Synthesis and antimicrobial activities of His(2-aryl)-Arg and Trp-His(2-aryl) classes of dipeptidomimetics

Article abstract of DOI:10.1039/c4md00041b

In this communication, we report the design, synthesis and in vitro antimicrobial activity of ultra short peptidomimetics. Besides producing promising antibacterial activities against Staphylococcus aureus and methicillin-resistant S. aureus (MRSA), the dipeptidomimetics exhibited high antifungal activity against C. neoformans with IC₅₀ values in the range of 0.16-19 μg mL^-1. The most potent analogs exhibited 4-fold higher activity than the currently used drug amphotericin B, with no apparent cytotoxicity in a panel of mammalian cell lines. This journal is the Partner Organisations 2014.

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Synthesis and Antimicrobial Activities of His(2ꢀaryl)ꢀArg and Trpꢀ
His(2ꢀaryl) Classes of Dipeptidomimetics
Amit Mahindra,a Krishna K. Sharma,a Dinesh Rathore,a Shabana. I. Khan,b Melissa R. Jacob,b and Rahul
5
Jain*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
many invasive fungal infections.12 These drugs have markedly
In this communication, we report the design, synthesis and in
changed the approach to antifungal therapy, sometimes even
allowing oral treatment of chronic mycoses.
vitro antimicrobial activity of ultra short peptidomimetics.
10 Besides producing promising antibacterial activities against
Staphylococcus aureus and Methicillinꢀresistant S. aureus
(MRSA), the dipeptidomimetics exhibited high antifungal
activity against C. neoformans with IC50 values in the range of
0.16ꢀ19 ꢁg/mL. The most potent analogs exhibited 4ꢀfold
15 higher activity than the currently used drug amphotericin B,
with no apparent cytotoxicity in a panel of mammalian cell
lines.
50 Unfortunately, the present repertoire of antifungal agents is
limited, particularly in comparison to the number of agents
available for bacterial infections.13 In fact, it took 30 years for the
newest class of antifungal drugs, the echinocandins, to progress
from benchꢀtoꢀbedside.14 Furthermore, it is sobering to consider
55 that the gold standard therapy for cryptococcal meningitis is
based on medications (amphotericin B and flucytosine) that were
discovered nearly 50 years ago. Although no ‘‘offꢀtheꢀshelf’’
antifungal drugs have emerged from ‘‘repurposing’’ studies, the
antifungal scaffolds with known pharmacological properties
60 could serve as useful leads for further development.15
Over the past several years, peptides form the basis for a vast
majority of antiꢀinfective therapies in current clinical use.16 For
example, broadꢀspectrum antimicrobial peptide pexiganan (a 22ꢀ
amino acid membrane disruptor analog of the Xenopus peptide
65 magainin), used for the topical treatment of diabetic foot ulcers
has reached phase III in clinical trial.17 A synthetic mimic of
indolicidin, named omiganan has reached in clinical trial phase
II.18 Novexatin, the lead product of NovaBiotics, UK, a cyclic
and highly cationic (arginineꢀrich) peptide based on human α and
70 βꢀdefensins (among others), targets stubborn fungal infections in
toenails.19 Other wellꢀknown peptides in various stages of clinical
trials includes, OPꢀ145, NVB302 and arenicin.20 Some of the
challenges facing these peptideꢀbased drugs are poor metabolic
stability, oral bioavailability, membrane permeability and high
75 production costs.
One possible answer for these problems is to design peptides of
shorter length, while keeping the essential pharmacophore intact.
In this direction, Svendsen and coꢀworkers have synthesized a
range of peptides of variable chain length.21 More recently, they
80 disclosed a short synthetic peptidomimetic, LTXꢀ109 exhibiting
potent antimicrobial activity.22ꢀ23 In the recent past, we have
reported the dipeptides having the motifs HisꢀArg and TrpꢀHis as
potent antimicrobial agents and tripeptides ArgꢀHis(2ꢀaryl)ꢀArg
as potent antifungal agents.24ꢀ25 Keeping these facts in mind, we
85 developed a series of dipeptides without increasing the length of
the lead peptide. In this regard, we kept the arginine and
The invasive antimicrobial infections are devastating, and often
classified as opportunistic or primary.1 Opportunistic infections
20 develop mainly in the immunocompromised hosts whereas
primary infections can develop in the immunocompetent hosts.2
The causes of immunocompromisation include AIDS, azotemia,
diabetes mellitus, bronchiectasis, emphysema, TB, lymphoma,
leukemia, other hematologic cancers, burns, and therapy with
25 corticosteroids or immunosuppressants.
Many fungi are opportunistic, which are usually not pathogenic
except in an immunocompromised host.3 Despite stateꢀofꢀtheꢀart
antifungal therapy, the mortality rates for invasive infections with
the three most common species of human fungal pathogens are —
30 Candida albicans (20ꢀ40%),4 Aspergillus fumigatus (50ꢀ90%),5
and Cryptococcus neoformans (20ꢀ70%).6 C. neoformans is one
of the leading causes of opportunistic fungal infections in
immunocompromised individuals worldwide.7 Cryptococcus is
responsible for over 625,000 deaths annually within a growing
35 cohort of susceptible individuals, particularly the AIDS
population;8 therefore, the need for new agents to target its
growing threat is vital.
The few available antimycotic agents target a limited repertoire of
fungalꢀspecific cell wall or membrane components, and have high
40 toxicity, and poor efficacy.9 Drugs for the systemic antifungal
treatment include, amphotericin B (and its lipid formulations),
various azole derivatives, echinocandins, and flucytosine.10
Amphotericin B, an effective but relatively toxic drug, has long
been the mainstay of antifungal therapy for invasive and serious
45 mycoses.11 However, newer potent and less toxic triazoles and
echinocandins are now often recommended as firstꢀline drugs for
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DOI: 10.1039/C4MD00041B
tryptophan residues intact in the respective dipeptide motif, and
coupling of Lꢀarginine (1) with benzylamine using 1,1’ꢀ
modified the histidine residue by placing a substitution at the Cꢀ2 35 carbonyldiimidazole (CDI) in water as described earlier.30
Lꢀ
position of the imidazole ring. It was reasoned that aryl
substitution at the Cꢀ2 would provide the required bulk and
hydrophobicity for membrane insertion without increasing the
overall sequence length. The general scaffolds of the designed
Arginine benzylamide (2) upon solventꢀfree reaction with BocꢀLꢀ
His(2ꢀAr)ꢀOH (3aꢀg) using a coupling reagent combination of
DIC, DIEA and HONB at 60 oC for 15 min under MW irradiation
gave protected dipeptides 4aꢀg. The removal of Boc group using
5
peptides are shown in Figure 1. A wide variety of lipophilic aryl 40 aqueous 3N HCl at ambient temperature for 15 min cleanly
substituents at the Cꢀ2 position of ꢀhistidine were explored. To afforded the designed dipeptides 5aꢀg (Scheme 1).
observe the effect of Cꢀterminus capping four series of peptides ꢀArginine methyl ester dihydrochloride (6) required for the
synthesis of 8aꢀg was obtained by the reaction of ꢀarginine (1)
L
L
10 were synthesized having NHBzl group and OMe group at the Cꢀ
terminus.
L
o
with anhydrous HCl gas at 4 C in methanol. Compound 6 was
45 first neutralized in situ using DIEA as a base and then subjected
to coupling with BocꢀHis(2ꢀAr)ꢀOH using a coupling reagents
o
combination of DIC, and HONB at 60 C for 15 min under MW
irradiation to afford dipeptides 7aꢀg. The removal of Boc group
as described above afforded the designed dipeptides 8aꢀg
50 (Scheme 2).
Fig. 1 Generalized structures of the synthesized dipeptides
Starting materials NꢀαꢀBocꢀ2ꢀarylꢀ
15 the synthesis of series 1ꢀ4 peptides, were obtained
regioselectively from Nꢀαꢀtrifluoroacetylꢀ ꢀhistidine methyl ester
Lꢀhistidines 3aꢀg required for
L
by a recently developed homolytic free radical reaction using
arylboronic acids.26 The synthesis of designed peptides was
accomplished using a recently developed environmentally benign
20 microwave (MW) assisted peptide synthesis protocol under
solventꢀfree conditions.27ꢀ29 This method provides
a new
paradigm in solventꢀfree peptide synthesis assisted by microwave
irradiation, using DIC–HONB as the coupling reagents
combination. Key features of this original protocol are solventꢀ
25 free synthesis, very short reaction time and racemizationꢀfree
synthesis in high purity. To confirm the racemization
Scheme 2. Synthesis of His(2ꢀaryl)ꢀArgꢀOMe (8aꢀg, series 2)
NꢀαꢀBocꢀ2ꢀarylꢀ
with benzylamine in the presence of DIC and HONB afforded Nꢀ
55 αꢀBocꢀ2ꢀarylꢀ ꢀhistidine benzylamides (9aꢀg). Compounds 9aꢀg,
upon acidolysis afforded the salts of 2ꢀarylꢀ ꢀhistidine
Lꢀhistidines 3aꢀg upon condensation reaction
predicament, BocꢀLꢀHisꢀArgꢀOMe, BocꢀDꢀHisꢀArgꢀOMe, and
L
BocꢀD,LꢀHisꢀArgꢀOMe were synthesized under solventꢀfree MW
irradiation. As evident from the HPLC chromatograms, purified
30 peptides were free of racemization (see Supporting Information).
L
benzylamide 10aꢀg, which were neutralized in situ with DIEA
and then coupled with BocꢀTrpꢀOH using DIC and HONB at 60
oC for 15 min under MW irradiation to give protected dipeptides
60 11aꢀg. The latter compounds 11aꢀg upon acidolysis afforded
desired peptides 12aꢀg (Scheme 3).
Scheme 3. Synthesis of TrpꢀHis(2ꢀaryl)ꢀNHBzl (12aꢀg, series 3)
Scheme 1. Synthesis of His(2ꢀaryl)ꢀArgꢀNHBzl (5aꢀg, series 1)
The synthesis of
Lꢀarginine benzylamide (2) was achieved by
2
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In similar fashion, 2ꢀarylꢀ
were obtained by passing HCl gas to a mixture of 2ꢀarylꢀ
histidine (13aꢀg) in methanol at 4 C for 2h followed by in situ
neutralization using DIEA. The coupling of latter compounds
with BocꢀTrpꢀOH using DIC, and HONB at 60 C for 15 min
under MW irradiation to give protected dipeptides 15aꢀg, which
upon acidolysis produced 16aꢀg (Scheme 4).
Lꢀhistidine methyl esterꢁ2HCl (14aꢀg)
Candida species, A. fumigatus and E. coli (results not included).
The minimum inhibitory concentration (MIC) was measured
using a protocol suggested by the Clinical and Laboratory
Standard Institute (previously known as the National Committee
20 for Clinical Laboratory Standards, NCCLS).31 Amphotericin B
and ciprofloxacin served as positive controls in these studies.32
The results of antifungal evaluation of His(2ꢀaryl)ꢀArg peptides
(Series 1 and 2) against C. neoformans are shown in Table 1. In
general, we observed that the peptides with an NHBzl group at
25 the Cꢀterminus are more potent compared to their counterparts
having a methyl ester linkage. For example, peptide 5e (Ar = 4ꢀ
tertꢀbutylphenyl, R1= NHBzl) displayed a much lower IC50 value
of 0.16 ꢂg/mL as compared to peptide 8e (Ar = 4ꢀtertꢀ
butylphenyl, R1= OMe) exhibiting IC50 value of 10.16 ꢂg/mL.
30 This difference in potency appears to be a consequence of
enhanced hydrophobicity imparted by the NHBzl group. We also
L
ꢀ
o
o
5
examined the effect of substitution at the Cꢀ2 position of Lꢀ
histidine residue. It is noteworthy that peptides, which contained
bulky substituents like 4ꢀtertꢀbutylphenyl, biphenyl and naphthyl
35 groups (5e, 5f, 5g) exhibited high activity against C. neoformans
with IC50 values in the range of 0.16ꢀ0.62 ꢂg/mL. The most
potent peptide 5e, which contained 4ꢀtertꢀbutylphenyl at the Cꢀ2
position of imidazole of histidine and NHBzl group at the Cꢀ
terminus exhibited IC50, MIC, MFC values of 0.16 and 0.31
40 ꢂg/mL, respectively. The activity of 5e is >4ꢀfold higher than
amphotericin B (IC50 = 0.69 µg/mL, MIC = 1.25 µg/mL, MFC =
1.25 µg/mL).
Scheme 4. Synthesis of TrpꢀHis(2ꢀaryl)ꢀOMe (16aꢀg, series 4)
10 Both free and Bocꢀprotected peptides (4ꢀ5, 7ꢀ8, 11ꢀ12 and 15ꢀ16)
were evaluated for in vitro growth inhibition activity against
fungal (Candida albicans, C. glabrata, C. krusei, Aspergillus
fumigatus and C. neoformans) and bacterial (Escherichia coli,
Staphylococcus aureus and methicillinꢀresistant S. aureus) strains
15 (Tables 1ꢀ3). All the peptides were found to be inactive against
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Table 1. Antiꢀcryptococcal activity of dipeptides (Series 1 and 2)
Peptide
Ar
R1
R2
C. neoformansd
Cytotoxicitye
Selectivity Indexf
a
IC50
MICb
MFCc
C. neoformans
CTX (µg/mL)
4a
4b
4c
4d
4e
4f
4g
5a
5b
5c
5d
5e
5f
5g
7a
7b
7c
7d
7e
7f
7g
8a
8b
8c
8d
8e
8f
H
C6H5
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Boc
Boc
Boc
Boc
Boc
Boc
Boc
H
H
H
H
H
NA
NA
5
10
NA
5
10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
–
2.56
6.04
5.77
2.42
1.09
2.79
NA
1.22
2.11
NA
0.16
0.2
0.62
NA
16.58
16.94
NA
>3.9
>1.7
>1.7
>4.1
>9.2
>3.6
–
>8.2
>4.7
–
>62.5
>50
>16.1
–
–
–
–
–
>2.0
>8
–
–
–
–
–
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
10
10
2.50
2.5
5
2.50
2.5
5
NA
2.5
2.50
NA
0.31
0.31
1.25
NA
NA
NA
NA
NA
10
2.5
NA
NA
NA
NA
20
NA
2.5
2.50
NA
0.31
0.31
1.25
NA
NA
NA
NA
NA
10
2.5
NA
NA
NA
NA
20
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
H
H
Boc
Boc
Boc
Boc
Boc
Boc
Boc
H
H
H
H
H
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
NA
4.91
1.25
NA
10.56
NA
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
OMe
OMe
OMe
OMe
NA
10.16
5.49
1.38
0.69
H
H
10
2.5
1.25
10
2.5
1.25
>1.8
>7.3
8g
OMe
Amphotericin B
b
aIC50 is the concentration (ꢂg/mL) that affords 50% inhibition of growth; MIC (Minimum Inhibitory Concentration) is the lowest test concentration
c
(ꢂg/mL) that allows no detectable growth; MFC (Minimum Fungicidal Concentration) is the lowest test concentration (ꢂg/mL) that kills the organism;
dHighest tested concentration was 20 ꢂg/mL; ehighest tested concentration was 10 ꢂg/mL; fselectivity index was calculated as CTX divided by IC50 values
5
for C. neoformans. For cytotoxicity experiments, peptides the CTX value was set to >10 ꢂg/mL, in order to calculate a selectivity index. NA, not active.
The relatively less bulky peptide 5f (Ar = biphenyl, R1= NHBzl)
showed the second highest antifungal potency against
Cryptococcus with IC50 value of 0.20 ꢂg/mL, and MIC and MFC
value of 0.31 ꢂg/mL. While, peptide 5g (Ar = naphthyl, R1=
Apart from promising activities against C. neoformans, the
peptides also showed encouraging activity against S. aureus and
methicillinꢀresistant S. aureus (MRSA) as shown in Table 3.
Analogs 4e, 4f and 12f produced promising activity against S.
10 NHBzl) showed activity (IC50 = 0.62 ꢂg/mL, MIC = 1.25 ꢂg/mL, 30 aureus with IC50 values of 5.95, 7.30 and 2.60 ꢂg/mL,
MFC = 1.25 ꢂg/mL) comparable to that of amphotericin B. Other
peptides of the series, 5b (Ar = phenyl, R1= NHBzl) and 5c (Ar =
tolyl, R1= NHBzl) showed good activity with IC50 values of 2.20
and 1.22 ꢂg/mL, respectively. Surprisingly, peptide 5d (Ar =
respectively. At the same time, analogs 12f and 4e were also
effective against MRSA with IC50 values of 4.31 and 9.32 ꢂg/mL,
respectively.
All synthesized peptides were also evaluated for cytotoxicity in a
15 anisolyl, R1= NHBzl) was inactive against C. neoformans. The 35 panel of mammalian cell lines to determine their safety profile.
remaining peptides 4bꢀ4g, 7bꢀ7c, 7fꢀ7g, 8b and 8eꢀ8g also
showed promising activity with IC50s in the range of 1ꢀ17 ꢂg/mL.
The results for the TrpꢀHis(2ꢀaryl) peptides (Series 3 and 4) are
shown in Table 2. These peptides in general are less active
The in vitro cytotoxicity was determined against four human
cancer cell lines (SKꢀMEL, KB, BTꢀ549, and SKꢀOVꢀ3) and two
noncancerous mammalian cells (VERO and LLCꢀPK1) by neutral
red uptake assay.33 The results demonstrated that the synthesized
20 against C. neoformans as compared to His(2ꢀaryl)ꢀArg class of 40 peptides were nonꢀtoxic up to a concentration of 10 ꢂg/mL,
peptides. A possible explanation for this observation is significant
reduction in cationicity of peptides due to the incorporation of
Trp residue. In nutshell, the most potent peptides from these
series have exhibited activity against C. neoformans with IC50
25 values in the range of 0.54ꢀ19 ꢂg/mL.
which is indicative of a higher selectivity index for some
compounds and their safety against mammalian cells.
45
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Table 2. Antiꢀcryptococcal activity of dipeptides (Series 3 and 4)
Peptide
Ar
R1
R2
C. neoformansd
Cytotoxicitye
Selectivity Indexf
a
IC50
MICb
MFCc
C. neoformans
CTX (µg/mL)
11a
11b
11c
11d
11e
11f
11g
12a
12b
12c
12d
12e
12f
12g
15a
15b
15c
15d
15e
15f
15g
16a
16b
16c
16d
16e
16f
H
C6H5
NHBzl
NHBzl
NHBzl
NHBzl
Boc
Boc
Boc
Boc
Boc
Boc
Boc
H
H
H
H
H
NA
NA
NA
NA
NA
NA
NA
NA
2.04
7.84
10.82
4.53
0.54
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
5
NA
NA
NA
NA
NA
NA
NA
NA
5
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
–
–
–
–
–
–
–
–
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4 NHBzl
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
NHBzl
>4.9
>1.3
–
>2.21
>18.5
–
–
–
–
–
–
–
–
–
>1.3
20
10
10
NT
NA
10
4ꢀC(CH3)3ꢀC6H4 NHBzl
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
NHBzl
NHBzl
OMe
OMe
OMe
H
H
1.25
NA
NA
NA
NA
NA
NA
NA
NA
NA
20
NA
NA
NA
10
NA
1.25
1.25
NA
NA
NA
NA
NA
NA
NA
NA
NA
20
NA
NA
NA
20
NA
1.25
Boc
Boc
Boc
Boc
Boc
Boc
Boc
H
H
H
H
H
OMe
4ꢀC(CH3)3ꢀC6H4 OMe
18.68
NA
NA
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
H
C6H5
4ꢀCH3ꢀC6H4
4ꢀOCH3ꢀC6H4
4ꢀC(CH3)3ꢀC6H4 OMe
4ꢀC6H5ꢀC6H4
1ꢀNaphthyl
OMe
OMe
OMe
OMe
OMe
OMe
NA
7.97
11.88
NA
18.84
3.82
17.3
0.69
–
–
–
>2.6
–
OMe
OMe
H
H
16g
Amphotericin B
b
aIC50 is the concentration (ꢂg/mL) that affords 50% inhibition of growth; MIC (Minimum Inhibitory Concentration) is the lowest test concentration
c
(ꢂg/mL) that allows no detectable growth; MFC (Minimum Fungicidal Concentration) is the lowest test concentration (ꢂg/mL) that kills 100% of the
organism; dHighest tested concentration was 20 ꢂg/mL; e Highest tested concentration was 10 ꢂg/mL; fselectivity index was calculated as CTX divided by
IC50 values for C. neoformans. For cytotoxicity experiments, peptides the CTX value was set to >10 ꢂg/mL, in order to calculate a selectivity index. NA,
not active. NT, not tested.
5
Table 3. Antibacterial activity of peptides against S. aureus and MRSA
Peptide
S. aureus
Methicillinꢀresistant S. aureus (MRSA)
IC50
MIC
MBCa
IC50
MIC
MBCa
4e
5e
4f
5f
5.95
15.86
7.30
10
NA
10
NA
20
NA
NA
5
NA
NA
20
NA
20
NA
NA
20
NA
NA
0.50
9.32
NA
8.97
NA
13.20
NA
NA
4.31
NA
NA
20
NA
20
NA
20
NA
NA
10
NA
NA
NA
NA
NA
NA
NA
20
12.84
16.37
18.98
15.06
2.60
11.65
10.15
0.08
4g
15f
15e
12f
12e
11e
Cipro
20
NA
0.25
NA
NA
0.25
NA
NA
0.50
0.09
a MBC (Minimum Bactericidal Concentration) is the lowest test concentration (ꢂg/mL) that kills 100% of the organism. NA, not active.
10 To measure the hydrophobicity of the synthesized peptides (series than their amidated counterparts. From Table 4, we can clearly
1 and 2) ClogP values were measured using ACD labs 12 15 understand the hydrophobic nature of various aryl substituents
software and compiled in Table 4. As expected, dipeptides having
and their effect on the activity against C. neoformans. From the
an ester linkage at Cꢀterminus were found to be less hydrophobic
values it is indicated that the high hydrophobic character was
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DOI: 10.1039/C4MD00041B
† Electronic Supplementary Information (ESI) available: [Experimental
procedures, spectral data, HPLC chromatogarms, and details of biological
assay]. See DOI: 10.1039/b000000x/
imparted by the 4ꢀtertbutylphenyl among all the incorporated aryl
groups at the Cꢀ2 position of ꢀhistidine. From the results, it can
L
be concluded that among the biaryl groups (biphenyl and
naphthyl), biphenyl imparts more hydrophobicity as compared to
naphthyl group. In conclusion, peptide 5e being most
hydrophobic in nature displays highest activity with IC50 value of
0.16 µg/mL against C. neoformans and the results are in good
correlation.
45
50
1. D. M. Dixon, M. M. McNeil, M. L. Cohen, B. G. Gellin and J.
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Hsueh, J. Infect. Chemother., 2008, 14, 77ꢀ85.
10 Table 4. Correlation of hydrophobicity (ClogP) with antiꢀ
cryptococcal activity
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26, 334ꢀ338.
Peptide
ClogP
C. neoformans IC50 (ꢂg/mL)
55
5a
5b
5c
5d
5e
5f
5g
8a
8b
8c
8d
8e
8f
ꢀ1.36
0.68
1.14
0.85
2.37
2.33
1.92
ꢀ2.51
ꢀ0.47
ꢀ0.01
ꢀ0.30
1.22
1.18
0.76
NA
1.22
2.11
NA
0.16
0.2
0.62
NA
10.56
NA
60
65
NA
10.16
5.49
1.38
8g
12. A. EspinelꢀIngroff, J. Clin. Microbiol., 1998, 36, 2950ꢀ2956.
13. H. Jenssen, P. Hamill and R. E. W. Hancock, Clin. Microbiol.
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70
14. D. W. Denning, J. Antimicrob. Chemother., 2002, 49, 889ꢀ
891.
Conclusions
15. A. Butts and D. J. Krysan, PLoS Pathog., 2012, 8, e1002870.
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Drug Des., 2013, 81, 136ꢀ147.
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Wexler and M. Zasloff, Antimicrob. Agents Chemother., 1999,
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21. M. B. StrÖm, Ø. Rekdal and J. S. Svendsen, J. Pept. Sci.,
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In summary we have prepared four series of dipeptides that were
15 based on the pharmacophore model of short antimicrobial
peptides. The peptides exhibited potent antifungal activity against
C. neoformans. The results also demonstrated that the peptides of
His(2ꢀaryl)ꢀArg class are more potent compared to TrpꢀHis(2ꢀ
aryl) class owing to a delicate balance required between
20 hydrophobicity and hydrophilicity in the peptidic structure. A
combination of dual hydrophobicꢀhydrophilic amino acid (His),
highly hydrophilic Arg residue and placement of NHBzl group at
the Cꢀterminus appeared to be ideal for strong antimicrobial
activity.
75
80
85
25 Acknowledgment
90
Amit Mahindra thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi for the award of Senior Research
Fellowship. The authors wish to thank Ms. Marsha Wright and
Mr John Trott for biological testing. This work was supported by
30 the NIH, NIAID, Division of AIDS, Grant No. AI 27094
(antifungal) and the USDA Agricultural Research Service
25. Mahindra, A.; Bagra, N.; Wangoo, N.; Khan, S. I.; Jacob, M.
R.;
(DOI: 10.1021/ml500011v)
26. A. Mahindra and R. Jain, Synlett, 23, 1759ꢀ1764.
Jain,
R.
ACS
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Chem.
Lett.
2014,
95
27. A. Mahindra, K. K. Sharma and R. Jain, Tetrahedron Lett.
2012, 53, 6931ꢀ6935.
28. A. Mahindra, K. Nooney, S. Uraon, K. K. Sharma and R. Jain,
RSC Adv., 2013, 3, 16810ꢀ16816.
29. A. Mahindra, N. Patel, N. Bagra and R. Jain, RSC Adv. 2014,
4, 3065ꢀ3069.
30. R. K. Sharma and R. Jain, Synlett, 2007, 603ꢀ606.
31. Reference Method for Broth Dilution Antifungal
Susceptibility Testing of Yeasts: Approved Standard, 2nd ed.;
NCCLS document M27ꢀA2; National Committee for Clinical
Laboratory Standards: Wayne, PA, 2002.
32. S. Kagan, D. Ickowicz, M. Shmuel, Y. Altschuler, E. Sionov,
M. Pitusi, A. Weiss, S. Farber, A. J. Domb and I. Polacheck,
Antimicrob. Agents Chemother., 2012, 56, 5603ꢀ5611.
Specific
Cooperative
Agreement
No.
58ꢀ6408ꢀ1ꢀ603
(antibacterial).
100
105
110
Notes and references
35 a*Department of Medicinal Chemistry, National Institute of
Pharmaceutical Education and Research, Sector 67, S. A. S. Nagar,
Punjab 160 062, India Corresponding author. Tel.: +91 (172) 2292024;
Fax: +91 (172) 2214692; E-mail: rahuljain@niper.ac.in
bNational Center for Natural Products Research, School of Pharmacy,
40 The University of Mississippi, University, Mississippi 38677, USA
6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 7 of 8
Medicinal Chemistry Communications
View Article Online
DOI: 10.1039/C4MD00041B
33. Borenfreund, E.; Babich, H.; MartinꢀAlguacil, N. In Vitro Cell
Dev. Biol. 1990, 26, 1030ꢀ1034.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

Medicinal Chemistry Communications
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DOI: 10.1039/C4MD00041B
Synthesis and Antimicrobial Activities of His(2-aryl)-Arg and
Trp-His(2-aryl) Classes of Dipeptidomimetics
Amit Mahindra, Krishna K. Sharma, Dinesh Rathore, Shabana. I. Khan, Melissa R. Jacob, and
Rahul Jain*
H
N
O
H
N
H2N
N
H
O
N
N
HN
HN
HN
N
O
O
H
H
N
N
H2N
N
H2N
N
Amphotericin B. IC50 = 0.69 µg/mL;
MIC = MFC = 1.25 µg/mL
H
H
<
<
>
O
O
HN
HN
HN
HN
NH2
NH2
5e. IC50 = 0.16 µg/mL;
MIC = MFC = 0.31 µg/mL
5f. IC50 = 0.2 µg/mL;
MIC = MFC = 0.31 µg/mL
12f. IC50 = 0.54 µg/mL;
MIC = MFC = 1.25 µg/mL