complexes part 13

The enhancement of solubility by urea and its derivatives was a result of hydrophobic bonding rather than complexation. Urea broke up the hydrogen-bonded water clusters surround¬ing the nonpolar solute molecules, increasing the entropy of the system and producing a driving force for solubilization of benzoic and salicylic acids
The interaction of drugs with proteins in the body may involve hydrophobic bonding at least in part, and this force in turn may affect the metabolism, excretion, and biologic activity of a drug.
Self-association
Some drug molecules may self-associate to form dimers, trimers, or aggregates of larger sizes. A high degree of as¬sociation may lead to formation of micelles, depending on the nature of the molecule. Doxorubicin forms dimers, the process being influenced by buffer composition and ionic strength. The formation of tetramers is favored by hydrophobic stacking aggregation. Self-association may af¬fect solubility, diffusion, transport through membranes, and therapeutic action. Insulin shows concentration-dependent self-association, which leads to complications in the treat¬ment of diabetes. Aggregation is of particular importance in long-term insulin devices, where insulin crystals have been observed. The initial step of insulin self-association is a hy¬drophobic interaction of the monomers to form dimers, which further associate into larger aggregates. The process is fa¬vored at higher concentrations of insulin. Addition of urea at nontoxic concentrations has been shown to inhibit the self-association of insulin. Urea breaks up the "ice¬bergs" in liquid water and associates with structured water by hydrogen bonding, taking an active part in the formation of a more open "lattice" structure.
Factors affecting complexation and protein binding
A significant correlation was found between the stabil¬ity constant of the water-soluble complexes and the hydrophobicity of the ligands. Electrostatic forces were not considered as an im¬portant because all compounds studied were uncharged un¬der the conditions investigated. Donor-acceptor properties and relative donor-acceptor strengths corre¬lated poorly with the formation constants of the complex. It was suggested that ligand hydrophobicity is the main contri¬bution to the formation of water-soluble complexes. It is found that, the more hydrophobic chlorobiocin analogues showed the highest percentage of drug bound to human serum albumin. Chloro¬biocin analogues bind to human albumin at the same site as warfarin. This site consists of two noncoplanar hydrophobic areas and a cationic group. Warfarin, an anticoagulant, serves as a model drug in protein-binding studies because it is ex¬tensively but weakly bound. Thus, many drugs are able to compete with and displace warfarin from its binding sites. The displacement may result in a sudden increase of the free (unbound) fraction in plasma, leading to toxicity, because only the free fraction of a drug is pharmacologically active, also the displacement of warfarin by non steroidal anti inflammatory drugs was investigated. The results showed variation of the stability constant, K, and the number of bind¬ing sites, n, of the complex albumin-warfarin after addition of competing drugs. Azapropazone and Phenylbutazone markedly decreases the K value, suggesting that both drugs, compete with warfarin for the same binding-site on albumin. also competes strongly for the binding site on albumin. Conversely, tolmetin may increase K, as suggested by a conformational change in the albumin molecule that favors warfarin binding.