Synthesis and biological evaluation of 9-deazaguanine derivatives connected by a linker to difluoromethylene phosphonic acid as multi-substrate analogue inhibitors of PNP

Article abstract of DOI:10.1016/j.bmcl.2007.05.054

9-(5′,5′-Difluoro-5′-phosphonopentyl)-9-deazaguanine (DFPP-DG) was designed as a multi-substrate analogue inhibitor against purine nucleoside phosphorylase (PNP) on the basis of X-ray crystallographic data obtained for a binary complex of 9-(5′,5′-difluor

Full text of DOI:10.1016/j.bmcl.2007.05.054

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4173–4177
Synthesis and biological evaluation of 9-deazaguanine derivatives
connected by a linker to difluoromethylene phosphonic acid
as multi-substrate analogue inhibitors of PNP
Sadao Hikishima,a Mariko Hashimoto,a Lucyna Magnowska,b
Agnieszka Bzowskab and Tsutomu Yokomatsua,*
aSchool of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
bDepartment of Biophysics, Institute of Experimental Physics, Warsaw University, Zwirki i Wigury 93, 02 089 Warsaw, Poland
Received 3 April 2007; revised 25 April 2007; accepted 16 May 2007
Available online 21 May 2007
Abstract—9-(50,50-Difluoro-50-phosphonopentyl)-9-deazaguanine (DFPP-DG) was designed as a multi-substrate analogue inhibitor
against purine nucleoside phosphorylase (PNP) on the basis of X-ray crystallographic data obtained for a binary complex of 9-
(50,50-difluoro-50-phosphonopentyl)guanine (DFPP-G) with calf spleen PNP. DFPP-DG and its analogous compounds were
adjusted by length of the linker achieved by the Sonogashira-coupling reaction between a 9-deaza-9-iodoguanine derivative and
x-alkynyldifluoromethylene phosphonates as a key reaction.DFPP-DG is a very potent PNP inhibitor with apparent inhibition con-
stants (in the presence of 1 mM phosphate) of 4.4 and 8.1 nM versus calf spleen and human erythrocyte PNPs, respectively. One of
its analogues, homo-DFPP-DG, with longer chain linking phosphonate and 9-deazaguanine is even more potent versus human
enzyme, with an apparent inhibition constant of 5.3 nM (in the presence of 1 mM phosphate).
Ó 2007 Elsevier Ltd. All rights reserved.
Purine nucleoside phosphorylase (PNP, E.C.2.4.2.1) is a
ubiquitous enzyme of purine metabolism that functions
in the salvage pathway, thus enabling cells to utilize pur-
ine bases recovered from metabolized purine ribo- and
deoxy-ribonucleosides to synthesize purine nucleotides.1
In mammals, homotrimeric PNP catalyzes the reversible
phosphorolytic cleavage of the glycosidic bond of 6-keto
purine nucleosides. PNP is crucial for the integrity of the
immune system, since PNP deficiency in humans leads
to defective T-cell response.1 Therefore, inhibitors of
PNP may be useful in the treatment of various autoim-
mune diseases, other T-cell proliferative disorders, and
T-cell cancers.1
(nucleoside and orthophosphate) in their structure are
expected to act as a ‘multi-substrate analogue’ inhibitor
for PNP. Therefore, a number of metabolically stable
acyclic nucleotides containing a purine and phosphate-
like moiety connected by a linker have been synthesized.2
Of the PNP inhibitors reported, 9-(50,50-difluoro-50-
phosphonopentyl)guanine (DFPP-G) developed by
Halazy et al. is one of the most potent and structurally
simple multi-substrate analogue inhibitors of PNP.3
During our previous studies directed toward the design
and synthesis of a multi-substrate analogue inhibitor
of trimeric PNP based on the use of difluoromethylene-
phosphonic acid as a phosphate mimic, we have exam-
ined structural modification of the linker and base
moieties of DFPP-G.4 In these studies, we have identi-
fied several conformationally constrained analogues of
DFPP-G that show better inhibition motifs than those
observed for DFPP-G.4,5
Since PNP accomplishes the reversible phosphorylation
of purine nucleosides via a ternary complex of the en-
zyme, nucleoside, and orthophosphate, compounds that
contain covalently linked elements of both substrates
In an effort to logically extend on our previous results,
we have recently succeeded in crystallizing a binary com-
plex with DFPP-G and calf spleen PNP (Fig. 1).6 High-
resolution X-ray differentiation data confirmed for the
first time that DFPP-G acts as a multi-substrate
Keywords: Purine nucleoside phosphorylase; Multi-substrate analogue
inhibitors; 9-Deazaguanine; Difluoromethylene phosphonic acids; So-
nogashira reactions.
*
Corresponding author. Tel./fax: +81 426 76 3239; e-mail: yokomatu
@ps.toyaku.ac.jp
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.054

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S. Hikishima et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4173–4177
derivatives.1,7 In addition, similar hydrogen-bond pat-
N
O
terns were detected in a ternary complex with the en-
zyme, phosphate, and immucillin G, a transition-state
analogue inhibitor of PNP.6,8 In this paper, we now de-
scribe the synthesis and biological evaluation of DFPP-
DG and its related analogues as a multi-substrate ana-
logue inhibitor of PNP.
N
H2O3PF2C
NH
NH2
N
DFPP-G
To synthesize analogous compounds of DFPP-DG, a
strategy which has high diversity for adjusting the length
of the linker is desirable. In this context, a cross-cou-
pling reaction between a 9-deazaguanine unit and a
CF2-phosphonate unit appropriately functionalized at
the x-position is one of the reactions to be chosen. This
strategy would make a variety of analogues of DFPP-
DG upon adjusting the number of carbon atoms
between ‘CF2PO3Et2’ moiety and x-functional group
(G) within the CF2-phosphonate unit (Scheme 1).
Keeping this strategy in mind, we first examined synthe-
sis of the 9-deazaguanine unit. Inspections of literature
revealed that suitably protected 9-deazaguanine deriva-
tive 2 has been previously prepared from readily accessi-
ble pyrimidinone 1,9 and compound 2 was transformed
to 9-iodo-9-deazaguanine derivative 3 by NIS-mediated
selective iodination at the 9-position.10 Then, according
to the literature procedure, we were able to obtain
multi-grams of 3 without any difficulty. To protect the
acidic proton at the 7-position, 3 was nosylated by con-
ventional method to give 4 in a quantitative yield
(Scheme 2).
Figure 1. Structure of DFPP-G and its interactions with calf spleen
PNP in the binary complex (figure from Ref. 6).
analogue inhibitor as it binds to both nucleoside- and
phosphate-binding sites. In addition, the putative hydro-
gen bonds identified in the base-binding site indicate
that the contact of guanine O6 with Asn243 Od1 is not
a direct contact but is mediated by a water molecule
(Figs. 1 and 2). It is apparent that the bridging water
molecule is entropically disfavored, and exclusion of
the water molecule from the complex may induce tighter
binding to PNP.
Multi-grams of suitably protected 9-iodo-9-deazagua-
nine 4 in hand, we next focused on the synthesis of
On the basis of these findings and hypothesis, we de-
signed a new candidate, 9-(50,50-difluoro-50-phosphono-
pentyl)-9-deazaguanine (DFPP-DG), as an inhibitor
against PNP, in which the guanine base of DFPP-G is
replaced by 9-deazaguanine (Fig. 2). Newly designed
DFPP-DG is expected to bind the nucleoside-binding
site through direct hydrogen bonds to Asn243 as shown
in Figure 2. Such hydrogen-bond patterns in which N(7)
of the base is protonated and donates a hydrogen to
Asn243Od1, while the Asn243Nd1 donates a hydrogen
to the exocyclic O6 of the base, are generally observed
with binary complexes with PNP and ground-state
PNP inhibitors such as 9-arylmethyl-9-deazaguanine
O
Y
N
W
N
+
G
(CH2)n CF2PO3Et2
N=Z
9-deazaguanine unit
N
9
X
CF2-phosphonate unit
,
,
G=
Bu3Sn
Y, W, Z=protective group; X=halogen
Scheme 1. Cross-coupling strategy for synthesis of DFPP-DG
analogues.
Asn 243
Asn 243
O
H
N
H
O
O
O
H
H
H
O
O2N
H3C
POM
NIS/DMF
ref.10
N
4-steps
ref. 9
NH
NH2
N
H
N
O
N
H
O
H
N
N=CHNMe2
N
N
N
N
NH
NH2
NH
NH2
1
2
N
O
Ns
I
O
H
POM
N
NsCl/NaH
CH2Cl2
N=CHNMe2
POM
N
N
N
N=CHNMe2
N
CF2P(O)(OH)2
DFPP-G
N
CF2P(O)(OH)2
I
3
4
DFPP-DG
POM:pivaloyloxymethyl
Scheme 2. Preparation of 9-iodo-9-deazaguanine derivatives.
Figure 2. Binding patterns of DFPP-G and expected binding patterns
of DFPP-DG with the base-binding site of PNP.

S. Hikishima et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4173–4177
4175
CF2-phosphonate units. Preliminary studies of the cou-
pling reaction of 4 with simple linker units such as 3-bu-
ten-1-ol, tri-n-butylvinylstanane, and 3-butyn-1-ol under
Heck, Still, and Sonogashira reaction conditions,11
respectively, revealed that only the Sonogashira reaction
with 3-butyn-1-ol gave the corresponding coupling-
product 5 in good yield, while the Still and Heck reac-
tions gave no desired products (Scheme 3).
3–5 carbon atom linker were prepared as shown in
Scheme 4.
Treatment of TMS-protected bromopropyne 6 with
BrZnCF2PO3Et2 in the presence of CuBr in DMF
according to our previously reported protocols12,13 gave
7a in a 70% yield. Tetrabutylammonium fluoride
(TBAF)-induced protodesilylation of 7a in THF in the
presence of AcOH gave x-alkenyldifluoromethylene
phosphonate unit 8a (C3-phosphonate unit) in a 97%
yield.14 To synthesize C4-phosphonate unit 8b and
C5-phosphonate unit 8c, alkylation of LiCF2PO3Et2
with readily available triflates 9 and 10 was applied
according to the method described by Berkowitz.13,15
While the reaction with 10 under the conditions pro-
ceeded smoothly to give 7c in an 89% yield, the reaction
with 9 was problematic due to competitive formation of
large amounts of enyne products by b-elimination of HF
under the conditions. However, 7b could be isolated in a
very low yield (8%). The compounds 7b and c were trea-
ted with TBAF in THF to give the requisite
C4-phosphonate unit 8b and C5-phosphonate unit 8c
in 75% and 90% yields, respectively.
Therefore, we decided to examine the cross-coupling
reaction of 4 with x-alkynyldifluoromethylenephsopho-
nate units under Sonogashira conditions for the
synthesis of DFPP-DG analogues. A series of requisite
x-alkynyldifluoromethylene phosphonate units having
O
Ns
OH
POM
N
N
4
N=CHNMe2
PdCl2(PPh3)2
CuI, Et3N, 60˚C
N
76% yield
5
OH
Scheme 3. Sonogashira reaction of 4 with 3-butyn-1-ol.
The Sonogashira-coupling reaction between the 9-
deazaguanine unit 4 and the phosphonate units (8a–c)
was first examined with C5-phosphonate unit 8c to ver-
ify proper reaction conditions, since 8c was readily avail-
able (Table 1). When the Sonogashira reaction of 4 was
carried out with a slight excess of 8c in Et3N in the pres-
ence of PdCl2(PPh3)2 (2 mol%) and CuI (1 mol%) at
60 °C for 15 h according to the procedure reported by
Larock,16 desired coupling product 11 c17 was obtained
in a 58% yield along with a large amount of recovered 4
(36%) (run 1). Yield of 11c was not increased upon using
the Pd(0) catalysis under the same conditions (run 2). In
an effort to increase the yield, modified conditions using
Br
(a)
TMS
6
CF2PO3Et2
n
R
7a-c
8a-c
R=TMS
R=H
OTf
(c)
(b) or (d)
n
TMS
a: n=0; b: n=1; c: n=2
n=1
n=2
9
10
Scheme 4. Synthesis of x-alkynyldifluoromethylenephosphonate units.
Reagents and conditions: (a) BrZnCF2PO3Et2, CuBr, DMF, rt; (b)
TBAF, THF, AcOH; (c) LiCF2PO3Et2, THF, ꢀ78 °C; (d) TBAF,
THF.
Table 1. Sonogashira reaction of 4 with phosphonate units (P-units)a
O
Ns
POM
N
N
CuI
Pd-cat.
CF2PO3Et2
N=CHNMe2
N
+
n
4
60˚C
8a-c
n
CF PO3Et2
2
a: n=0; b: n=1; c: n=2
11a-c
Run
P-unit
Pd-cat.b
Et3N (equiv)
Solvent
11
Yield, %c
1
8c
8c
8c
8c
8c
8a
8a
8b
PdCl2(PPh3)2
Pd(PPh3)4
>60
>60
2
None
None
THF
11c
11c
11c
11c
11c
11a
11a
11b
58 (36)
22 (78)
15 (69)
41 (23)
69 (5)
2
3
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
4
2
DMF
MeCN
MeCN
MeCN
MeCN
5
2
6
2
39 (22)
59 (41)
66 (8)
7d
8
1.2
2
a All reactions were carried out in the presence of CuI (1 mol%) at 60 °C for 15 h unless otherwise stated.
b Two mol% of Pd-catalysis was utilized.
c Yields in parentheses refer to recovered 4.
d Reaction time was 1 h.

4176
Ns
S. Hikishima et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4173–4177
Table 2. Inhibitory properties versus calf spleen and human erythro-
cyte PNPs determined, if not otherwise indicated, in the presence of
1 mM phosphatea
O
N
O
H
POM
N
N
N
NR
(a),(b)
N
n
Compound
IC50
(nM)
IC50
(nM)
Kai pp
[nM]
Kai pp
(nM)
CHNMe2
N
N=CHNMe2
calf PNPb human calf PNP human
CF2PO3Et2
PNPb
PNP
n
CF2PO3Et2
12a-c
13a-c
R=POM
R=H
DFPP-G
DFPP-DG
18.7
10.2
—
20.2
20.4
173
9.5
—
6.9 0.7c 10.8 0.7
(c)
11a-c
4.4 0.6
—
8.1 0.6
O
H
nor-DFPP-DG
homo-DFPP-DG
DFPP-Gd
DFPP-DGd
—
N
NH
NH2
—
—
5.7 0.6
2.7 0.2
1.0 0.2
5.3 0.4
(d)
a: n=0; b: n=1; c: n=2
—
N
n
—
—
1.0 0.2
a All reactions were carried out in 50 mM Hepes buffer, pH 7.0, at
25 °C, with 7-methylguanosine as a variable substrate, and, if not
otherwise indicated, in the presence of 1 mM phosphate.
b With 25 lM 7-methylguanosine.
CF2PO3H2
14 (nor-DFPP-DG) n=0
15 (DFPP-DG) n=1
16 (homo-DFPP-DG) n=2
c Data from Ref. 5.
Scheme 5. Synthesis of DFPP-DG analogues. Reagents: (a) thiophe-
nol, K2CO3, DMF; (b) H2, Pd-C, MeOH–CHCl3; (c) NaOMe, MeOH;
(d) concd HCl.
d With 0.025 mM phosphate.
Table 2). By contrast, there are almost no differences
in inhibition parameters versus human erythrocyte
PNP in the case of DFPP-G and DFPP-DG. The shorter
chain in nor-DFPP-DG results in about nine-fold weaker
binding versus human PNP. However, the longer chain
in homo-DFPP-DG which allows more favorable inter-
actions with base-, pentose-, and phosphate-binding
sites yields IC50 two-fold lower than observed in the case
of DFPP-DG, 9.5 nM as compared with 20.4 nM (at
1 mM phosphate concentration, see Table 2). Also
apparent inhibition constant for homo-DFPP-DG is
lower than that observed for the DFPP-DG,
(5.3 0.4) and (8.1 0.6) nM.
representative solvents composed by 2 equivalents of
Et3N were examined (runs 3–6 and 8). This survey iden-
tified MeCN was a good solvent to induce a good yield
(69%) of 11c and minimize the recovered yield of 4 (run
5). Under these conditions, phosphonate units 8a and b
were coupled with 4 to give 11a and b in 39% and 66%
yields, respectively (runs 6 and 8). The yield of 11a was
significantly improved to 59%, when the coupling reac-
tion was carried out in the presence of a slight excess
of Et3N for 1 h (run 7). It should be noted that 11a
did not form upon conducting the coupling in a large
excess of the amine without the solvent, due to rapid
decomposition of 11a under these conditions.
DFPP-G and some of its analogues, with cyclic linkers
between the guanine and difluoromethylene phosphonic
acid groups, significantly slow down proliferation of T-
lymphocytes isolated from patients with autoimmune
thyroid disease—Hashimoto’s thyroiditis, compared to
the inhibitory effects on the growth of human blood T-
lymphocytes isolated from healthy donors.19 It is there-
fore very likely that the effect of the newly synthesized
compounds on the proliferation of autoimmune lym-
phocytes (from patients affected by Hashimoto’s thy-
roiditis and other autoimmune diseases) will be even
more significant. Such studies are now underway.
The compounds 11a–c thus obtained were readily trans-
formed to DFPP-DG and its homo- and nor-analogues
(Scheme 5). Thiophenol-mediated removal of the nosyl
protecting group for 11b, followed by hydrogenation,
gave 12b in a 90% yield. The compound 12b was treated
with NaOMe in MeOH to give 13b in an 82% yield.
Removal of the ethyl protecting group and the dimethy-
laminomethylene group for 13b was achieved in concd
HCl at reflux (17 h) to give DFPP-DG (15) as white sol-
ids in a 91% yield. The compounds 11a and c were,
respectively, transformed to nor-DFPP-DG (14) and
homo-DFPP-DG(16) by the same procedure as a series
of DFPP-DG in good overall yield.
Acknowledgments
Inhibitory activities of DFPP-DG and its analogues with
calf spleen and human erythrocyte PNPs were measured
as previously described for other PNP inhibitors5,18 and
evaluated in comparison with that of DFPP-G. The
results are summarized in Table 2.
This work was supported in part by a Grant-in-Aid for
Scientific Research (C) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and
by Polish Ministry of Science and Higher Education
Grant N301 003 31/0042.
DFPP-DG is about a two-fold more potent inhibitor of
calf spleen PNP than its counterpart with guanine base,
DFPP-G. The inhibition constant, as expected for the
multisubstrate analogue inhibitor competing with phos-
phate for the phosphate-binding site, decreases from
(4.4 0.6) nM at 1 mM phosphate concentration to
(1.0 0.2) nM at 0.025 mM phosphate concentration
(vs calf spleen and human erythrocyte enzymes, see
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3.66 (2H, td, JHF = 19.9 Hz, JHP = 7.2 Hz), 3.32, 3.09 (6H,
each s), 1.04 (6H, t, J = 8.6 Hz), 0.72 (9H, s); 31P NMR
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ꢀ47.28 (2F, dt,
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J = 7.9, 1.3 Hz), 8.01–7.92 (3H, m), 6.02 (2H, s), 4.23 (4H,
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JPF = 104.0 Hz); 19F NMR (376 MHz, DMSO-d6)
d
ꢀ50.74 (2F, dt, JFP = 104.0 Hz, JFH = 19.5 Hz); ESI-MS
m/z 743.2028 (calcd for C30H37F2N6O10PS: 743.2075).
Compound 11c: mp 115–121 °C; 1H NMR (400 MHz,
DMSO-d6) d 8.56 (1H, s), 8.32 (1H, dd, J = 7.9, 1.6 Hz),
8.10 (1H, dd, J = 7.7, 1.4 Hz), 8.00–7.91 (3H, m), 6.02
(2H, s), 4.19 (4H, dq, JHH = JHP = 7.2 Hz), 3.17, 2.99 (6H,
each s), 2.59 (2H, t, J = 6.8 Hz), 2.27 (2H, m), 1.80 (2H,
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ꢀ49.25 (2F, dt,
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JFP = 106.4 Hz,JFH = 20.4 Hz); ESI-MS m/z 757.2220
(calcd for C31H39F2N6O10PS: 757.2232). nor-DFPP-DG:
mp >300 °C; 1H NMR (400 MHz, D2O, NaOD) d 7.17
(1H, s), 2.65 (2H, t, J = 7.2 Hz), 2.10 (2H, m), 1.87 (2H,
m); 31P NMR (162 MHz, D2O, NaOD) d 7.24 (1P, t,
JPF = 87.7 Hz); 19F NMR (376 MHz, D2O, NaOD)
ꢀ47.80 (2F, dt, JFP = 87.7 Hz, JFH = 21.4 Hz). ESI-MS
m/z 323.0717 (calcd for C10H13F2N4O4P: 323.0720); UV
0.1 N HCl 229 (23,000), 274 (13,600), pH 7.0 Hepes buffer
230 (23,100), 273 (10,500), 0.1 N NaOH 230 (24,600) 265
(7100), 287 (6200). DFPP-DG: mp >287 °C; 1H NMR
(400 MHz, D2O, NaOD) d 7.15 (1H, s), 2.62 (2H, t,
J = 7.0 Hz), 2.01 (2H, m), 1.69–1.62 (4H, m); 31P NMR
(162 MHz, D2O, NaOD) d 7.33 (1P, t, JPF = 87.7 Hz); 19
NMR (376 MHz, D2O, NaOD)
ꢀ47.96 (2F, dt,
F
d
JFP = 87.7 Hz, JFH = 21.4 Hz); ESI-MS m/z 337.0856
(calcd for C11H15F2N4O4P: 337.0877); UV 0.1 N HCl
230 (17,000), 274 (12,700), pH 7.0 Hepes buffer 231
(17,500), 273 (10,100), 0.1 N NaOH 231 (18,400), 267
(6800), 285 (6100); pKa1 = 5.1, pKa2 = 10.6 (from spectro-
photometric titrations). homo-DFPP-DG: mp >300 °C; 1H
NMR (400 MHz, D2O, NaOD) d 7.10 (1H, s), 2.56 (2H, t,
J = 7.4 Hz), 1.98 (2H, m), 1.64–1.55 (4H, m), 1.40 (2H, m);
31P NMR (162 MHz, D2O, NaOD) d 7.35 (1P, t,
JPF = 88.0 Hz); 19F NMR (376 MHz, D2O, NaOD) d
ꢀ47.80 (2F, dt, JFP = 88.0 Hz, JFH = 21.4 Hz). ESI-MS m/
z 351.1047 (calcd for C12H17F2N4O4P: 351.1033); UV, due
to poor solubility extinction coefficients for this analogue
were not determined; for calculation of the inhibitor
concentration, extinction coefficients obtained for DFPP-
DG were used.
12. Yokomatsu, T.; Ichimura, A.; Kato, J.; Shibuya, S.
Synlett 2001, 287.
13. For a review of fluorinated phosphonates: Romanenko,
V.; Kukhar, V. P. Chem. Rev. 2006, 106, 3868.
14. AcOH was necessary to induce a good yield; large
amounts of enyne products were produced in the absence
of AcOH.
15. (a) Berkowitzt, D. B.; Eggen, M.-J.; Shen, Q.; Sloss, D. G.
J. Org. Chem. 1993, 58, 6174; (b) Shen, Q.; Sloss, D. G.;
Berkowitz, D. B. Synth. Commun. 1994, 24, 1519, Berko-
witzt reported coupling reactions of LiCF2PO3Et2 with
triflates proceeded well in THF in the presence of HMPA.
However, yields of 7b and 7c were found to decrease upon
using HMPA as a co-solvent.
16. Zhang, H.; Larock, R. C. J. Org. Chem. 2002, 67, 7048.
17. All new compounds gave satisfactory spectroscopic and
analytical data. Compound 11a: mp 132–137 °C; 1H NMR
(400 MHz, DMSO-d6) d 9.86 (1H, s), 9.34 (1H, dd,
J = 9.6, 1.7 Hz). 9.29 (1H, dd, J = 9.4, 1.1 Hz), 9.18–9.07
(3H, m), 6.76 (2H, s), 4.61 (4H, dq, JHH = JHP = 8.6 Hz),
18. (a) Bzowska, A. Biochim. Biophys. Acta 2002, 1596, 293;
(b) Wierzchowski, J.; Bzowska, A.; Ste˛pniak, K.; Shugar,
D. Z. Naturforsch. 2004, 59c, 713; (c) Wielgus-Kutrowska,
B.; Bzowska, A. Biochim. Biophys. Acta 2006, 1764, 887.
ˇ
19. Glavas-Obrovac, L.; Suver, M.; Hikishima, S.; Yokoma-
tsu, T.; Bzowska, A. Nucleosides Nucleotides and Nucleic
Acids (in press).