Aumentar la imagen. Macrocycles : Construction, Chemistry and Nanotechnology Applications distils the essence of this important topic for undergraduate and postgraduate students, and for researchers in other fields who are interested in getting a general insight into this increasingly important class of molecules.
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Imagen del editor. Higson, Frank Davis. Brand new Book. Recently, we employed dynamic combinatorial chemistry 18 and constructed a pair of dynamic configurational macrocycles which can interconvert.
Molecular recognition using tetralactam macrocycles with parallel aromatic sidewalls
Research on similar but covalent receptors confirms the two isomers have very different guest preferences and binding abilities. How can we unify several interconvertible cavities into one kinetically inert macrocycle?
Conformational change may be resorted to. Calix[ n ]arene 21 and related compounds are well known to possess several conformations, but not all the conformers have a well-defined cavity. Even though they can interconvert, not all of them show satisfactory guest binding properties.
To the best of our knowledge, this kind of smart macrocycles have not been well demonstrated. In the present research, we report a naphthalene-based macrocyclic receptor with four interconvertible and deep cavities. This macrocycle can choose an appropriate cavity for a specific guest and achieve the optimal binding. Naphthalene-based macrocycles are relatively rare in the supramolecular literature.
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In the present research, we designed and synthesized a naphthalene-based macrocycle Fig. Oxatubarene has the following features in its structure: a a longer linker is used instead of the often-seen methylene linker. The short linker is believed to cause steric hindrance between neighboring naphthalenes, and the macrocycles may twist, resulting in ill-defined cavities.
A longer linker may solve this problem, leading to a macrocycle with a deep cavity. Thus, macrocyclization might be readily achieved through Williamson ether synthesis. In addition, the linker may not be innocent and may get involved in binding when the ether oxygen atoms are inwardly-directed. The synthesis of oxatubarene is straightforward Fig. Both the precursors 1 and 2 can be obtained in three steps from commercial materials with or without simple column chromatography. TA4 is a flexible macrocycle and thus possesses numerous conformations. However, the flipping of naphthalene rings as the major activation barriers separates four representative conformations or conformational sets Fig.
In order to show the cavity's size and dimension, the models of these conformers are presented in the most expanded form by forcing all the methylene protons in the linkers directed outwards and the oxygen atoms inwards. In contrast to calixarenes, all four conformers possess well-defined cavities which may offer good guest binding properties.
Moreover, the cavity of each conformer is different.
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The diameter and length of conformer I D 2d are 6. This is similar to conformer II which, however, has the lowest symmetry C 2. The differences in cavity sizes and lengths are expected to result in different guest binding preferences. According to their different symmetries, 1 H NMR can be used to distinguish them when the exchange of guests is slow on the NMR timescale. It seems there is only one conformer I or IV according to their symmetries.
Only one explanation can account for this: the two protons on the same methylene group may exchange their positions very quickly and this can only occur through a rapid flipping of the naphthalene rings. In order to slow down the ring flipping, variable-temperature NMR experiments Fig. One may argue this is caused by aggregation at low temperatures. Consequently, there is a conformational ensemble of TA4 in solution.
These conformers undergo quick interconversion at room temperature.
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Computations indicated all the conformers are electron-rich in their cavities Fig. Control experiments Fig. The NMR integrals and mass spectrum Fig. Meanwhile, the flipping of the naphthalene rings is slow, since the cavity is occupied and the guest exchange is slow. This was further supported by the splitting of proton signals a, b and c diastereotopic protons. The aromatic protons of the hosts appear as two doublets. According to the conformers' symmetries, the host in this solution can be assigned to a single conformer: either I D 2d or IV D 4.
That is, only one conformer was selected from the conformational ensemble. The peaks are slightly broadened. Accordingly, the new conformer can be assigned to either conformer I or IV. Aromatic protons appear as two peaks, again suggesting the existence of either conformer I or IV. For the guests with longer alkyl groups, the sharper signals become predominant. We compared the NMR spectra of these two guest series Fig. These results strongly support conformers I and IV are mainly involved in the binding of these two guest series.
As shown in Fig. It is also supported by the peak line-shapes in the NMR spectra: the NMR peaks of conformer IV are all broadened while the peaks of conformer I remain sharp, since conformer IV has a larger cavity with faster guest exchange kinetics. With even longer linkers, this trend was reversed Fig. This is the conformational response of the host to the structural changes of guests. This conformation can be assigned to conformer II , since conformer II has the lowest symmetry C 2 and should show eight doublets for its aromatic protons.
Compared to the 1,4-diazabicyclo[2. However, the guest is too thin and undergoes fast exchange at the NMR timescale. This results in broadened NMR signals Fig. Two measures were taken to circumvent this problem: a decreasing the temperature; b attaching sizable but not too large end groups. Both ways worked efficiently to slow down the exchange. The peak pattern suggests conformer III C 2h symmetry to be the selected conformation. Conformer III has a long and narrower cavity and fits perfectly to such thin guests. The structures of TA4 Fig. This further consolidates the above assignments.
For conformer IV , both enantiomers appear in pairs in the solid state Fig. For all the guests shown above, one conformer or a combination of two or three conformers of TA4 is selected, and some examples are shown in Fig.ovvacdansba.cf
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That is, different conformers have different binding affinities towards the same guest. The host also shows conformational responses to the structural change of guests. What are the thermodynamic consequences for such conformational responses? In view of the conformational equilibrium, the association free energy surely would undergo a larger decrease were the conformation fixed.
If the conformation is fixed, the increase should be smaller. However, their association free energies are not very different. Consequently, the conformational reservoir of the host shows conformational responses to different guests and thus maximizes the association free energy.