Stability and structure of some organic molecular complexes in aqueous solution

Semi-quantitative evaluation of molecular meshing via surface analysis with varying probe radii

Molecular meshing in molecular recognition and assembly can be assessed by plotting the distribution of contact surfaces against the contact distance.

Drug solubilization by complexation

Drugs must possess some solubility in water to be therapeutically effective after oral or topical administration to the eye, and drugs must be soluble to be formulated as aqueous solutions for, for example, parenteral delivery. A variety of methods can be applied to enhance aqueous solubility of poorly soluble drugs one of which is the usage of solubilizing complexing agents. There are numerous types of complexes and some are more water-soluble than others. Coordination complexes consist of drugs that act as complexing agents (i.e. ligands) and metal ions (i.e. substrates). Examples of coordination complexes are some water-soluble tetracycline-metal ion complexes. Organic molecular complexes can consist of a small substrate (i.e. the drug) and a small (e.g., caffeine) or a large (e.g., polyvinylpyrrolidone) ligand. In inclusion complexes the substrate is partly or completely enveloped by the complexing agent (e.g., cyclodextrin). Finally, pharmacosomes are drug-phospholipid complexes that can not only enhance aqueous solubility of poorly soluble drugs but also their solubility in organic solvents. This is a mini-review of solubilizing complexing agents that are or can be used in pharmaceutical products.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Energetic and entropic factors determining binding affinity in protein-ligand complexes

Understanding of non-covalent interactions in protein-ligand complexes is essential in modern biochemistry and should contribute toward the discovery of new drugs. The affinity of a ligand toward its receptor falls into a range of 10-80 kJ/mol. It is related to the binding constant and corresponds to a free energy. Accordingly enthalpic and entropic effects determine binding affinity. Hydrogen bonds and lipophilic contacts are the most important contributions to protein-ligand interactions. They are governed by changes in entropy and enthalpy. Solvation and desolvation effects either of the ligand and the protein binding site play a key role in the binding process. Prerequisite for a quantitative description and subsequently for a prediction of protein-ligand interactions is a partitioning in additive group contributions. In many cases, this additivity seems to be a good approximation, however, phenomena such as conformational pre-organizations give rise for a non-additive behavior. Flexibility and mobility of the bound ligand influence binding affinity. The rare experiments separating enthalpic and entropic contributions to the binding affinity sometimes reveal surprisings results, e.g. the loss of a hydrogen bond parallels with a loss in entropy.

Water soluble complexes of the antiviral drugs, 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine and acyclovir: the role of hydrophobicity in complex formation

We investigated water-soluble complexes of various ligands with the antiviral drugs, 9-[(2-hydroxyethoxy)methyl]guanine (acyclovir) and 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (DHPG). For comparison, we also examined the "parent" compounds, guanine and guanosine, as substrates for complex formation. Using the phase-solubility technique, we measured formation constant (K1) values at 23 degrees C in pH 7 buffer. For a single substrate, formation constants with different ligands varied in the order: caffeine greater than pyridoxine approximately cytidine greater than nicotinamide greater than sucrose. With caffeine as the ligand, formation constants with different substrates varied in the order: guanine greater than guanosine approximately acyclovir greater than DHPG. The largest formation constant observed was 58 M-1 (for guanine-caffeine), and the smallest formation constant was 0.29 M-1 (for DHPG-sucrose). Examining the literature for formation constant data on compounds related to DHPG, and comparing literature data with our own, reveals a significant correlation between formation constants and ligand hydrophobicity. For 41 substrate-ligand pairs, least squares linear regression analysis of log K1 values versus various parameters reflecting donor-acceptor abilities (e.g., substrate and ligand HOMO and LUMO values, or substrate oxidation potentials) failed to significantly correlate. We conclude that ligand hydrophobicity is a general determinant of water soluble complex formation, but not necessarily the exclusive or dominant controlling factor for all complexes. Charge-transfer interactions are not important determinants of complex formation for the substrate-ligand combinations that we have considered.

Thermodynamic parameters of molecular complexes in aqueous solution: enthalpy-entropy compensation in a series of complexes of caffeine with beta- naphthoxyacetic acid and drug-related aromatic compounds

Stability constants and thermodynamic parameters have been evaluated for the complexation reaction in aqueous solution of caffeine with beta-naphthoxy acetic acid. The values were higher than those previously reported for the complexation of other ligands with methyl xanthines. In nearly all aromatic ligands complexing with caffeine and theophylline for which data are available, both entropy and free energy of complexation were linearly related to the enthalpy, giving an isoequilibrium relationship. Salicylamide, sodium benzoate and cis-methyl cinnamate exhibited slight deviations on the delta G-delta H plot; the non-aromatic dehydroacetic acid showed the largest deviation. The isoequilibrium relationship was shown to be valid statistically (349-365 K, caffeine systems; 353-372 K, caffeine and theophylline systems) indicating underlying chemical causation. Thermodynamic equations are presented for analysis of the factor involved, which are attributed to a combination of substrate-ligand interactions and solvent effects. The substrate-ligand overlap area is considered as a common parameter through which the solvent and interaction forces might cooperate to give rise to linearity in the isoequilibrium relationship. The increasingly negative experimental values of the enthalpy and entropy with increase in ligand planar overlap area are discussed in relation to the underlying forces involved in the complexation.

trans-Cinnamic acid--alpha-cyclodextrin system as studied by solubility, spectral, and potentiometric techniques

Complex formation in aqueous solutions of trans-cinnamic acid or trans-cinnamate ion (the substrate, S) and alpha-cyclodextrin (the ligand, L) can be described quantiatively as the 1:1 and 1:2 complexes, SL and SL2. The solubility, spectral, and potentiometric data over a wide range of ligand concentrations yielded consistent estimates of the complex association constants. For cinnamic acid at 25 degrees K11 = 2260 M-1, delta H degree 11 = 9.3 kcal/mole, and delta S degree 11 = -8 e.u.; and K12 = 60 M-1, delta H degree 12 = -12 kcal/mole, and delta S degree 12 = -26 e.u. For cinnamate ion at 25 degrees, K11 = 110 M-1, delta H degree 11 = -1.9 kcal/mole, and delta S degree 11 = +11 e.u.; and K12 = 15 M-1, delta H degree 12 = 9 kcal/mole, and delta S degree 12 = -15 e.u. (all entrophy changes are unitary quantities). Thermodynamic cycles for the complexes, using solubility data, reveal that complex formation in the solid phase is thermodynamically spontaneous but that complex stability is greater in ageous solution than in the solid phase.

Stability of salicylamide-caffeine complex at different temperatures and its thermodynamic parameters

The stability constants for formation of complexes of salicylamide with caffeine have been measured between 15 and 45degrees, by means of the solubility method. There was a linear solubility increase at all temperatures but phase diagrams indicated that at 15 and 25degrees an additional phase was formed which was found to be an insoluble 1 : 1 complex. The enthalpies and entropies of interactions were evaluated. They indicate that the interaction is exothermic and enthalpy controlled.