Current Organic Chemistry - Volume 8, Issue 1, 2004

In this issue of Current Organic Chemistry several analytical methods to solve problems associated with the keto-enoltautomerism, molecular recognition and the stereochemistry of small and macromolecules are covered. E. Iglesias reviews work done on keto-enol / enolate equilibria in constrained media, such as aqueous solutions of surfactants forming micelles or cyclodextrins. The discussion offers new insights into enolization which is the standard textbook example of tautomeric rearrangements. K. Yannakopoulou and I. M. Mavridis describe the dependency of the host-guest recognition from structural parameters in the formation of inclusion complexes between cyclodextrins and guest molecules. The power of NMR spectroscopy and solid state x-ray crystallography for the analysis of the host-guest adducts is emphasized. The article of Y. Liu, B-H. Han and H-Y Zhang also deals with molecular recognition using modified cyclodextrins as model systems. Fluorescence spectroscopy, circular dichroism, UV-vis spectroscopy and 2D-NMR spectroscopy are applied to study the conformation and the complexation behavior of native and modified cyclodextrins. In the contribution of D. M. Hatch, J. O. Boles, Z. Li and L. A. “Pete” Silks, the incorporation of Se and Te containing amino acids in proteins is addressed. The advantage of introducing these heavy-atoms in proteins and peptides are to be seen in their ability to reduce the phase problem of macromolecular crystallography. Furthermore they are useful probes for vibrational and NMR spectroscopy. Finally, J. Gawronski and P. Skowronek present new achievments for stereochemical analysis by excition-coupled circular dichroism obtained by introducing extended π-chromophores. The aim of these reviews is to enable the reader to up-date his knowledge in advanced applications of well-established analytical techniques. The editors thank all authors for their efforts in preparing their articles.

Keto-enol tautomerism has been the subject of continuous interest in chemistry. Enols and enolate ions are essential intermediates in many important chemical reactions, and a number of biological transformations also involve enol formation. Enolization, moreover, has long been of interest as an example of tautomeric rearrangement. One of the things that one would like to know about enols is the magnitude of keto (KH) / enol (EH) equilibrium constants (KH EH, KE). This tautomeric equilibrium has been studied for many years and a large number of methods have been applied for investigating the keto-enol conversion. We review some of the work that we have been doing in the field of keto-enol / enolate equilibria characterization and on enol reactivity in constrained media, such as aqueous solutions of surfactants forming micelles or cyclodextrins. A recently developed method of studying keto-enol equilibrium that combines the UV-vis spectroscopy and aqueous solutions of organized microstructures will be presented. The emphasis of the exposition is the comparative analysis of the several methods used in continuing research to evaluate keto-enol equilibrium constants, by paying detailed attention to the difficulties, and advantages / disadvantages of this new method in comparison with previous others. Therefore, after a brief description of common properties of 1,3- dicarbonyl compounds and of micro-organized media, we report studies of keto-enol / enolate equilibria and the methods and techniques used.

The length of guest molecules determines the stoichiometry of inclusion complexes of long endfunctionalized molecules with cyclodextrins in aqueous solution. Molecules up to 10-12 Å form primarily 1:1 host:guest adducts, even if they have to curve inside the cavity, whereas longer molecules form mixtures of 1:1 and 2:1 adducts in equilibrium with each other. The width of the cyclodextrin cavity determines the host-guest recognition, therefore aliphatic chains have a preference for α-cyclodextrin, whereas aromatic rings fit better to β-cyclodextrin. If the size of the cyclodextrin cavity is large relative to the thickness of the guest, and the guest has a propensity to dimerise, then 2:2 host:guest adducts, may be observed. Out of the various mixtures in the aqueous solutions, crystallization of single-stoichiometry inclusion complexes takes place and the guest's end-groups play a key role in the crystal packing. It seems, therefore, that the length of the guest influences critically the first-order organization of cyclodextrins in the aqueous solution, whereas the end-functional groups influence heavily the further organization of the initial assemblies into larger supramolecular arrays, as expressed by the packing modes in the crystal.

This review describes the spectroscopic studies on molecular recognition of a wide variety of guest molecules with native and modified cyclodextrins, including monomodified cyclodextrins, and functional group-bridged bis(cyclodextrin)s. Steady-state and time-resolved fluorescence spectroscopy, circular dichroism spectroscopy, UV-vis spectroscopy, and two-dimensional NMR spectroscopy can be taken to determine the absolute conformation and inclusion complexation behavior of native and modified cyclodextrins. To investigate quantitatively molecular recognition of modified cyclodextrins, different spectrophotometric titration and / or competitive inclusion approach are used to determine the binding constant of inclusion complexes of various guest molecules with native and modified cyclodextrins.

Biosynthetic incorporation of selenium- and tellurium- containing amino acids has been exploited to produce both heavy-atom derivatives and nuclear magnetic resonance probes in biomacromolecules. These derivatives have played a significant role in the elucidation of both the local and global structures of a variety of biomacromolecules such as proteins and oligomeric nucleic acids. Although the replacement of methionine with selenomethionine (SeMet) in a protein was reported as early as 1957, it was not until recently that a seleniumcontaining amino acid was successfully used to simplify the phase problem of macromolecular crystallography through the use of multiwavelength anomalous diffraction (MAD) techniques. Boles and coworkers reported the first incorporation of telluromethionine (TeMet) into a protein. More recent research has included the incorporation of [4,5]selenatryptophan ([4,5]SeTrp) and [6,7]selenatryptophan ([6,7]SeTrp) into recombinant proteins, adding two additional tools to the arsenal of heavy-atom derivatives available to probe the structure and dynamics of biomacromolecules. Methods for the construction of Te / Se-Cys and Se-tyrosine and their potential use in biomolecular studies have been reported.

Circular dichroism spectroscopy based on exciton coupling (ECCD) is a powerful tool for stereochemical analysis of organic compounds, i.e. for the determination of absolute configuration as well as conformation. Although the method is widely used for more than three decades, only in the past decade advances have been made toward making the ECCD method more sensitive and user-friendly. The newly developed extended π-conjugated chromophores with high molecular extinction coefficient (ε >105) allow to make CD measurements with less than 0.1 mg amounts of the analyte. Furthermore, tailor-made chromophoric derivatives allow unobstructed CD measurements in a chosen spectral window extending well into the visible region. In addition, chromophore can now be attached to the analyzed molecule without resorting to the formation of a covalent bond between the functional group of an analyte and the chromophoric molecule. These and other applications, preceded by a short introduction describing foundations of the ECCD method, are covered by the review. This review is dedicated to Professor Maciej Wiewiorowski on the occasion of his 85th birthday and in recognition of his contribution to the field of natural products chemistry.