1 / HSA
5.0 mol equiv.
-10
-20
-30
-40
-50
0 mol equiv.
θ
θ
0 mol equiv.
5.0 mol equiv.
0
-1
-2
-3
-10
-20
θ
-4
-5
340
360
400
Wavelength3/80nm
250
300
350
400
450
500
250
300
350
400
450
500
Wavelength / nm
Wavelength / nm
Fig. 3 UV–visible and CD spectral titration of: a (R)-1 (0, 0.0075, 0.015, 0.023, 0.030, 0.038, 0.045,
0.053, 0.060, 0.075, 0.083, 0.090, 0.12, and 0.15 mM) and b (S)-1 (0, 0.0075, 0.015, 0.023, 0.030, 0.038,
0.045, 0.060, 0.090, and 0.15 mM) with 0.03 mM HSA in PBS at pH 7; the insets show the new bands
emerging at the absorption edge
123

Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
377
Complexation stoichiometry
To elucidate the complexation stoichiometry of (R) and (S)-1 at the strongest
binding site of BSA and HSA, Job analysis was performed by examining the CD
spectral changes at different guest fractions with a fixed total concentration of
0.06 mM. Thus, the net ellipticity changes (Dh) at 365 nm were plotted as a
function of the molar fraction of 1 to afford the Job plots for BSA (Fig. 4a) and
HSA (Fig. 4b). The plots, although somewhat scattered because of the small
absolute ellipticity values, maximized at 0.5, indicating 1:1 stoichiometry for all the
combinations of (R) and (S)-1 with BSA and HSA.
Binding constant
Because Job analysis confirmed 1:1 stoichiometry for complexation of (R) and (S)-1
with BSA and HSA, the ellipticity changes (Dh values) at 365 nm obtained in the
CD spectral titration (Figs. 2, 3) were plotted against the concentration of (R) and
(S)-1; from these the binding constants (K) were calculated by use of non-linear
(a)
(c) 0.4
0.4
0.3
0.2
0.1
0
0.3
0.2
Δ
Δ
0.1
0
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
(R)-1 / [(R)-1 + BSA]
(R)-1 / [(R)-1 + HSA]
(b)
(d)
0
-0.5
-1
0
-0.2
-0.4
-0.6
-0.8
-1
-1.5
-2
Δθ
Δθ
-2.5
-1.2
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
(S)-1 / [(S)-1 + BSA]
(S)-1 / [(S)-1 + HSA]
Fig. 4 CD spectral Job plots of the net ellipticity change (Dh) at 365 nm induced upon complexation of
BSA with: a (R)-1 and b (S)-1, and of HSA with c (R)-1 and d (S)-1, where the total concentration
([1] ? [SA]) was kept constant at 0.06 mM
123

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M. Nishijima et al.
least-squares fitting (Fig. 5). As can be seen from Table 1, the binding constants
obtained for complexation of binaphthol enantiomers with the strongest binding site
of BSA/HSA are rather modest, of the order of 103–106 M-1, significantly smaller
than the values reported for complexation of AC with BSA/HSA (107–108 M-1
)
[26, 27] or of warfarin (IUPAC name: 4-hydroxy-3-(3-oxo-1-phenylbutyl)-2H-
chromen-2-one) with HSA (1.9 9 105 M-1) [36], presumably because of the
bulkier nonplanar structure of binaphthol and/or the different binding sites involved.
It is interesting to note that both BSA and HSA bind (R)-1 only weakly at
K = 6 9 103 M-1 and strongly favor (S)-1, giving K values 1–2 orders of
magnitude higher (Table 1), indicative of substantial S/R selectivity of 8.9 and 160
for BSA and HSA, respectively. Although it is not immediately feasible to speculate
about the exact binding site involved in binaphthol binding, the very similar CD
spectral behavior in peak position and sign would suggest the existence of a
common binding site preserved in BSA and HSA.
(R)-1 / BSA
(R)-1 / HSA
(a)
(c)
0
0.4 0.8 1.2 1.6
2
2.4 2.8 3.2
0
0.4 0.8 1.2 1.6
2
2.4 2.8 3.2
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
Δ
θ
KR, BSA = (6.5 1.4) x 103 M-1
KR, HSA = (6.2 1.8) x 103 M-1
0
0
2x10-5 4x10-5 6x10-5 8x10-5 0.0001
0
0
2x10-5 4x10-5 6x10-5 8x10-5 0.0001
[(R)-1] / M
[(R)-1] / M
(S)-1 / HSA
(S)-1 / BSA
0.4 0.8 1.2 1.6
2
2.4 2.8 3.2
0.4 0.8 1.2 1.6
2
2.4 2.8 3.2
(b)
0
0
-0.5
-1
KS, BSA = (5.8 0.4) x 104 M-1
KS, HSA = (9.8 5.9) x 105 M-1
-0.5
-1
-1.5
-2
Δθ
Δθ
-2.5
-3
-1.5
2x10-5 4x10-5 6x10-5 8x10-5 0.0001
0
2x10-5 4x10-5 6x10-5 8x10-5 0.0001
0
[(S)-1] / M
[(S)-1] / M
Fig. 5 Non-linear least-squares fits of ellipticity changes at 365 nm obtained upon titration of BSA with:
a (R)-1 and b (S)-1, and of HSA with c (R)-1 and d (S)-1 in PBS at 25 °C (the original titration data are
from Figs. 2 and 3)
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Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
379
Table 1 Binding constant (K) and preference (KS/KR) of BSA and HSA for (R) and (S)-1 in pH 7 PBS at
25 °C
Albumin
K/M-1
KS/KR
(R)-1
(S)-1
BSA
HSA
6.5 ( 1.4) 9 103
6.2 ( 1.8) 9 103
5.8 ( 0.4) 9 104
9.8 ( 5.9) 9 105
8.9
160
Table 2 Conversion dependence of the enantiomeric excess (ee) of 1 remaining after photoirradiation of
rac-1 in the presence of BSA or HSA in pH 7 PBS at 25 °C
Albumin
Irradiation time/min
% Conversion
% eea
BSA
1
2
52
83
93
94
99
99
17
31
65
29
36
67
79
98
97
11
26
46
4
6
8
10
2
HSA
4
10
Irradiation at[320 nm using a 300-W high-pressure mercury lamp, fitted with a uranium filter, in pH 7
PBS at 25 °C; [rac-1] = [SA] = 0.15 mM
a
Enantiomeric excess of remaining (R)-1
Photoreaction of racemic binaphthol (rac-1) with BSA and HSA
In their pioneering work, Levi-Minzi and Zandomeneghi used an unbuffered
aqueous solution for photoreaction of rac-1 with BSA and retrieved (R)-1 in 99 %
ee after 77 % consumption of the starting material [21]. In this study, the same
photoreaction was conducted in a physiologically more benign buffer solution to
examine whether or not the nature of aqueous solution alters the stereochemical
outcome of this enantioselective photoreaction. It is also of particular interest to
compare the performance of BSA versus HSA as chiral mediator, because the latter
had much higher binding preference for (S)-1.
Thus, photoirradiation of rac-1 in the presence of BSA or HSA was performed at
wavelengths [320 nm (effective wavelength 365 nm) in 33 mM PBS (pH 7) at
25 °C. The irradiated solutions were extracted with diethyl ether and the ether
extracts were subjected to chiral HPLC analysis on a Daicel Chiralcel AD column
with 8:2 hexane–isopropanol containing 0.1 % TFA as mobile phase; the
conversion and ee values obtained are listed in Table 2.
BSA efficiently mediated photoreaction of rac-1; 52 % conversion was achieved
after irradiation for 1 min and 99 % conversion after 8 min. However, the ee of
remaining (R)-1 developed less efficiently with 29 % ee being achieved even at
52 % conversion, 36 % ee at 83 % conversion, eventually reaching 98 % ee only
123

380
M. Nishijima et al.
100
80
60
40
20
0
Fig. 6 The ee of remaining 1 as
a function of conversion upon
photoreaction of rac-1 in the
presence of BSA (dots) and
HSA (squares) in pH 7 PBS at
25 °C (the original data are from
Table 2)
0
20
40
60
80
100
% conv.
after 99 % of the starting material was consumed. These ee values are much lower
than those reported for photoreaction in unbuffered aqueous solution, which
afforded 80 % ee at 55 % conversion and 99 % ee at 79 % conversion [21],
revealing the significant effects of the reaction media. These results may be
accounted for in terms of the different pH and/or ionic strength of PBS and
unbuffered water, which are likely to affect the three-dimensional structure and
complexation behavior of SA binding sites and the photochirogenic behavior of
binaphthol accommodated therein. Because the isoelectric point of BSA (4.7) is not
very far from the pH of unbuffered water (5.4–5.7), the ionic state of basic amino
acid residues in SA would differ in PBS and unbuffered water, affecting
photochirogenic outcome [31].
In contrast, photoreaction of rac-1 in the presence of HSA proceeded much more
slowly (by a factor of ca 8) with a mere 17 % conversion being achieved after 2 min
irradiation and 65 % conversion even after 10 min (Table 2); the lower (by a factor
of ca 2) absorbance of HSA at 365 nm could be partially responsible. However, the
ee obtained at 65 % conversion was 46 % ee, which is significantly higher than the
36 % ee obtained with BSA at 83 % conversion.
Figure 6 compares the growth of ee as a function of conversion upon
photoreaction of rac-1 with BSA and HSA. Although direct comparison of ee at
the same conversion is not feasible (because we used a series of fixed irradiation
periods for both albumins to obtain significantly different conversions), it is obvious
that HSA, rather than BSA, results in better performance, yielding better ee values
at lower conversion. The better performance of HSA observed in the enan-
tioselective photoreaction of rac-1 could be attributed, at least in part, to the
higher (S)-selectivity of HSA than BSA.
Conclusion
The complexation behavior of enantiomeric binaphthol, (R) and (S)-1, with BSA
and HSA in phosphate buffer has been examined for the first time, revealing
123

Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
381
significant binding preference for the (S) enantiomer; the relative binding constant,
KS/KR, was 8.9 and 160 for BSA and HSA, respectively. Enantioselective
photoreaction of rac-binaphthol mediated by BSA in phosphate buffer was then
investigated. Enantiomeric excess values were substantially lower than those
reported for the same photoreaction in unbuffered solution by Levi-Minzi and
Zandomeneghi [21], revealing the significant effect of the reaction medium,
particularly in photoreactions mediated by biomolecules.
We also demonstrated that the photoreaction of rac-1 mediated by HSA,
although the rate is slower, affords higher ee values than those obtained with BSA at
comparable conversions. These results encourage the use of biomolecules as chiral
supramolecular hosts or mediators for a variety of photochemical reactions.
Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (A) (no.
21245011; YI) and (B) (no. 23350018; TM) from the Japan Society for the Promotion of Science, and a
Grant for Basic Science Research from The Sumitomo Foundation (no. 080435; MN), which are
gratefully acknowledged.
References
1. Y. Inoue, Asymmetric photochemical reactions in solution. Chem. Rev. 92, 741–770 (1992)
2. Y. Inoue, Synthetic chemistry: light on chirality. Nature 436, 1099–1100 (2005)
3. Y. Inoue, V. Ramamurthy, Chiral Photochemistry (Marcel Dekker, New York, 2004)
4. A. Nakamura, Y. Inoue, Supramolecular catalysis of the enantiodifferentiating [4 ? 4] photocyc-
lodimerization of 2-anthracenecarboxylate by c-cyclodextrin. J. Am. Chem. Soc. 125, 966–972
(2003)
5. A. Nakamura, Y. Inoue, Electrostatic manipulation of enantiodifferentiating photocyclodimerization
of 2-anthracenecarboxylate within c-cyclodextrin cavity through chemical modification. Inverted
product distribution and enhanced enantioselectivity. J. Am. Chem. Soc. 127, 5338–5339 (2005)
6. C. Yang, G. Fukuhara, A. Nakamura, Y. Origane, K. Fujita, D.-Q. Yuan, T. Mori, T. Wada, Y. Inoue,
Enantiodifferentiating [4 ? 4]photocyclodimerization of 2-anthracenecarboxylate catalyzed by
6A,6X-diamino-6A,6X-dideoxy-c-cyclodextrins: manipulation of product chirality by electrostatic
interaction, temperature and solvent in supramolecular photochirogenesis. J. Photochem. Photobiol.
A: Chem. 173, 375–383 (2005)
7. C. Yang, A. Nakamura, T. Wada, Y. Inoue, Enantiodifferentiating photocyclodimerization of
2-anthracenecarboxylic acid mediated by c-cyclodextrins with a flexible or rigid cap. Org. Lett. 8,
3005–3008 (2006)
8. C. Ke, C. Yang, W. Liang, T. Mori, Y. Liu, Y. Inoue, Critical stereocontrol by inter-amino distance
of supramolecular photocyclodimerization of 2-anthracenecarboxylate mediated by 6-(O-amin-
oalkylamino)-c-cyclodextrins. New J. Chem. 34, 1323–1329 (2010)
9. A. Joy, V. Ramamurthy, J.R. Scheffer and D.R. Corbin, Enantioselective photoelectrocyclization
within zeolites: tropolone methyl ether in chirally modified NaY, Chem. Commun. 1379–1380
(1998)
10. A. Joy, J.R. Scheffer, V. Ramamurthy, Chirally modified zeolites as reaction media: photochemistry
of an achiral tropolone ether. Org. Lett. 2, 119–121 (2000)
11. J. Sivaguru, A. Natarajan, L.S. Kaanumalle, J. Shailaja, S. Uppili, A. Joy, V. Ramamurthy, Asym-
metric photoreactions within zeolites: role of confinement and alkali metal ions. Acc. Chem. Res. 36,
509–521 (2003)
12. Y. Kawanami, T.C.S. Pace, J. Mizoguchi, T. Yanagi, M. Nishijima, T. Mori, T. Wada, C. Bohne, Y.
Inoue, Supramolecular complexation and enantiodifferentiating photocyclodimerization of
2-anthracenecarboxylic acid with 4-aminoprolinol derivatives as chiral hydrogen-bonding templates.
J. Org. Chem. 74, 7908–7921 (2009)
13. T. Bach, H. Bergmann, B. Grosch, K. Harms, Highly enantioselective intra- and intermolecular
[2 ? 2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host: host-guest
123

382
M. Nishijima et al.
interactions, product configuration, and the origin of the stereoselectivity in solution. J. Am. Chem.
Soc. 124, 7982–7990 (2002)
14. B. Grosch, C.N. Orlebar, E. Herdtweck, W. Massa, T. Bach, Highly enantioselective Diels-Alder
reaction of a photochemically generated o-quinodimethane with olefins. Angew. Chem. Int. Ed. 42,
3693–3696 (2003)
15. M. Fleck, C. Yang, T. Wada, Y. Inoue and T. Bach, Regioselective [2 ? 2]-photocycloaddition
reactions of chiral tetronates-influence of temperature, pressure, and reaction medium, Chem.
Commun. 4422 (2007)
16. Y. Gao, T. Wada, K. Yang, K. Kim, Y. Inoue, Supramolecular photochirogenesis in sensitizing chiral
nanopore: enantiodifferentiating photoisomerization of (Z)-cyclooctene included and sensitized by
POST-1. Chirality 17, S19–S23 (2005)
17. G. Fukuhara, T. Nakamura, C. Yang, T. Mori, Y. Inoue, Dual chiral, dual supramolecular diaste-
reodifferentiating photocyclodimerization of 2-anthracenecarboxylate tethered to amylose scaffold.
Org. Lett. 12, 3510–3513 (2010)
18. G. Fukuhara, T. Nakamura, C. Yang, T. Mori, Y. Inoue, Diastereodifferentiating photocyclodimer-
ization of 2-anthracenecarboxylate tethered to cellulose scaffold. J. Org. Chem. 75, 4307–4310
(2010)
19. T. Wada, N. Sugahara, M. Kawano and Y. Inoue, First asymmetric photochemistry with nucleosides
and DNA: enantiodifferentiating Z–E photoisomerization of cyclooctene, Chem. Lett. 1174–1175
(2000)
20. M. Zandomeneghi, Photochemical activation of racemic mixtures in biological matrices. J. Am.
Chem. Soc. 113, 7774–7775 (1991)
21. N. Levi-Minzi, M. Zandomeneghi, Photochemistry in biological matrices: activation of racemic
mixtures and interconversion of enantiomers. J. Am. Chem. Soc. 114, 9300–9304 (1992)
22. A. Ouchi, G. Zandomeneghi, M. Zandomeneghi, Complexation with albumins of chiral aromatic
substrates and their chemistry in ground and excited states. Catalytic and chirality recognition
properties of the protein in the cases of binaphthol, its photoisomers, and ketoprofen. Chirality 14,
1–11 (2002)
23. V. Lhiaubet-Vallet, Z. Sarabia, F. Bosca, M.A. Miranda, Human serum albumin-mediated stereo-
differentiation in the triplet state behavior of (S)- and (R)-carprofen. J. Am. Chem. Soc. 126,
9538–9539 (2004)
24. J. Rohacova, M.L. Marin, M.A. Miranda, Complexes between fluorescent cholic acid derivatives and
human serum albumin. A photophysical approach to investigate the binding behavior. J. Phys. Chem.
B 114, 4710–4716 (2010)
25. M. Marin, V. Lhiaubet-Vallet, M.A. Miranda, Site-dependent photo-fries rearrangement within
serum albumins. J. Phys. Chem. B 115, 2910–2915 (2011)
26. T. Wada, M. Nishijima, T. Fujisawa, N. Sugahara, T. Mori, A. Nakamura, Y. Inoue, Bovine serum
albumin-mediated enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate. J. Am.
Chem. Soc. 125, 7492–7493 (2003)
27. M. Nishijima, T. Wada, T. Mori, T.C.S. Pace, C. Bohne, Y. Inoue, Highly enantiomeric supramo-
lecular [4 ? 4] photocyclodimerization of 2-anthracenecarboxylate mediated by human serum
albumin. J. Am. Chem. Soc. 129, 3478–3479 (2007)
28. M. Nishijima, T.C.S. Pace, A. Nakamura, T. Mori, T. Wada, C. Bohne, Y. Inoue, Supramolecular
photochirogenesis with biomolecules. Mechanistic studies on the enantiodifferentiation for the
photocyclodimerization of 2-anthracenecarboxylate mediated by bovine serum albumin. J. Org.
Chem. 72, 2707–2715 (2007)
29. T.C.S. Pace, M. Nishijima, T. Wada, Y. Inoue, C. Bohne, Photophysical studies on the supramo-
lecular photochirogenesis for the photocyclodimerization of 2-anthracenecarboxylate within human
serum albumin. J. Phys. Chem. B 113, 10445–10453 (2009)
30. K. Bando, T. Zako, M. Sakono, M. Maeda, T. Wada, M. Nishijima, G. Fukuhara, C. Yang, T. Mori,
T.C.S. Pace, C. Bohne, Y. Inoue, Bio-supramolecular photochirogenesis with molecular chaperone:
enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by prefoldin.
Photochem. Photobiol. Sci. 9, 655–660 (2010)
31. T. Peters Jr, All About Albumin: Biochemistry, Genetics, and Medical Applications (Academic Press,
San Diego, 1996)
32. T. Peters Jr, Adv. Protein Chem. 37, 161–245 (1985)
33. D.C. Carter, J.X. Ho, Adv. Protein Chem. 45, 153–203 (1994)
123

Chiral recognition and supramolecular photoreaction of 1,10-binaphthol
383
34. E.L. Gelamo, C.H.T.P. Silva, H. Imasato, M. Tabak, Interaction of bovine (BSA) and human (HSA)
serum albumins with ionic surfactants: spectroscopy and modeling. Biochim. Biophys. Acta 1594,
84–99 (2002)
35. L.F.M. Ismail, M.S.A. Abdel-Mottaleb, Fluorescence studies of photoactivated acidity of 2,20-
dihydroxy-1,10-binaphthalene. Int. J. Photoenergy 5, 249–254 (2003)
36. H. Vorum, B. Honore, Influence of fatty acids on the binding of warfarin and phenprocoumon to
human serum albumin with relation to anticoagulant therapy. J. Pharm. Pharmacol. 48, 870–875
(1996)
123