|
|
|
|
|
| |
 |
|
| |
| |
N-Phthaloyl-L-Glutamic
anhydride
A chiral γ-L-Glutamyl
transfer reagent
|
| |
ABSTRACT
A readily available γ-glutamyl transfer reagent, namely,
N-phthaloyl-L-glutamic anhydride (NPGLA) offers a simple
solution for the racemization free synthesis of γ-glutamyl
amino acids with usefully enhanced properties
γ-glutamyl dipeptides (γ-Glu AA) are defined in this context
as the compounds derived by the acylation of an amino acid
(AA) through the γ-carboxyl carbon of L-glutamic acid. The
resulting amide linkage has sometimes been referred as a
pseudo-peptide bond. The general structure of a typical
γ-glutamyl dipeptide is given
in Figure 1. Interest in
γ-Glutamyl peptides and
analogs has persisted for
long (1). The aim of this short
review is to highlight the
useful improvement of
properties of the amino acid
which is γ-glutamylated. Also
a brief review of the chemistry leading to γ-glutamyl dipeptide
in a practical and efficacious way requiring the least number
of protection/deprotection steps is presented.
|
| LFigure
1. γ-Glutamyl amino acid
(γ-Glu AA or γ-Glutamyl dipeptide). |
Enzymes belonging to the class called γ-glutamyl transpeptidases
lead either to the formation of this pseudopeptide bond or
the hydrolysis of the γ-glutamyl dipeptide into its constituent
amino acids depending on conditions.
IMPACT OF γ-GLUTAMYL SUBSTRUCTURE
ON PROPERTIES OF AMINO ACIDS
Suzuki et al found (2) that the bitter taste of valine was highly
attenuated on transformation to its γ-glutamyl derivative.
Similar glutamylation has been carried out on isoleucine
and tyrosine.
|
Figure 2. γ-Glutamylvaline
(γ- Glu Val). |
| |
|
Figure 3. NÂ-(γ-Glutamyl)lysine. |
Ne-(γ-Glutamyl)lysine (Figure
3) is important amino acid
component forming part of
food products. Nutritional
improvement of bread has
been noticed with Ne-(γ-
Glutamyl)lysine (3) wherein.
Ne-(γ-Glutamyl)lysine serving
as a nutritional source of lysine.
In another study it was found
that scanning electron
microscopy indicated that the
physical properties of dry
noodles were improved through
the formation of cross-links, Ne-(γ-Glutamyl)lysine, brought
about by MTGase (4).
Molecules (Figure 2 and Figure 4) possessing the properties
of mouthfulness and thickness and increasing continuity of
food taste perception have been termed by the Japanese as
"kokumi" flavor compounds and such properties are ostensibly
conferred by the γ-glutamyl residue on the amino acids (5).
Glutathione, itself a γ-glutamyl peptide, γ-Glu-Cys-Gly, is
also a "kokumi" compound. Another mouthfulness enhancing
"kokumi" compound with a γ-glutamyl substructure is
γ-Glutamyl-trans-S-propenyl-L-cysteine sulfoxide (Figure 5)
occurring in garlic and onion.
|
Figure 4. γ-Glutamylleucine and Homoglutathione. |
| |
|
Figure 5. γ-Glutamyl-trans-S-propenyl-Lcysteine
sulfoxide. |
Occurrence of these
γ-glutamyl "kokumi"
compounds attest to
the use of beans,
garlic, onion being
served often with
meat dishes for
enhancing the
sensory perception
as well as a longer lasting food sensation. |
| |
|
| Figure 6. γ-Glutamyl-Se-methyl-Lselenocysteine. |
Selenium amino acids rank among the top of the list on
selenium supplements. Selenium has been recognized as
an essential micronutrient the lack of which leads to various
disease states (6), ascribed to the deficiency of selenium
containing amino acids in the body. Certain selenium
compounds are offensive in smell and this property may
become a deterrent factor for oral and topical use. For
example, while Se-Methyl-L-selenocysteine has an odor
associated with it, the γ-glutamyl derivative is practically
odorless (Figure 6). Still
the biological characteristics
of Se-methyl-Lselenocysteine
are not
considerably changed
by this structural variation.
It has been
well recognized that
γ-glutamyl-Se-methyl-L
selenocysteine is a carrier molecule as effective
as its parent, Se-methyl-L-selenocysteine, in its
anticancerous properties (7). This improvement
of olfactory property has another important
application in cosmetics. While the not-so-acceptable
smell of some of the selenium compounds may
have contributed to a slightly muted acceptance
of selenium compounds in cosmetic industry,
olfactory improvement by γ-glutamylation should
lead to a favorable acceptance of these compounds by the
industry.
|
Figure 7. Typical Sequence for γ-glutamylation of amino acids. |
| |
|
Figure 8. N-phthaloyl-Lglutamic
anhydride
(NPGLA). |
|
| |
γ-GLUTAMYLATION AND PHYSICAL PROPERTIES |
| |
Since γ-glutamylation introduces more polar functional groups
with hydrophilic characteristics, it can be anticipated that
the solubility of γ-glutamyl amino acid in water may be
enhanced compared to the corresponding underivatized
amino acid. Such has been the case in the duo, Se-methyl-
L-selenocysteine and its glutamylated product, γ-Glutamyl-
Se-methyl-L-selenocysteine. The solubility of the L-Semethylselenocysteine
is ca. 10 percent with dissolution
occurring in two hours in water. The solubility of γ-L-glutamyl-
L-Se-methylselenocysteine is ca. 25 percent with dissolution
occurring in thirty minutes in water. This solubility modifying
characteristic has important ramifications on the flexibility
of formulation of these materials. |
| |
MISCELLANEOUS EXAMPLES |
| |
Amino acids such as γ-glutamyl-taurine (Glutaurine)
isolated from bovine parathyroids has been reported
to influence the metabolism of Vitamin A and to
possess radioprotective properties in addition to
other useful therapeutic properties. Again such as
an array of beneficial properties would appear
to result from the modification of the taurine structure
with γ-glutamyl group appendage (8). γ-D-Glutamyl-
L-tryptophan (SCV-07) is a prospective medicine
for the treatment of tuberculosis, that reached upto
phase II clinical trial. Here the optical antipode
(D) is used (9).
Another stellar example is Theanine. Even though
it is not a γ-glutamyl aminoacid, rather a γ-glutamyl
ethyl amine, the γ-glutamyl structural fraction is
the most important feature ofTheanine. γ-Glutamyl derivatives
have been well recognized for their immunomodulating
properties. Several amino acid and amine analogs have
been described in the literature. |
| |
CHEMISTRY OF γ-GLUTAMYLATION |
| |
γ-Glutamyl group can be incorporated onto a typical amino
acids in a several ways. Basically this usually requires the
protection of two of the three carboxylic groups in the reactant
molecules as esters (leaving of course the third γ-COOH of
the glutamic acid free). Also the amino group of the L-glutamic
acid residue should be suitably protected. Figure 7 shows a
sequence wherein only two protecting groups are used. The
trityl group on the glutamic acid functions plays a dual role
wherein it protects the amino group as well as rendering the
neighboring -COOH as sterically unreactive. The carboxyl
of the other component, namely, Se-methyl-L-selenocysteine is protected as a methyl ester. After the formation of pseudopeptide
bond, the two protecting groups need to be removed
sequentially without affecting the amide linkage (10).
|
Figure 9. Reaction Sequence for γ-Glutamylation on Unprotected Amino Acids using.
NPGLA |
| |
|
Figure 10. Single Pot γ-Glutamylation of Taurine. |
However the availability of N-phthaloyl-L-glutamic anhydride
(NPGLA) (Figure 8) in commercial quantities in high chemical
and chiral purity has changed this scenario (11). The
γ-glutamylation sequence becomes simpler and higher yielding
when N-phthaloyl-L-glutamic anhydride is used.
As the γ-glutamylating agent. The amino acid, that needs
to be γ-glutamylated, can be used directly without any
protecting groups. A solvent system incorporating an aqueous
polar solvent can be employed. The products are free from
any possible racemization. Amines also can be employed
on their conversion to γ-glutamyl derivatives. Finally phthaloyl
group can be simply removed by standard methods such as
use of hydrazine (see below Figure 9).
An improved synthesis of γ-L-glutamyl-taurine (Glutaurine)
similarly involves the direct reaction of NPGLA (Figure 8)
with taurine solubilized in acetonitrile with tetramethyl
guanidine. Deprotection using hydrazine in the same pot
afforded the dipeptide (Figure 10). In the synthesis of
γ-glutamylmarasmine, NPGLA has been used effectively
(12). An enantioselective synthesis of chloroquine and the
establishment of configuration used NPGLA as a pivotal
chiral agent (13).
Enzymatic methods of introduction of γ-glutamyl group have
also been used (1). A bacterial γ-glutamyltranspepdidase
enzyme was used. This enzymic technology still has to reach
industrial levels since only low concentrations of substrates
only could be used. |
| |
DEPROTECTION OF PHTHALIMIDES
(REMOVAL OF PHTHALOYL PROTECTING
GROUP FROM PRIMARY AMINO FUNCTION) |
| |
Variety of methods are available for the removal of the
phthaloyl group to liberate the primary amine functionality.
The classical and most widely employed method is through
the use of hydrazine whereby the phthaloyl group is removed
as phthalhydrazide or sometimes as N-aminophthalimide
(14). The deprotection can be carried out in alcoholic solvents
wherein the phthalhydrazide is easily removed as illustrated
in Figure 9.
Sometimes co-occurring oxidation of hydrazine and the
subsequent formation of diimide results in the reduction of double bonds present elsewhere in the substrate.
To avoid such side-reactions, additionally a
sacrificial olefin such as (Z)-2-buten-1-ol is
employed that is more preferentially reduced
to scavenge adventitious diimide (15). Also
other hydrazines can be employed such as
methylhydrazine where such diimide formation
is averted (16). Alternatively phenylhydrazine
can be employed in place of hydrazine.
In stead of hydrazines, various primary amines
including ammonia can be employed for the
removal of phthaloyl group. Amines such as
methyl amine, butyl amine, ethylene diamine
or propylenediamine derivatives have been
employed (14).
In addition to these basic amine reagents, acidic conditions
have also been employed (14).
An interesting reductive removal of phthalimide protecting
group using borohydride has been reported by Ganem et
al. as illustrated in the Figure 11.
Ganem et al note that this reductive removal followed by
acetic acid treatment was a one-flask operation and no
racemization of the chiral centers was detected (17).
|
Figure 11. Reductive Removal of Phthaloyl Protecting Group. |
|
| |
CONCLUSIONS |
| |
A readily available chiral γ-glutamyl transfer reagent, namely
N-Phthaloyl-L-glutamic anhydride (NPGLA) onto amino acids
and also onto primary amines, offers a simple solution for
installing L-glutamyl functionality in a straightforward way.
The amino acid need not bear any protecting groups and
the reaction is free from racemization. |
| |
REFERENCES AND NOTES |
| |
1. S. Wilk, H. Mizoguchi et al., J. Pharmacol. Exp. Ther. 206, p. 227
(1978).
2. H. Suzuki, Y. Kajimoto et al., J. Agri. Food Chem. 50, p. 313 (2002);
H. Suzuki, K. Kato et al., J. Agri. Food Chem. 52, p. 577 (2004) and
references therein.
3. M. Friedman, P-A. Finot, J. Agri. Food Chem. 38, p. 2011 (1990).
4. J. Wu, H. Corke, J. Sci. Food Agri. 85, p. 2587 (2005).
5. A. Dunkel, J. Koster et al., J. Agri. Food Chem. 55, p. 6712, (2007).
6. N. Kalyanam, M. Majeed, Chem Oggi 25(5), p. 36 (2007).
7. Y. Dong, D. Lisk et al., Cancer Research 61, p. 2923 (2001).
8. J. Gulyas, F. Sebestyen et al., Organic Prep. Proced. Int. 19, p. 64
(1987).
9. H. Suzuki, K. Kato et al., J. Biotechnology 111, p. 291 (2004).
10. E. Block, M. Birringer et al., J. Agri. Food Chem. 49(1), p. 458
(2001); J. F. Carson, F. F. Wong, JCS Perkin I, p. 685 (1974); E.
Khalifa, H. H. Bieri et al., Helv. Chim. Acta 56, p. 2911 (1973); M.
Itoh, Chem. Pharm. Bull. 17, p. 1679 (1969).
11. Available from Sabinsa Corporation, www.sabinsa.com.
12. L.A.G.M. Van den Broek, M. Breuer et al., J. Org. Chem. 52, p.
1511 (1987).
13. J. C. Craig, H. N. Bhargava et al., J. Org. Chem. 53, p. 1167
(1988).
14. For a brief review and leading references see, Greenes's Protecting
Groups in Organic synthesis, P. G. Wuts, T. W. Greene, 4th Edition,
John Wiley & Sons, Inc., Hoboken, NJ, pp 790-793 (2006).
15. B.E. Maryanoff, M.N. Greco et al., J. Am. Chem. Soc. 117, p. 1225
(1995).
16. A.L. Smith, C.K. Hwang et al., J. Am. Chem. Soc. 114, p. 3134
(1992).
17. J.O. Osby, M.G. Martin et al., Tetrahedron Lett. 25, p. 2093 (1984). |
| |
NAGABUSHANAM KALYANAM,
MUHAMMED MAJEED
Corresponding author
Sabinsa Corporation
70 Ethel Road West Unit 6
Piscataway, NJ, USA |
| |
|
| |
|
|
|
|
