“…Receding contact angles were 2-4°lower for water and 3-6°lower for hexadecane. The measured film thickness seems to be quite consistent with the close-packed monolayer structure of approximately vertical, fully extended alkyl chains, and our results for contact angle measurements are in good agreement with values previously published.8 We note, however, that our ellipsometrically determined film thickness of ~34 A is greater by 6 A than the value of 28 A found by using X-ray diffraction techniques.8 This may indicate that we have obtained more densely packed monolayers.…”
Monolayer and multilayer films were formed by self-assembly of methyl 23-(trichlorosilyl)tricosanoate (1) from organic solution. In agreement with published results, this compound was found to form good quality, close-packed monolayers on silicon surfaces. We have, however, found that films significantly thicker than the three monolayers previously obtained can be formed from continued chemisorption of monolayers of this compound followed by reduction of the surface ester with LiAlH4 in THF to form an "alcohol surface". The quality of monolayer formation in the multilayer films was monitored in detail by ellipsometry, contact angle, and FTIR measurements, and, although generally increasing disorder can be detected, films of up to 25 discrete monolayers can be successfully made. These results indicate that self-assembly is a viable alternative to the Langmuir-Blodgett transfer technique for the construction of relatively thick (O.l-gm scale), ordered, multilayer films.
“…Receding contact angles were 2-4°lower for water and 3-6°lower for hexadecane. The measured film thickness seems to be quite consistent with the close-packed monolayer structure of approximately vertical, fully extended alkyl chains, and our results for contact angle measurements are in good agreement with values previously published.8 We note, however, that our ellipsometrically determined film thickness of ~34 A is greater by 6 A than the value of 28 A found by using X-ray diffraction techniques.8 This may indicate that we have obtained more densely packed monolayers.…”
Monolayer and multilayer films were formed by self-assembly of methyl 23-(trichlorosilyl)tricosanoate (1) from organic solution. In agreement with published results, this compound was found to form good quality, close-packed monolayers on silicon surfaces. We have, however, found that films significantly thicker than the three monolayers previously obtained can be formed from continued chemisorption of monolayers of this compound followed by reduction of the surface ester with LiAlH4 in THF to form an "alcohol surface". The quality of monolayer formation in the multilayer films was monitored in detail by ellipsometry, contact angle, and FTIR measurements, and, although generally increasing disorder can be detected, films of up to 25 discrete monolayers can be successfully made. These results indicate that self-assembly is a viable alternative to the Langmuir-Blodgett transfer technique for the construction of relatively thick (O.l-gm scale), ordered, multilayer films.
“…These unexpectedly fast host-guest exchange kinetics are in agreement with observations made previously for a family of arene-binding dendrophanes with a narrower apolar pocket [8]. To more precisely address the host-guest exchange kinetics, quantitative studies, based on fluorescence relaxation techniques [23], are now on their way.…”
The H,O-soluble dendritic cyclophanes (dendrophanes) 5 5 of first to third generation with molecular weights up to nearly 20 kD were synthesized, purified, and characterized. Cyclophane 2, which served as the initiator core (generation zero), was prepared from tetrabromocyclophane 10 in a four-step sequence which involved as the first transformation a high-yielding, four-fold Pd(0)-catalyzed Suzuki cross-coupling reaction with 4-(benzyloxy)-phenyl-boronic acid to give 18. The X-ray crystal-structure analysis of tetrabromocyclophane 10 displayed an open, nearly rectangular box with opposite aromatic walls being 8.3 and 11.4 A apart and of suitable size for the incorporation of steroidal substrates. 'H-NMR Binding titrations in borate-buffered D20/CD30D 1 : 1 showed that cyclophane-tetracarboxylate 2 forms 1 : 1 inclusion complexes with steroids ( Table 2). Complexation was found to be enthalpically driven with higher binding affinities measured for the more apolar substrates. 'H-NMR Titrations in the same solvent also provided clear evidence for core-selective complexation of testosterone (21) by the dendrophanes 3 (Ist), 4 (2nd), and 5 (3rd generation) carrying up to 108 carboxylate surface groups. The stability of the corresponding 1 : 1 complexes was hardly affected by the size of the dendritic shell, although some generation-dependent conformational changes in the receptor cavity seemed to take place. Remarkably, host-guest exchange kinetics in all recognition processes were fast on the 'H-NMR time scale.Together with the naturally abundant cyclodextrins [I], cyclophanes do form the major part of synthetic receptors for inclusion complexation of apolar substrates [2]. Only a limited number of synthetic hosts are capable of steroid recognition [3] [4], a process of fundamental importance in biology [5]. Recent X-ray structural data for steroid-binding proteins, enzymes, and antibodies [6] revealed that natural receptors, similar to cyclophane hosts, prefer complexing the voluminous steroidal substrates in binding sites largely shaped by aromatic amino-acid side chains, thus taking advantage of favorable desolvation processes and apolar dispersion as well as polar CH. . -z interactions.
“…Since cyclodextrins tend to form aggregates, , it is not surprising that the stoichiometry of cyclodextrin−steroid complexes varies and is often higher in receptor, with 2:1, ,, 3:2, or 3:1 host−guest complexes being formed at elevated cyclodextrin concentrations. 10 β- (a) and γ-Cyclodextrin−chromophore conjugates (b) in complexes with steroids for which association constants for 1:1 host−guest stoichiometry have been reported (Table ). 11 Steroids in cyclodextrin complexes for which association constants for 1:1 host−guest stoichiometry have been reported (Table ). 12 Fluorescent steroids for the examination of protein−steroid and cyclodextrin−steroid interactions, described by Kempfle et al , …”
Section: Cyclodextrins As Hosts For Natural and
Synthetic
Steroidsmentioning
confidence: 99%
“…Fluorescent steroids for the examination of protein−steroid and cyclodextrin−steroid interactions, described by Kempfle et al , …”
Section: Cyclodextrins As Hosts For Natural and
Synthetic
Steroidsmentioning
confidence: 99%
“…A fluorescence assay, which had been previously developed by Kempfle et al to study protein−steroid interactions, was also applied to determine the stability of cyclodextrin complexes. , In this assay, the quenching of the emission of fluorescent steroids 91 − 93 (Figure ) upon transfer from the more polar aqueous phase into the less polar receptor binding site is evaluated. It was found that, within the concentration limits of the assay, steroids 91 − 93 did not undergo stable association with α-cyclodextrin ( 21 ) while β- and γ-cyclodextrins ( 22 and 23 ) underwent complex formation with 1:1 (Table , entries 98, 99, and 132−134) or 1:2 host−guest stoichiometry.…”
Section: Cyclodextrins As Hosts For Natural and
Synthetic
Steroidsmentioning
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