Jean-Paul Behr

The binding of divalent cations by purified gangliosides

The gangliosides are glycosphingolipids containing sialic acids. They are mainly found in certain membrane fractions of the central nervous systems [ 11; but they may also be detected in the visceral system where they are implicated in intercellular interactions and seem to inhibit the proliferation of malignant cells ([2] and references therein). Numerous data suggest that they may play an important role in the nervous system, especially via the carboxyl groups of the sialic acids [ 1, see discussion]. These carboxylates contribute to the membrane electric charge and interact with various cationic species [3]. Divalent metal cations, especially calcium, play a fundamental role in the release of biogenie amines and in secretory processes in general [4]. It seemed thus of interest to study the stability constants and the nature of the complexes formed between such cations and ganglioside aggregates, which could be considered as simplified membrane models. Cation association with sialic(N-acetylneuraminic)acid itself has been studied previously [5].

Stability constants for the complexation of alkali and alkaline-earth cations by N-acetyl-neuraminic acid

N-Acetylneuraminic acid (NANA) is found in numerous glycoproteins and glycolipids. The carbohydrate components of these molecules, which may be either free or associated with membrane structures, take part in a variety of biological phenomena such as contact inhibition, antigenic and hormonal action, fibrinogin coagulation and receptor functions for neurotransmitters (see reviews in [ l-41 ). Removal of the NANA residues with neuraminidase affects markedly their properties (see review in [l-5] ): thus plasma glycoproteins and gonadotropic hormones are rapidly eliminated from the circulation [6], and gangliosides loose their receptor properties [7]. On the other hand, the behaviour of most of these molecules is closely related to the presence of mono and divalent metal cations, especially of calcium ions. As a first step in a study of the interaction of gangliosides with various cationic species it was of interest to determine the association constants between metal cations and the most widely distributed component of these macromolecules bearing an anionic site: Nacetylneuraminic acid. Such data also bear on the role of gangliosides in cation transport phenomena [I]. In addition, much interest has been shown in recent years for the alkali and alkaline earth metal cation complexing properties of various natural [8,9] and synthetic [ 1 O-l 21 substances.

Molecular dynamics of acetylcholine and of choline

Abstract A Fourier Transform 13C and 14N NMR study of the behaviour of acetylcholine in solution has been performed. The local molecular motions and the corresponding activation parameters at each carbon have been obtained. The nitrogen-14 quadrupolar coupling constant, which reflects the electric properties of the quaternary ammonium site, has been determined. Results about the conformation of the molecule are also presented. No complexation with either AMP, ATP or CMP in the presence or absence of alcaline earth ions, could be detected.

Enhanced rates of dihydropyridine to pyridinium hydrogen transfer in complexes of an active macrocyclic receptor molecule

The complexes formed by the dihydropyridine-bearing macrocyclic receptor molecule (I) with the pyridinium substrates (S1) and (S2) display enhanced rates of internal H-transfer which may be inhibited by addition of other complexable cations.

Stable ammonium cryptates of chiral macrocyclic receptor molecules bearing amino-acid side-chains

The chiral 18-crown-6-tetracarbonyl-[1]-cryptands (3), (5), and (6) are molecular receptors for primary ammonium salts, forming stable complexes whose stability and selectivity depends on interactions with the side chains of the ligands.