Effects of ring size and polar functional groups on the glutathione peroxidase-like antioxidant activity of water-soluble cyclic selenides

Article abstract of DOI:10.1021/acs.joc.5b00544

To elucidate the effects of ring structure and a substituent on the glutathione peroxidase- (GPx-) like antioxidant activities of aliphatic selenides, series of water-soluble cyclic selenides with variable ring size and polar functional groups were synthesized, and their antioxidant activities were evaluated by NADPH-coupled assay using H₂O₂ and glutathione (GSH) in water and also by NMR spectroscopy using H₂O₂ and dithiothreitol (DTT^red) in methanol. Strong correlations were found among the GPx-like activity in water, the second-order rate constants for the oxidation of the selenides, and the HOMO energy levels calculated in water. The results support the conclusion that the oxidation process is the rate-determining step of the catalytic cycle. On the other hand, such correlations were not obtained for the activity observed in methanol. The optimal ring size was determined to be five. The type of substituent (NH₂ < OH < CO₂H) and the number can also control the activity, whereas the stereoconfiguration has only marginal effects on the activity in water. In methanol, however, the activity rank could not be explained by the simple scenarios applicable in water.

Full text of DOI:10.1021/acs.joc.5b00544

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Article
Effects of a Ring-Size and Polar Functional Groups on the Glutathione
Peroxidase-Like Antioxidant Activity of Water-Soluble Cyclic Selenides
Kenta Arai, Fumio Kumakura, Motoi Takahira, Natsumi
Sekiyama, Nozomi Kuroda, Toshiki Suzuki, and Michio Iwaoka
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00544 • Publication Date (Web): 05 May 2015
Downloaded from http://pubs.acs.org on May 11, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Organic Chemistry is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Effects of a RingꢀSize and Polar Functional Groups on the
Glutathione PeroxidaseꢀLike Antioxidant Activity of WaterꢀSoluble
Cyclic Selenides
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Kenta Arai, Fumio Kumakura, Motoi Takahira, Natsumi Sekiyama, Nozomi Kuroda, Toshiki
Suzuki, and Michio Iwaoka*
Department of Chemistry, School of Science, Tokai University, Kitakaname, Hiratsukaꢀshi,
Kanagawa 259ꢀ1292, Japan.
Corresponding Author
*Eꢀmail: miwaoka@tokai.ac.jp (M. I.).
TOC/Abstract graphic
Magnitude of GPx-like catalytic activity
in water: OH > NH2
in methanol: NH > OH
2
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT: To elucidate the effects of ring structure and a substituent on the glutathione
peroxidase (GPx)ꢀlike antioxidant activity of aliphatic selenides, series of waterꢀsoluble cyclic
selenides with a variable ringꢀsize and polar functional groups were synthesized, and their
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2 2
antioxidant activities were evaluated by the NADPHꢀcoupled assay using H O and glutathione
red
(
2 2
GSH) in water and also by the NMR assay using H O and dithiothreitol (DTT ) in methanol.
Strong correlations were found among the GPxꢀlike activity in water, the secondꢀorder rate
constants for the oxidation of the selenides, and the HOMO energy levels calculated in water.
The result supported that the oxidation process is a rateꢀdetermining step of the catalytic cycle.
On the other hand, such correlations were not obtained for the activity observed in methanol. The
2 2
optimal ringꢀsize was determined to be five. A sort of a substituent (NH < OH < CO H) and the
count can also control the activity, whereas the stereo configuration has only marginal effects on
the activity in water. In methanol, however, the activity rank could not be explained by the
simple scenarios applicable in water.
Introduction
Selenium (Se), a member of group 16 chalcogen elements, is an essential micronutrient for
living organisms. Generally, selenium is involved in proteins as selenocysteine (Sec), which is a
selenium analog of natural amino acid cysteine (Cys) and is genetically coded like other
2
ACS Paragon Plus Environment

Page 3 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
1
proteinogenic amino acids. Significant roles of seleniumꢀcontaining proteins are implicated in
maintenance of redox homeostasis in cells. Glutathione peroxidase (GPx), a representative
selenoenzyme, catalyzes reduction of reactive oxygen species (ROS), such as hydrogen peroxide
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2‒5
2 2 2
(H O ), to harmless water (H O) using glutathione (GSH) as a reducing cofactor. In the past
few decades, synthesis and application of low molecular weight organoselenium compounds as
6‒11
possible GPx mimics have been explored in fields of chemical biology.
For example,
2
aromatic diselenides (ArSeSeAr) having a polar substituent, such as OH and NH , near the
selenium atom can imitate the catalytic cycle of GPx and exhibit significant GPxꢀlike antioxidant
12‒19
activity.
For such GPx mimics, selection of a polar substituent is one of important factors to
control the activity because it can regulate the reactivity and stability of the intermediates, which
appear during the catalytic cycle, through weak nonbonded Se···X (X = O, N, etc.) interaction
and/or inductive electronic perturbation.
On the other hand, monoselenides (RSeR) have less frequently been focused as potential GPx
2 2
mimics. However, selenides can be oxidized with H O to the corresponding selenoxides, and the
selenoxides can be reduced back to the selenides with thiols (RSH). This selenide–selenoxide
20,21
redox system indeed provides a unique catalytic cycle (Scheme 1) analogous to GPx.
Rahmanto and Davies investigated the GPxꢀlike activity of selenomethionine to reduce protein
2
2
hydroperoxides. Back reported high GPxꢀlike activity of bis(3ꢀhydroxypropyl) selenide, which
23
forms a reactive spiro dioxoselenurane species, instead of a selenoxide, when oxidized. Braga
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
recently characterized in the oxidation products of aryl benzyl selenides in methanol a highly
reactive hydroxy perhydroxy selenane species (RR’Se(OH)OOH), which would be a key
2
4
intermediate for the observed significant GPxꢀlike catalytic activity.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1: A GPxꢀlike redox cycle of selenides.
We recently found that the GPxꢀlike antioxidant activity of waterꢀsoluble aliphatic selenides
having various polar functional groups (X) (Scheme 2) critically depends on X: the activity
2 2
increases in the order of 2 (NH ) < 1 (OH) < 3 (CO H) in an aqueous medium, while the order is
2
1
inverted in methanol, i.e., 3 (CO
2
H) < 1 (OH) < 2 (NH
2
). Similar substituent effects were
25
reported on the GPxꢀlike activity of other series of simple aliphatic selenides. It was also
indicated that the rateꢀdetermining step of the catalytic cycle is the oxidation of the selenide (i.e.,
Reaction A in Scheme 1) and the reduction of the selenoxide (i.e., Reaction B in Scheme 1)
2
1,26
red 27
proceeds rapidly.
Furthermore, cyclic selenide 4 (transꢀDHS ) exhibited higher GPxꢀlike
4
ACS Paragon Plus Environment

Page 5 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
activity in both water and methanol than openꢀchain selenide 1. The activity enhancement
observed for 4 was attributed to the cyclic structure, which would modify the HOMO so that the
21
oxidation of 4 takes place more effectively. In this context, the cyclic selenides having a polar
functional group other than OH would be interesting synthetic targets to enhance the GPxꢀlike
activity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Se
Se
X
X
HO
OH
1: X = OH
2: X = NH2
trans-DHSred (4)
3: X = CO H
2
Scheme 2: Waterꢀsoluble selenides previously studied.
In the present study, to elucidate more clearly the effects of ring structure and polar functional
groups on the GPxꢀlike antioxidant activity of selenides, series of waterꢀsoluble cyclic selenides
5ꢀ11 (Scheme 3) with a variable ringꢀsize and substituents have been synthesized, and their redox
behaviors are compared with those of 1ꢀ4 as reference compounds. The effects of stereo
configuration (i.e., 4 vs. 6 and 9 vs. 10) and a count of a substituent (i.e., 8 vs. 9 or 10), as well as
the solvent effects, are discussed based on reducing ability of the selenides (i.e., the ability as
reducing agents) and the HOMO energy levels.
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 3: Waterꢀsoluble cyclic selenides examined in this study.
Results and Discussion
Synthesis of waterꢀsoluble cyclic selenides. Hydroxyꢀsubstituted fourꢀmembered cyclic
28
29,30
selenide 5, hydroxylꢀsubstituted sixꢀmembered cyclic selenide 7,
and carboxyꢀsubstituted
20
sixꢀmembered cyclic selenide 11 were synthesized according to the literature procedures.
red
Fiveꢀmembered cyclic selenide 8 (monoaminoselenolane, MAS ) was obtained as a
3
1
hydrochloride salt from Lꢀaspartic acid in six steps. On the other hand, cyclic selenide 6
red
red
(cisꢀdihydroxyselenolane, cisꢀDHS ), which corresponds to a cis stereoisomer of transꢀDHS
(
4), was synthesized according to Scheme 4. Dibenzyl ether 12 was prepared from
3
2
cisꢀ2ꢀbuteneꢀ1,4ꢀdiol in two steps by following the literature procedures. After conversion of
3
3
1
2 to an acetonide form (13), the benzylic groups (Bn) were removed reductively. Produced
6
ACS Paragon Plus Environment

Page 7 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
3
4
diol 14 was then treated with methanesulfonyl chloride (MsCl) to give dimesylate 15.
red
cisꢀDHS (6) was obtained by selenation of 15 with sodium hydrogen selenide (NaHSe) and
subsequent deprotection of the acetonide group. The total yield of 6 was 37% from 12.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
red
cisꢀDHS (6) is a known compound, but the synthetic route in this study is different from the
35
previous one .
red
Scheme 4: Synthesis of cis-DHS (6). Reagents and conditions: i) DMP, pꢀTsOH, acetone, rt,
1
.5 h; ii) H
2
, Pd/C, MeOH, rt, 23 h; iii) MsCl, pyridine, 0 °C, 2.5 h; iv) NaHSe, EtOH/THF,
a
b
c
d
reflux, 4 h; v) AcOH, MeOH, reflux, 19 h. Ref 32, Ref 33, Ref 34, Ref 35
red
red
Diaminoselenolanes, i.e., transꢀDAS (9) and cisꢀDAS (10), with fiveꢀmembered ring
structure were synthesized as dihydrochloride salts according to Scheme 5. Starting from diols
1
7a and 17b, which were prepared in eight and seven steps, respectively, from
36‒39
cisꢀ2ꢀbuteneꢀ1,4ꢀdiol by following the literature procedures,
target selenides 9 and 10 were
obtained in three steps through mesylation and selenation of the hydroxyl groups and subsequent
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
deprotection of the Boc groups. The total yields of 9 and 10 were 73 and 85%, respectively, from
red
36
1
7. Synthesis of cisꢀDAS (10) was previously reported, but our synthetic route was slightly
modified from previous one. All cyclic selenides (5ꢀ11) obtained in this study were
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
13
77
unambiguously characterized by H, C, and Se NMR spectroscopy and HRMS (APCI+)
analyses (see the experimental section for details).
red
Scheme 5: Synthesis of cis- and transꢀDAS . Reagents and conditions: i) MsCl, pyridine, 0 °C,
a
b
3
2
.5 h; ii) NaHSe, EtOH/THF, reflux, 4 h; iii) HCl, H O/THF, 30 °C, 18 h. Ref 36. Ref 39
Redox properties of selenides/selenoxides. In order to confirm the redox properties of the
synthesized cyclic selenides (5ꢀ11), the oxidation and the subsequent reduction (Scheme 1) were
7
7
monitored by Se NMR spectroscopy in D O. The spectral changes observed for 9 are shown in
2
Figure 1. When 9 was reacted with one equivalent of H O for 120 min, the absorption for 9 (δ =
2
2
ox
122 ppm) completely disappeared, and a new signal that corresponds to selenoxide transꢀDAS
8
ACS Paragon Plus Environment

Page 9 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
red
(
20) appeared at δ = 935 ppm. Subsequently, one equivalent of dithiothreitol (DTT ) was added
1
to the resulting solution as a dithiol substrate to observe quantitative recovery of 9. In H NMR
ox
1
spectrum, concomitant formation of disulfide DTT (DTT ) was observed (see H NMR spectra
in supporting information). These experiments clearly showed that selenide 9 has capacity of a
redox catalyst as a GPx mimic. The other selenides also showed similar redox behaviors. The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
77
Se NMR chemical shifts observed for the selenides and the corresponding selenoxides are
summarized in Table 1. Although all selenides could be transformed quantitatively to selenoxides,
ox
ox
stability of the selenoxides varied significantly: cisꢀDHS (26), 27, transꢀDAS (20), and
ox
ox
ox
cisꢀDAS (29) were stable as a solid material like transꢀDHS (24), while MAS (28)
gradually decomposed when precipitated from the solution and selenoxides 25 and 30 degraded
slowly even in solution upon prolonged preservation. It should be noted that some selenoxides
show two absorption signals due to pyramidalization of the selenoxide moiety.
9
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
7
Figure 1: Se NMR spectral changes upon oxidation (Reaction A) and subsequent reduction
red
(
Reaction B) of transꢀDAS (9) in D O at 25 °C. Reaction conditions were [9] = [H O ] = 48
2 0 2 2 0
red
mM for reaction A, and one equivalent of DTT was added in reaction B.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
10
ACS Paragon Plus Environment

Page 11 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
7
7
Table 1: Se NMR chemical shifts of selenides and the corresponding selenoxides
a
a
Selenides
δSe
Selenoxides
δSe
1
80
838
872
865
926
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
3
4
78
179
74
5
6
26
84
799 (major) and 850
923 (major) and 925
7
8
9
80
829
135
122
128
945 and 958 (major)
935
10
930 and 970 (~1:1)
11
250
753
11
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a 77
2
Se NMR chemical shifts in D O.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
12
ACS Paragon Plus Environment

Page 13 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
GPxꢀlike antioxidant activity in an aqueous medium. GPxꢀlike catalytic activity of
selenides 5ꢀ11 in a phosphate buffer solution at pH 7.4 was evaluated at 25 °C by the standard
nicotinamide adenine dinucleotide phosphate (NADPH)ꢀcoupled method in the presence of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
40
glutathione reductase (GR). In this assay, H
2
O
2
is reduced with GSH in the presence of a
catalytic amount of a selenide, and produced disulfide GSSG is reduced with NADPH by the
2 2
function of GR enzyme. The velocity of H O reduction can be monitored by the decreased
amount of NADPH, which has UV absorption at 340 nm. Figure 2 shows the decrease of the
0
absorbance at 340 nm as a function of the reaction time. The initial velocities (v ) estimated from
Figure 2 are summarized in the second column of Table 2.
The activities of openꢀchain selenide 1 and cyclic selenides 4–7 are compared in Figure 2A to
investigate the ringꢀsize effects on the GPxꢀlike activity. All selenides exhibited the GPxꢀlike
2 2
activity under the applied conditions, where excess H O with respect to NADPH was added. It
is notable that the activity of cyclic selenides 4–7 was higher than that of selenide 1,
reconfirming our previous observation that cyclic selenides are more efficient GPx mimics than
21
openꢀchain selenides. In addition, the order of the activity, 4 ≈ 6 > 5 > 7 suggested that an
optimal ringꢀsize for the GPxꢀlike activity of cyclic selenides is five. The relative configuration
red
of the polar substituents (i.e., OH) did not affect the activity because transꢀDHS (4) and
red
cisꢀDHS (6) showed almost the same activity.
Figure 2B compares the GPxꢀlike activities of 2‒4 to 8–11. As observed above in the case of a
13
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
OH substituent, the GPxꢀlike activities of cyclic selenides 8–10 were higher than that of
openꢀchain selenides 2 when the substituent was NH . However, in the case of CO H, the
2
2
activity of sixꢀmembered cyclic selenide 11 was similar to or slightly lower than that of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
openꢀchain 3, probably due to the less number of CO H present in 11, in addition to the
2
nonꢀoptimal ringꢀsize.
In the meantime, as to the effects of the functional group, the activity of cyclic selenides
increased in the order, 9 and 10 (NH2
×2) < 4 (OH ×2) < 8 (NH2 ×1) < 11 (CO H ×1). This order
2
is almost consistent with the order observed for openꢀchain selenides in an aqueous medium, i.e.,
21
2 (NH ) < 1 (OH) < 3 (CO H). Generally, it can be assumed that a negatively charged
2
2
–
2
carboxylic group (ꢀCO
) at pH 7.4 would enhance reducing ability of the selenide moiety
+
through inductive electronꢀreleasing, while a positively charged amino group (ꢀNH
3
) would
diminish the reducing ability through inductive electronꢀwithdrawing. Therefore, the trend
observed for the cyclic selenides can be explained by this simple consideration, except for the
red
case of MAS (8). The reason for the unusual redox behavior of 8 is not clear at this moment.
14
ACS Paragon Plus Environment

Page 15 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
1.4
.2
1.0
1.4
A
B
1
1.2
10
• • • • •
1.0
•
9
0
0
0
.8
.6
.4
blank
0.8
0.6
0.4
blank
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
6
3
2
red
trans-DHS (4)
11
1
0
.2
5
0.2
0.0
8
7
red
trans-DHS (4)
0.0
0
100
200
300
400
0
100
200
300
400
Reaction time (sec)
Reaction time (sec)
Figure 2: NADPHꢀcoupled GPx assay for waterꢀsoluble selenides 1 and 4ꢀ7 (A) and 2ꢀ4 and
ꢀ10 (B). Reaction conditions were [GSH] = 1.0 mM, [H O ] = 2.5 mM, [NADPH] = 0.3 mM,
8
0
2
2 0
0
[GR] = 4 units/mL, and [selenide] = 0.2 mM in pH 7.4 phosphate buffer at 25 °C. The results for
selenide 1ꢀ4 were quoted from Ref 21.
15
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2: Summary of GPxꢀlike catalytic activity of selenides in water and in MeOH along with the
secondꢀorder rate constants for the oxidation and the HOMO energy levels.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ
1 a
ꢀ1 ꢀ1 c
Selenides
v
0
(ꢁM min )
t
50
k
ox (M s )
HOMO
Substituents HOMO
in water
Substituents
b
(
min)
in vacuo
d
e
(
ꢀ
eV)
(eV)
No catalyst
32.2 (± 3.2)
44.2 (± 2.4)
34.5 (± 1.1)
71.1 (± 6.9)
>300
90
ꢀ
1
2
3
0.26 (± 0.02)
0.06 (± 0.01)
0.87 (± 0.02)
0.57 (± 0.03)
–6.07
‒5.83
‒6.10
‒6.12
‒5.90
‒5.86
‒6.20
OH
NH
‒6.20
‒6.69
‒5.72
‒6.16
‒6.06
‒6.01
‒6.21
OH
+
10
2
NH
3
–
100
40
CO
H
CO
2
2
red
f
transꢀDHS (4) 54.2 (± 4.0)
OHax, OHax
OHeq
OHax, OHax
OHeq
g
5
50.7 (± 2.5)
54.3 (± 3.7)
45.7 (± 1.5)
95
ND
red
g
cisꢀDHS (6)
15
ND
OHeq, OHax
OHeq, OHeq
OHeq
OHeq, OHax
OHeq, OHeq,
g
7
275
ND
,
OHeq
red
+
MAS (8)
59.8 (± 1.1)
20
6
0.47 (± 0.05)
0.14 (±0.02)
0.13 (±0.02)
‒5.50
‒5.32
‒5.52
‒6.02
NH2ax
‒6.44
NH
NH
NH
3
3
3
ax
red
+
+
+
+
transꢀDAS (9) 47.2 (± 4.3)
NH2ax, NH2ax ‒6.87
NH2eq, NH2ax ‒6.88
ax, NH
3
3
ax
red
cisꢀDAS (10)
48.2 (± 5.3)
63.8 (± 1.0)
65
120
eq, NH
ax
g
–
11
ND
CO
2
H
eq
‒5.74
CO2 eq
16
ACS Paragon Plus Environment

Page 17 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
a
Initial velocities (v ) of H O reduction in phosphate buffer at pH 7.4 and 25 ºC. The values
0
2
2
were calculated from the slope of the UV absorbance changes in the range of 0 to 60 sec in
Figures 2A and 2B.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
b
c
red
ox
Reaction times for 50% conversion of DTT to DTT in MeOH. See Figure 3 for the
reaction conditions.
Secondꢀorder rate constants for the oxidation of selenides with H O in water at 25 ºC (i.e.
2
2
reaction A in Scheme 1).
d
e
HOMO energy levels calculated at B3LYP/6ꢀ31+G(d,p) in vacuo.
HOMO energy levels calculated at B3LYP/6ꢀ31+G(d,p) in water using the polarizable
continuum model (PCM).
Data was quoted from Ref 21.
Not determined.
f
g
17
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
GPxꢀlike antioxidant activity in methanol. The activity was evaluated by applying the
20
red
2 2
NMR method as previously described. The redox reaction between H O and DTT was
red
initiated by addition of H
2
O
2
3
to a solution of DTT in CD OD containing a catalytic amount of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
a selenide. The reaction progress was monitored by H NMR spectroscopy at 25 ºC. The relative
red
ox
populations of DTT and DTT were estimated by integration of the peaks at δ = 2.63 and 3.67
red
ox
red
ppm of DTT and at δ = 2.87, 3.03, and 3.49 ppm of DTT . The remaining amounts of DTT
are plotted in Figure 3 as a function of the reaction time. The times requisite for 50% conversion
red
ox
3
of DTT to DTT in CD OD (t50) are summarized in the third column of Table 2.
In methanol, all selenides exhibited GPxꢀlike antioxidant activity as in a phosphate buffer (i.e.,
in water), but the activity was remarkably enhanced in methanol compared to the blank. It is also
obvious that the activity varied largely depending on the structure and the substituents. When
cyclic selenides 4ꢀ7 are compared to openꢀchain selenide 1 (Figure 3A), it is seen that the
activities of fiveꢀmembered ring selenides 4 and 6 are higher than that of 1, while the activity of
fourꢀmembered ring selenide 5 is nearly equal to that of 1 and for the case of sixꢀmembered ring
selenide 7 it is even lower. The trend of the activities observed for the cyclic selenides are similar
to that observed in water, indicating that the ringꢀsize effect on the GPxꢀlike activity of cyclic
selenides is not affected by the solvent (i.e., the optimal ringꢀsize is five). However, including
openꢀchain selenide 1, the order of the activity in methanol (i.e., 7 < 1 ≈ 5 < 4 < 6) is
significantly different from that observed in water (i.e., 1 < 7 < 5 < 4 ≈ 6). Moreover, as to the
18
ACS Paragon Plus Environment

Page 19 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
stereo configuration of the two OH substituents (i.e., 4 vs 6) the effect is striking in methanol.
This is in large difference from the marginal effect observed in water. Thus, solvent effect would
be an important factor that can control the relative GPxꢀlike activities of selenides.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In Figure 2B, the GPxꢀlike activities of selenides 2‒4 and 8‒11 are compared to investigate
the effect of the substituents on the activity. When the selenides with two functional groups are
focused, the activity increases in the order of 3 (CO H
×
2) < 10 (NH2×2) < 4 (OH×2) < 2
2
(NH2
×2) < 9 (NH2×2), showing that the substitution of the acidic functional groups (CO H)
2
with neutral (OH) or basic ones (NH
2
) enhances the activity in methanol. In general, the
21
substituent effect on the GPxꢀlike activity can be inverted in methanol from that in water
because in water the substituent is protonated or deprotonated while it would exist as an
electronically neutral form in methanol. Comparison of the activity between 4 and 10, however,
shows confliction to this general assumption. As to the effect of cyclization, it seems that the ring
structure basically increases the activity according to comparison of the activity between 2 (NH2
×
2) and 9 (NH2×2). However, 10 (NH2×2) and 8 (NH2×1) with fiveꢀmembered ring structure
showed lower activity than openꢀchain selenide 2. Moreover, cyclic diaminoselenolane with
trans configuration (9) exhibits the higher activity than the cisꢀisomer (10) in methanol. This
trend is opposite to the case of DHS (i.e., 4 vs. 6 in methanol). These results strongly suggest that
2
stereo configuration and a count of an NH substituent have more complex effects on the
2
GPxꢀlike activity in methanol than those in water. As to a CO H substituent, on the other hand,
19
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the activity of 3 with linear structure is slightly higher than that of 11 with a sixꢀmembered ring.
This trend is consistent with that observed in water.
Based on the overall results obtained from the assays in water and in methanol, it is obvious
that the effects of substituents, including those of the count and the stereo configuration, on the
GPxꢀlike antioxidant activity of selenides in methanol are significantly different from those in
water. This suggests the possibility that the activity can be controlled by changing the solvent.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
A
B
1
00
100
80
Blank
8
0
Blank
2
1
3
60
40
trans-DHSred (4)
60
trans-DHSred (4)
MASred (8)
trans-DASred (9)
5
4
0
0
0
cis-DHSred (6)
7
cis-DASred (10)
11
2
0
0
2
0
60
120
180
0
60
120
180
Reaction time (min)
Reaction time (min)
red
Figure 3: Percentages of residual DTT as a function of the reaction time in the oxidation of
red
DTT with H O in the presence of a selenide catalyst (1ꢀ11) in CD OD. (A) A selenide catalyst
2
2
3
was 1, 4, 5, 6, or 7. (B) A selenide catalyst was 2, 3, 4, 8, 9, 10, or 11. Reaction conditions were
red
0 2 2 0
[DTT ] = [H O ] = 0.14 mM and [selenide] = 0.014 mM at 25 °C.
Competition of the reducing ability of selenides. Since the rateꢀdetermining step in the
2 2
GPxꢀlike catalytic cycle of a selenide is the oxidation process with H O to the selenoxide (i.e.
20
ACS Paragon Plus Environment

Page 21 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
2
1
reaction A in Scheme 1), the reducing ability of selenides should have strong correlation with
the GPxꢀlike activity. To confirm this hypothesis, competitive oxidation of the selenides with
77
H O in D O was monitored by Se NMR. When selenides 4 and 5 were simultaneously
2
2
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
oxidized with one equivalent of H O (Figure 4A), about 80% of 4 disappeared and the signal
2
2
ox
corresponding to the selenoxide (transꢀDHS , 24) appeared in a low magnetic field, while the
concentration of 5 decreased in only about 20%. The result indicated that the reducing ability of
a fiveꢀmembered ring selenide is higher than that of a fourꢀmembered ring selenide. This is
consistent with the order of GPxꢀlike activity observed for 4 and 5 in water (Figure 2).
Similarly, when oxidation of selenides 4 and 10 with H O was compared in D O (Figure 4B),
2
2
2
4 was more readily oxidized than 10. The result is again consistent with the order of the GPxꢀlike
activity in water (Figure 2). A similar result was obtained in the competitive experiment using
selenides 4 and 9. According to the results obtained from the competitive experiments for various
combinations of the selenides, the order of the reducing ability was eventually determined to be
3
≈ 11 > 4 ≈ 6 ≈ 8 > 1 ≈ 5 ≈ 7 ≥ 9 ≈ 10 > 2. This order is consonant with the order of the
GPxꢀlike activity in an aqueous medium (Figure 2 and Table 2), strongly supporting the fact that
the rateꢀdetermining step in the GPxꢀlike catalytic cycle of selenides is the oxidation process.
3
The same examination was taken place in CD OD as a solvent in order to obtain information
for the relationship between easiness of the oxidation step (reaction A in Scheme 1) and the
GPxꢀlike activity in methanol (Figure 3). Surprisingly, the order of reducing ability of the
21
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
selenides in methanol was completely identical to that determined in water (i.e., 3 ≈ 11 > 4 ≈ 6 ≈
8
> 1 ≈ 5 ≈ 7 ≥ 9 ≈ 10 > 2) and did not have any correlation with the order of the GPxꢀlike
activity in methanol (i.e., 9 ≈ 10 > 2 > 8 > 4 > 6 > 5 ≈ 1 > 3 > 11 > 7). This might be explained
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
by switching the rateꢀdetermining step of the catalytic cycle in methanol from reaction A to B.
However, it was confirmed that the reaction velocities for reaction A (~1 h) is obviously slower
red
than that for reaction B (<5 min) when the conversion between transꢀDHS
(4) and
ox
1
transꢀDHS (24) was monitored by H NMR. Although the oxidation velocity in methanol was
larger than that in water (~2 h), the rateꢀdetermining step was still reaction A in methanol.
Another possible reason for the observed discrepancy between the reducing ability of the
selenides and the GPxꢀlike activity in methanol might be that the reaction progresses through a
different cycle from that shown in Scheme 1. Recently, Braga reported that the oxidation of
2 2
PhSH with H O in methanol catalyzed by an aryl benzyl selenide/selenoxide redox system can
proceed via a hydroxy perhydroxy selenane intermediate, which is formed by further oxidation
24
of the selenoxide with H
2
O
2
. Back reported formation of a spiro selenium species for similar
23
openꢀchain selenides. Participation of such highly reactive species could explain the observed
discrepancy, but such species were not observed in our NMR analysis. At this moment, there is
no effective scenario to explain the order of the GPxꢀlike activity observed for selenides 1ꢀ11 in
methanol.
22
ACS Paragon Plus Environment

Page 23 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7
7
Figure 4: Se NMR spectral changes observed for the competitive oxidation of (A) selenides 4
and 5 (0.025 mmol each) and (B) selenides 4 and 10 (0.025 mmol each) with H (0.025 mmol)
2
O
2
2
in D O (0.7 mL) at 25 °C.
2
.6. Second-order rate constants for reaction A
In order to further investigate the relationship between the reducing ability of selenides and
the GPxꢀlike antioxidant activity in an aqueous medium, the secondꢀorder rate constants (kox) for
2 2
the oxidation of the selenides with H O to the corresponded selenoxides (i.e. reaction A in
Scheme 1) were estimated as follows. It is known that the UV spectrum of a selenide changes
2
1
during the conversion to the selenoxide. Indeed, the selenides employed in this study exhibited
clear UV spectral changes with a progress of the oxidation to the selenoxides. A representative
23
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
example is shown in Figure 5 for the case of oxidation of 9 to 20. The reaction rate was then
determined by analyzing the timeꢀcourse of the absorbance change at 234 nm. The secondꢀorder
rate constants (k ) for the oxidation of 1, 2, 3, 4, 8, 9, and 10 to the corresponding selenoxides
ox
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
1, 22, 23, 24, 28, 20, and 29, respectively, were determined by plotting the observed pseudo
2 2
firstꢀorder rate constants (kobs) against the concentration of H O (Figure 6). The values of kox
estimated from the slopes are summarized in the fourth column of Table 2. The magnitude
decreases in the order, 3 > 4 > 8 > 1 > 9 ≈ 10 > 2, which well matches with the order of the
GPxꢀlike activity in an aqueous medium (i.e., 3 > 8 > 4 > 1 ≈ 9 ≈ 10 > 2) except for the order of
4 and 8. The correlation plots i.e., v vs. k , are shown in Figure 7. The positive correlation
0
ox
confirmed that the reducing ability of a selenide can control the GPxꢀlike activity in an aqueous
medium.
3.0
0.08
2.5
2
.0
.5
0.04
1
0.00
1
.0
.5
190 210 230 250
0
0.0
190
210
230
250
270
Wavelength (nm)
24
ACS Paragon Plus Environment

Page 25 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Figure 5: UV spectral changes (A) and differential UV spectra (B) observed in the oxidation of
selenide 9 with H O . Reaction conditions were [9] = 0.3 mM and [H O ] = 2 mM in water at
2
2
0
2
2 0
2
5 °C. Reaction time was 0 to 50 min.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
.0100
.0080
Selenide
3
0
0.0060
4
8
0
.0040
.0020
k
1
9
0
1
0
2
0.0000
0
.000 0.002 0.004 0.006 0.008 0.010
Concentration of H O (M)
2
2
Figure 6: Linear plots showing the dependence of observed pseudo firstꢀorder rate constants
k ) for the oxidation of selenide 1, 2, 3, 4, 8, 9, and 10 (0.3 mM) with H O .
(
obs
2
2
25
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 26 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
80
70
60
50
40
3
8
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
9
1
1
0
2
v
30
0
.00 0.20 0.40 0.60 0.80 1.00
-
1
-1
kox (M s )
Figure 7: Correlation plot of the GPxꢀlike activity (v ) of selenides (1‒4 and 8‒10) in water vs.
0
the secondꢀorder rate constants (kox) for the oxidation of selenides to the corresponding
selenoxides.
HOMO energy levels The HOMO energy levels of selenides 1ꢀ11 were obtained by quantum
chemical calculation at the B3LYP/6ꢀ31+G(d,p) level in vacuo and also in water applying the
polarizable continuum model (PCM). The results are shown in the right side of Table 2. For all
selenides, HOMO is dominantly localized at the selenium atom, indicating that the oxidation
velocity of the selenides to the selenoxides should be controlled by the HOMO energy level.
Plots of the GPxꢀlike activity observed for selenides 1ꢀ11 in methanol (i.e., the reaction times
red
for 50% conversion of DTT , t50) against the HOMO energy levels in vacuo failed to obtain a
clear correlation curve (Figure 8). Thus, the GPxꢀlike activity in methanol should not be
26
ACS Paragon Plus Environment

Page 27 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
2 2
determined simply by the feasibility of the selenides to reduce H O . Despite this, there seems to
be a weak tendency that the higher HOMO results in the smaller t50 if one focuses on some series
of selenides. For example, the activity of openꢀchain selenides 1ꢀ3 increases with elevation of the
HOMO level (broken line a). Similar correlations seem to exist for DHS (4 and 6) and DAS (9
and 10) series (broken lines b and c, respectively).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
280
240
200
160
7
1
1
a
1
20
c
3
5
1
8
0
1
0
b
t
v
40
0
4
8
6
2
9
-
6.4 -6.2 -6.0 -5.8 -5.6 -5.4 -5.2
HOMO energy levels in vacuo (eV)
Figure 8: Correlation plots between HOMO energy levels of selenides 1ꢀ11 in vacuo and the
GPxꢀlike activity in methanol (t50).
On the other hand, a clear correlation was obtained in water between the HOMO energy level
0
and the GPxꢀlike activity (v ) as shown in Figure 9. The correlation supported that the GPxꢀlike
antioxidant activity in water is controlled by the reducing ability of selenides. For the selenides
27
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 28 of 41
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
red
red
red
with NH group(s), i.e., MAS (8), transꢀDAS (9), and cisꢀDAS (10), however, the plots are
2
obviously deviated from the correlation line. This may be explained by considering the
equilibrium between an ionized form and a neutral form in water. If this equilibrium exists, the
HOMO would be elevated and the plots should move toward the correlation line.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
80
3
70
60
50
40
11
8
6
4
7
9
5
1
0
1
v
2
30
-7.0
-6.5
-6.0
-5.5
HOMO energy levels in water (eV)
Figure 9: Correlation plots between HOMO energy levels of selenides 1ꢀ11 in water and the
0
GPxꢀlike activity in a phosphate buffer solution (v ).
Conclusions
In this paper, we synthesized various series of waterꢀsoluble cyclic selenides 5ꢀ11 and
compared their GPxꢀlike antioxidant activities to reference selenides 1ꢀ4. As a result, several
28
ACS Paragon Plus Environment

Page 29 of 41
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
clues were obtained for molecular design of cyclic selenides with enhanced GPxꢀlike activity. (1)
The optimal ring size would be five. (2) Substituents can control the activity. In general, the
2 2
activity increases by changing the substituent from NH to OH and then CO H in water, but this
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
order is inverted in methanol. (3) The count of a substituent should also control the activity, but
red
red
this effect does not always hold because MAS (8) shows higher activity than DAS (9 and 10)
in water. (4) Stereo configuration of the substituents does not affect the activity in water, but it
would have a strong impact in methanol. (5) As to the solvent effects, the activity is enhanced in
methanol, but the rank of the activity in methanol cannot be explained by simple scenarios based
on the reducing ability of the selenides and the HOMO energy levels. Although more extensive
study is necessary to fully understand the observed GPxꢀlike activity, especially in methanol,
these findings will be useful in future study for development of small molecular GPx mimics or
antioxidant selenium drugs.
Experimental Section
1
13
77
General. H (500 MHz), C (125.8 MHz), and Se (95.4 MHz) NMR spectra were recorded
at 298 K and coupling constants (J) are reported in Hz. Highꢀresolution mass spectra (HRMS)
were recorded under atmospheric pressure chemical ionization (APCI+) or electrospray
ionization (ESI+) conditions. All reactions for the synthesis of selenides were monitored by
29
ACS Paragon Plus Environment