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C Schmuck

Publications and source records attributed to C Schmuck.

7 recordsLinked to original sources
PubMed2001

Anion-dependent dimerization of a guanidiniocarbonyl pyrrole cation in DMSO.

With spherical counteranions such as chloride or hexafluorophosphate, the glycine-derived guanidiniocarbonyl pyrrole cation 1 self-assembles into discrete dimers in DMSO, as can be seen by NMR and ESI mass spectral analysis. According to concentration- and temperature-dependent NMR studies, the dimerization is endothermic and therefore entropy driven. Molecular modeling suggests that the dimers are held together by hydrogen bonding in combination with pi-pi interactions. In the presence of picrate anions, dimerization of cation 1 does not occur, probably due to the formation of pi-stacked ion pairs.

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PubMed2001

A self-threaded "molecular 8".

A new type of [1]rotaxanes containing two aliphatic bridges between axle and wheel is obtained in 39% yield in a one-step synthesis starting from a [2]rotaxane which contained one sulfonamide group each in both the wheel and the axle. Temperature controlled chemoselective substitution reactions first at these sulfonamide nitrogens and then subsequently at the various other carboxamide nitrogens in the wheel and axle give rise to the formation of an isomeric mixture of three double-bridged [1]rotaxanes which could be separated by HPLC. Structure determination of the main product 3a was possible by NMR experiments supported by molecular modeling calculations. Using different reaction conditions, a double-substituted but not yet bridged [2]rotaxane 4 could be isolated as an intermediate giving further evidence for the assigned structure of 3a and the way of its formation. The shape of this double-bridged [1]rotaxane 3a reminds of a self-intertwining chiral "molecular 8", in which any possible racemization due to deslipping is hindered by the two stoppers originating from the former rotaxane axle. Hence, to the best of our knowledge this is the first example of a molecule in which both concepts, cycloenantiomerism and helical chirality, are realised in one structure. Enantiomer separation of the main product was possible by further HPLC using chiral stationary phases. The Cotton effects of the circular dichrograms are different to those of the already synthesized [1]rotaxanes bearing just one aliphatic bridge between axle and wheel.

Journal Article↗
PubMed2000

Self-folding molecules: a well defined, stable loop formed by a carboxylate-guanidinium zwitterion in DMSO

Two zwitterions 1a,b have been synthesized, in which a carboxylate group is attached via a flexible alkyl chain of different length (butylene and ethylene, respectively) to a guanidiniocarbonyl pyrrole cation moiety. For 1b, no signs for either an intra- or intermolecular association between these two groups in polar solution (DMSO) could be found. In contrast to this, the 1H NMR spectrum of 1a shows clear evidence for a strong interaction between the carboxylate and the guanidiniocarbonyl pyrrole cation. According to variable-temperature and concentration-dependent concentration-dependent NMR studies, this interaction stems from an intramolecular complexation. It was shown by ROESY and H/D-solvent exchange experiments that 1a, even in DMSO, folds into a well-defined intramolecular loop conformation held together by multiple weak interactions.

Journal Article↗
PubMed2000

Carboxylate binding by 2-(guanidiniocarbonyl)pyrrole receptors in aqueous solvents: improving the binding properties of guanidinium cations through additional hydrogen bonds.

A series of guanidiniocarbonyl pyrrole receptors has been synthesized which bind carboxylates by ion pairing in combination with multiple hydrogen bonds. Their binding properties with various carboxylates have been investigated using NMR titration studies in 40% water/DMSO (v/v). The best receptor has association constants which are in the order of K approximately/= 10(3) mol(-1) and hence some 30 times larger than with the simple acetyl guanidinium cation. Through a systematic variation of the receptor structure, semiquantitative estimates for the energetic contributions of the individual binding interactions could be derived. These data show that the various hydrogen bonds are not equally important for the binding but differ significantly in their energetic contribution to the overall complexation process. Furthermore, the receptor can be made chiral and shows selectivity upon binding of enantiomeric amino acid carboxylates. Molecular modeling was used to obtain structural information for the various receptor carboxylate complexes and served as a basis to explain the observed differences in binding constants.

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