Journal of Photochemistry and Photobiology A: Chemistry 153 (2002) 121–128
Influence of methanol and cyclodextrin cavity size on stoichiometry and
binding constant of 3-[2-(9-anthryl)benzoxazol-5-yl]-alanine
Justyna Mrozek
a
, Katarzyna Guzow
a
, Mariusz Szabelski
a
,
Jerzy Karolczak
b
, Wieslaw Wiczk
a,∗
a
Faculty of Chemistry, University of Gda´ nsk, Sobieskiego 18, 80-952 Gda´ nsk, Poland
b
Quantum Electronic Laboratory, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Pozna´ n, Poland
Received 13 March 2002; received in revised form 24 June 2002; accepted 25 June 2002
Abstract
A newly synthesized amino acid derivative 3-[2-(9-anthryl)benzoxazol-5-yl]-alanine was applied to study the influence of methanol
concentration on equilibrium constant with -cyclodextrin (-CD) and -cyclodextrin (-CD). For both CDs the formation of ternary
CD/guest/methanol complexes were established. The increase of methanol concentration decreases the apparent equilibrium constant for
-CD, whereas for -CD an increase was observed up to about 1 M of MeOH and then monotonical decrease with further increase of
alcohol concentration. Calculated from the dependence of apparent equilibrium constant on methanol concentration, equilibrium constants
of binary complex between -CD and amino acid is equal to 780 M
-1
, between -CD and methanol 2 M
-1
and 410 M
-1
for ternary
complex. The apparent thermodynamic parameters for complex formation of amino acid studied with - and -CD in water/methanol
mixture (2%, v/v) are: S =-32.7 ± 2.5 J/mol K, H =-25.3 ± 0.7 kJ/mol; S = 24.9 ± 3.3 J/mol K, H =-2.7 ± 1.0 kJ/mol for
-CD and -CD, respectively.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Methanol; Cyclodextrin; Amino acid
1. Introduction
Cyclodextrins (CDs) are toroidally shaped polysaccha-
rides made up of six to eight d-glucose monomers con-
nected at the 1 and 4 carbon atoms. The cavities of CDs
are relatively hydrophobic and have an internal diameter of
4.7–8.3 Å [1,2]. This difference in cavity size allows bind-
ing specificity to be tailored based on substrate size and ge-
ometry. Therefore, CDs are ideal molecules for the study
of small molecule binding. There are many studies deal-
ing with inclusion complexes of CDs [3–23]. The structure
of a supramolecule consisting of CD as a host and organic
molecule along with several solvent molecules as guests is
quite well defined [24].
Imperfect fit of the guest to the CD cavity leads to asso-
ciation constant decrease. If the guest does not occupy the
whole internal space of the cavity, the void space will be
filled with solvent molecules. In the case of water, its incor-
poration is thermodynamically unfavourable. The efficiency
∗
Corresponding author. Tel.: +48-58-34-50-353;
fax: +48-58-341-03-57.
E-mail address: ww@chemik.chem.univ.gda.pl (W. Wiczk).
of CD complexation can be altered by addition of ternary
complexation agents, such as alcohols leading to either a
decrease [3,4,7,16] or increase [15,21] of the CD–guest as-
sociation constants. Enhancement of the complexation ef-
ficiency has been frequently attributed to the formation of
ternary complexes, where the third complexation agent re-
placed the water molecules in the void space of the CD cav-
ity. In contrast, when the equilibrium constants decrease the
third complexation agent may partially displace the guest
molecule within the CD. Recently, much attention has been
focussed on the use of alcohols as the third component, since
they could affect the formation of the original binary com-
plex. Alcohols are the simplest organic analogues of water,
which is known to be essential in inclusion process. The
size and geometry of the alcohol seem to be important fac-
tors [25,26] as is its ability to alter the hydrophobicity of
the cavity [27]. The presence of primary and secondary hy-
droxyl groups in the CD is also essential for the formation
of the ternary alcohol complexes [26], whose stoichiometry
depends on the specificity of the guest, the type of CD, and
the type of alcohol [3,4,7,15,26,28].
It is worth noting that the alcohols themselves are able to
form the complex with CD. The strength of the binding of
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