Biomedical Engineering Reference
In-Depth Information
2.1
Chemical Structures and Properties
Biosynthesis of TA is a stereoselective multi-step process generating secondary
metabolites, e.g.
S
-scopolamine and
S
-hyoscyamine, the latter one is synonymous for
L-hyoscyamine or (−)-hyoscyamine (Fig.
1
). Therefore,
S
-hyoscyamine is a chiral
molecule of esterified 3a-hydroxytropane and
S
-tropic acid (Fig.
1
) [
1
] . While alka-
line extraction of plant material for pharmaceutical purposes,
S
-hyoscyamine, easily
undergoes racemization thus producing
R
-hyoscyamine (Fig.
1
) to yield a mixture
named atropine [
10
]. However, sometimes atropine is misleadingly used to assign
the natural
S
-hyoscyamine content of plants especially in cases of intoxication [
11-
15
]. Both hyoscyamine and scopolamine serve as raw material for next generation
drugs representing derivatized analogues being mostly N-alkylated thus yielding
positively charged quaternary TA (QTA), e.g.
N
-butyl scopolamine and cimetropium
from scopolamine and ipratropium from hyoscyamine (Fig.
1
). Apart from those
QTA a large number TA has been synthesized exchanging the natural tropic acid
moiety against, e.g. paramethyl-benzenesulfonic acid (satropane, Fig.
1
), dichlo-
robenzoic acid (bemesetron, Fig.
1
), indole-3-carboxylic acid (tropisetron, Fig.
1
) or
mandelic acid (homatropine, Fig.
1
), thus regulating pharmacological activity.
In contrast to TA of the
Solanaceae
family cocaine from
Erythroxylaceae coca
contains a substituted 3b -hydroxytropane (3 b-tropine, pseudotropine) skeleton
(ecgonine methyl ester) (Fig.
1
).
For elucidation of biosynthesis and TA identification in plants mass spectromet-
ric procedures including LC-MS [
16,
17
] , LC-MS/MS [
18
] and DART MS (direct
analysis in real time MS) [
19
] were successfully applied.
A review on biosynthesis of TA is given by Humphrey and O´Hagan [
20
] and
derived TA drugs are reviewed by Grynkiewicz and Gadzikowska [
1
] .
2.1.1
Charge Properties of TA
Due to the presence of the tertiary amine in the tropane moiety, tertiary TA (TTA)
behave as weak bases allowing protonation of the N-atom. Table
1
summarizes cal-
culated p
K
b
-values of TTA being in the range from 4.9 to 6.8 indicating this charac-
ter. For comparison, NH
3
has a p
K
b
of 4.7. Calculations were based on SMILES
notation of the relevant TTA structures whose usefulness is briefly addressed below
(
SMILES notation
). Even though the listed theoretical p
K
b
calculations, which are
based on the SPARC software [
21
], do not represent experimental data, dimensions
and trends of p
K
b
will have a satisfying prognostic value. For comparison, the p
K
b
of atropine and scopolamine were experimentally determined to be 4.57 and 6.25,
respectively [
22
], and calculation resulted in sufficient congruency with 5.08 and
6.78, respectively (Table
1
).
The pH influences the ratio of protonated to non-protonated TTA, which has
impact on pharmacological behaviour (see Sect.
2.2
) as well as on sample prepara-
tion procedures (see Sect.
3.1
). Especially, when extracting by liquid-liquid extrac-
tion (LLE), the partition between an aqueous and an organic non-polar phase is
