Complexation of Synthetic Organic Dye Dapoxyl with cyclodextrins studied by fluorescence spectroscopy Daniel Granadero, Jorge Bordello, Mercedes Novo and Wajih Al ‐ Soufi Departamento de Química Física, Universidade de Santiago de Compostela, Facultade de Ciencias, Avda. Alfonso X O Sabio, 27002 Lugo (Spain) Abstract: Cyclodextrins (CDs) are natural ‐ occurring oligomers with a hydrophobic cavity which allow them to form inclusion complexes in water with a variety of organic molecules. These complexes are generally stabilized by van der Waals and hydrophobic interactions, although specific host ‐ guest interactions can also play an important role. [1] In addition, geometrical and orientational requirements of both the guest and the host may control the association process, whereas the dissociation rate is determined by the strength of the interactions. [2] Therefore, a change of the cavity size or the rigidity of the host has dramatic effects on both the association and the dissociation rate constants and in consequence on the stability of the complexes. [3] Moreover, for a certain guest, stoichiometry and geometry of the inclusion complexes may also be much dependent on the host cavity size. [4] In this work we study the effects of CD cavity size on the stoichiometry, stability and structure of the complexes formed between the synthetic organic probe Dapoxyl and three natural cyclodextrins ( α‐ CD, β‐ CD and γ‐ CD) differing in the size of their inner cavity, using steady ‐ state and time ‐ resolved fluorescence techniques. The ditopic structure of this fluorophore together with its high sensitivity to the surroundings make it a useful model molecule to study the geometrical effects on the complexation. Complexation provokes strong changes in the fluorescent properties of Dapoxyl with a large blue shift of its emission spectrum and a great increase of the fluorescence quantum yield. The differences observed for the three CDs regarding stoichiometry, stability and fluorescence properties of the complexes are discussed on the basis of a size ‐ selective complexation of the Dapoxyl. 13rd International Electronic Conference on Synthetic Organic Chemistry (ECSOC-13), 1-30 November 2009. http://www.mdpi.org/ecsoc-13/ & http://www.usc.es/congresos/ecsoc/13/ [f006]
Introduction: Dapoxyl sodium sulfonate Fluorescent probe Fluorescent probe: : 2 ‐ ‐ (4 (4’ ’ ‐ ‐ Sulfophenyl Sulfophenyl) ) ‐ ‐ 5 5 ‐ ‐ (4 (4’’ ’’ ‐ ‐ dimethylaminophenyl)oxazole dimethylaminophenyl)oxazole, , sodium sodium salt salt (DAP) (DAP) 2 Dimethylaminophenyl moiety Sulfo phenyl moiety › Higher sensitivity to the surroundings than other probes: Fluorescence maximum and fluorescence quantum yield very sensitive to solvent polarity › Other interesting properties: long emission wavelengths, high extinction coefficients and large Stokes shifts 2
Introduction: Cyclodextrins 1.53 nm 1.69 nm 1.37 nm 0.57 nm 0.78 nm 0.95 nm 0.78 nm Inclusion complexes are simple and useful models for the investigation of host ‐ guest interactions in supramolecular α‐ CD γ‐ CD β‐ CD systems › Natural ‐ ocurring cyclic oligosaccaharides obtained by enzymatic conversion of starch › With a toroidal shape, CDs have a highly hydrophobic central cavity which allows them to form inclusion complexes with many organic substrates › Many applications: Pharmaceutical industry (controlled release), food technology, chemical industry, agricultural and analytic chemistry, … 3
Methodology and results 1. ‐ ‐ Absorption Absorption and and steady steady ‐ ‐ state state fluorescence fluorescence titrations titrations 1. 0 .2 5 0 .2 5 Intensidad Fluorescencia /kcps 0 .6 1 5 0 0.04 M 0.04 M 0.01 M 0.01 M 0.025 M 0.025 M 2 5 0 Fluorescence Intensity /Kcps Fluorescence Intensity /Kcps Fluorescence Intensity /Kcps 1 5 0 0 .5 0 .2 0 0 .2 0 2 0 0 Absorbance Absorbance Absorbance 0 .4 1 0 0 0 .1 5 0 .1 5 1 0 0 1 5 0 0 .3 0 M 0 M 0 M 0 M 0 M 0 M 0 .1 0 0 .1 0 1 0 0 0 .2 5 0 5 0 0 .0 5 0 .0 5 5 0 0 .1 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 λ /n m λ /n m λ /n m α‐ CD γ‐ CD β‐ CD The figures show the variations of the absorption and fluorescence emission spectrum of Dapoxyl with increasing concentrations of the three CDs. Small but systematic red shifts are observed in the absorption spectra. Emission spectra show strong blue shifts and huge increase of the fluorescence intensity. 2. ‐ 2. ‐ Determination Determination of of the the number number of of fluorescent fluorescent species species: : Analysis of the series of absorption and emission spectra at different CD concentrations using Principal Component Analysis (PCA) yield the minimal number of species contributing to the experimental spectra. PCA α‐ CD: 2 species β‐ CD: 3 species γ‐ CD: 3 species 4
Methodology and results 3. ‐ ‐ Proposal Proposal of of the the complexation complexation mechanisms mechanisms and and fit fit functions functions 3. Taking into account the results of PCA, the following mechanisms are proposed to explain the behaviour of Dapoxyl in the presence of the different cyclodextrins. Only complexation equilibria in the ground state are considered since the association and dissociation processes are too slow to compete with the deactivation. α‐ CD: 2 species β‐ CD and γ‐ CD: 3 species Complexation mechanism 1:1 + 1:2 Complexation mechanism 1:1 DAP* DAP:CD* * DAP* DAP:CD 2 * DAP* DAP:CD DAP* DAP:CD* DAP:CD * DAP:CD 2 * I DAP I k DAP k I DAP:CD I I DAP k DAP I DAP:CD k DAP:CD k I k I k DAP:CD k I DAP:CD I k DAP:CD k DAP DAP DAP:CD DAP DAP DAP:CD DAP:CD DAP:CD DAP:CD 2 DAP:CD 2 2 2 K 1 K K 1 K 2 K K 1 1 2 DAP + CD DAP:CD DAP:CD DAP:CD + CD + CD DAP + CD DAP + CD DAP + CD DAP:CD DAP:CD 2 DAP:CD 2 [ ] [ ] [ ] λ + λ ⋅ ⋅ λ + λ ⋅ ⋅ + λ ⋅ ⋅ ⋅ 2 a ( ) b ( ) K CD a b K CD c K K CD ( ) ( ) ( ) λ = = λ = = 1 1 1 2 0 0 0 P ( ) P A or F P ( ) P A or F [ ] [ ] [ ] + ⋅ + ⋅ + ⋅ ⋅ 2 1 K CD 1 K CD K K CD 1 1 1 2 0 0 0 5
Methodology and results 4. ‐ ‐ Global Global Analysis Analysis (GA) (global (GA) (global fit fit to to all all wavelengths wavelengths) ) 4. Global analysis (GA) of the series of emission and absorption spectra as datasets yield precise values of the association equilibrium constants K 1 in the case of α‐ CD and K 1 and K 2 for β‐ CD and γ‐ CD. β‐ CD: 1:1 + 1:2 Complexation γ‐ CD: 1:1 + 1:2 Complexation α‐ CD: 1:1 Complexation 1 7 5 1 7 5 Fluorescence Intensity /Kcps 0 .1 8 5 Fluorescence Intensity /Kcps Flu o rescen ce In ten sity at 5 4 0 n m F lu o r e s c e n c e In t e n s it y a t 5 3 0 n m Fluorescence Intensity /Kcps Flu o re sce n c e In te n sity at 5 0 0 n m 0 .2 1 2 5 0 A b s o r b a n c e a t 3 8 0 1 5 0 A b so rb an ce at 4 0 0 n m A b so rb an ce at 3 7 0 1 5 0 0 .2 8 0 .1 8 0 0 .2 0 1 2 5 Absorbance Absorbance Absorbance 2 0 0 1 2 5 0 .1 7 5 0 .2 6 0 .1 9 1 0 0 1 0 0 1 5 0 0 .1 8 0 .1 7 0 0 .2 4 7 5 7 5 0 .1 7 0 .1 6 5 - 1 - 1 1 0 0 - 1 - 1 K 1 = = ( (1 8 6 5 1 8 6 5 ± 2 5 0 ) 2 5 0 ) M M K 1 = = ( (3 4 8 8 3 4 8 8 ± 1 4 1 4 ) M M 5 0 5 0 0 .2 2 0 .1 6 - 1 - 1 K 1 = ( = (6 6 .3 ± 0 . 0 .1 ) M M 0 .1 6 0 - 1 5 0 - 1 - 1 - 1 K 2 = = ( (1 1 3 . 1 1 3 .7 7 ± 0 . 0 .8 ) 8 ) M M 2 5 2 5 0 .1 5 K 2 = = (3 (3 .0 .0 ± 1 . 1 .7 ) 7 ) M M 0 .2 0 0 .1 5 5 4 6 4 Residuals Residuals 0 .0 0 1 Residuals 0 .0 02 2 0 .0 0 2 3 2 0 .0 0 0 0 0 .0 0 0 0 0 .0 00 0 - 2 - 3 - 0 .0 0 2 - 0 .0 02 - 2 - 0 .0 0 1 - 4 - 6 - 4 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 0 2 4 6 8 1 0 0 5 1 0 1 5 2 0 2 5 - 3 M -3 M [ β - C D ]/ 1 0 - 3 M [ γ - C D ]/1 0 [ α - C D ]/1 0 System DAP + α‐ CD DAP + β‐ CD DAP + γ‐ CD Species 2 : (DAP + DAP: α‐ CD) 3 : (DAP + DAP: β‐ CD + DAP: β‐ CD 2 ) 3 : (DAP + DAP: γ‐ CD + DAP: γ‐ CD 2 ) K 1 /M ‐ 1 66.3 ± 0.1 3488 ± 14 1865 ± 250 K 2 /M ‐ 1 ‐ 3.0 ± 1.7 113.7 ± 0.8 The stability constants of the complexes are very different for the three CDs showing the effect of the cavity sizes in the association processes as well as in the strength of the host ‐ guest interactions. 6
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