Biomedical Engineering Reference
In-Depth Information
Therefore, the two major products were hexyl galactoside and glucoside, at a total yield of
46%. The galactose monosaccharide unit was preferably catalyzed by the enzyme over the
glucose monosaccharide unit. Major issues for this approach were the limited miscibility
of the substrates, requiring that a medium- but not long-chain alcohol be used (C
8
or less),
and the higher water activities needed by glycosidases compared to lipases, which reduces
the yield at the approach of thermodynamic equilibrium.
Secondly, APGs were produced starting with alkylglycoside and a source of additional
saccharide units,
-cyclodextrins, using cyclodextrin glycosyl transferase from
Bacillus
macerans
(Svensson
et al
., 2009a, 2009b). The reaction took place in aqueous media at pH
5.2 and 60 °C, using an eightfold molar excess of cyclodextrin to dodecyl
α
β
-D-maltoside.
This reaction yielded mainly the dodecyl
-D-maltooctaoside with an overall yield of 50%.
The eight glucosidyl units result from two units present in the substrate's maltose unit plus
six present in the glucosidyl donor
β
-cyclodextrin.
An alternative approach to produce APGs was to react soluble starch (a homopolymer
of glucose with
α
α
-1,4-glycosidic linkages) with 1-butanol in aqueous media in the
presence of
-amylase at 50 °C (Larsson
et al
., 2005). This approach produced a mixture
of APGs, with the average degree of polymerization of the oligosaccharides residing
between two and four. Yields were generally low. In summary, significant progress has
been achieved for the enzymatic synthesis of APGs in recent years; however, the
enzymatic route will require significant improvement to be competitive with chemical
APG synthesis.
α
10.5.5 Enzyme-catalyzed synthesis of amino acid derivatives
Enzymes, particularly hydrolases, are capable of covalent attachment of fatty alkyl or acyl
groups onto the amino and carboxylic acid functional groups of amino acids. Two examples
will be described initially; these involve the covalent modification of arginine. For the first,
papain, a protease from papaya latex, catalyzed the attachment of the carboxylic acid moiety
of arginine to either a fatty amine or a fatty alcohol, producing an amide or ester bond,
respectively (Figure 10.5 ) (Infante
et al
., 2009b). The starting material contained the
N-benzyloxy (“Cbz”) protective group attached to the
-amino group; and, its carbonyl
group was esterified to methanol, the latter serving as a good leaving group. Amide bond
formation occurred using a 50-fold mole excess of
n
-C
8
-C
12
alkyl amine at room temperature
using a mixture of acetonitrile and 0.1 M borate buffer, pH = 8.2. Papain immobilized
via
adsorption onto polyamide was employed as biocatalyst. Typically, 70-90% conversion of
Cbz-argine into the amide occurred. Fatty alcohols of chain length 8-12 were esterified to
arginine's hydroxyl group using a solvent-free reaction medium conducted at 50-65 °C,
where the alcohol served as substrate and solvent. Ester yields of up to 80-90% occurred.
The surfactants have potential application as antimicrobial agents and for DNA compaction.
By a similar approach, the carboxylic acyl moieties of several N- (and for cysteine S-) Cbz-
protected amino acids (glycine, lysine, serine, aspartate, phenylalanine, tyrosine, and
cystine) were conjugated with fatty alcohols (to yield esters),
α
α
,
ω
-diols (yielding diesters
containing two amino acids), and
α
,
ω
-diamines (yielding diamides) (Valivety
et al
., 1998 ).
-carboxylic acid moiety of CBz-protected amino acids with
alcohols or amines has been the most successful approach for enzymic transformations. The
attempt to add fatty acyl groups to free amino acids has not been successful (Soo
et al
.,
2003 ; Valivety
et al
., 1997). The most successful addition of acyl groups to unprotected
amino acids has been reported for the
Covalent attachment of the
α
ε
-amino group of lysine, the
ε
-OH group of homoserine,
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