Estrategia de comunicación Definición de la categoría de estudio

Studies in aqueous media also present special challenges. One is the prominent role of the hydrophobic interaction. This large effect can often dominate binding studies in aqueous media,

effect can be difficult. Also, in order to make synthetic receptors water soluble, one must often append polar groups that are typically charged. The possibility of conventional electrostatic interactions between cationic guests and these polar groups can complicate analysis of binding

80 studies

It was discovered that, in all cases, the deoxygenation of certain hydroxyl groups essentially abolished binding, and this brought attention to the fact that all the epitopes were amphiphilic in

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character . In the case of complex oligosaccharides, the epitopes normally involved a cluster of two to four hydroxyl groups, and these were designated the key hydroxyl groups. Since, prior to complex formation, the polar groups of both the epitope and the receptor site were surely extensively hydrated, water molecules would have to be displaced for the complex to

QO QO

form ’ . In view of the directional demands for the formation of hydrogen bonds , a high degree of complementarity was necessary, otherwise dehydration would be energetically

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difficult and complex formation strongly discouraged . For this reason, the water molecules of the hydration shell were considered a hindrance to access to the receptor site by non-

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complementary structures, and the hydrated polar gate concept was proposed . Paraphrasing Emil Fischer^, the hydrated polar groups within the combining site were viewed as a locked gate that could be opened only by the key polar groups of the epitope. In this context, water molecules are intimately connected to the specificity of binding. With regard to structural requirements for effective binding, it is noteworthy that simple monosaccharides often display low but detectable activities . It was exciting to learn, through the use of deoxy congeners, that a strong recognition of an oligosaccharide normally involved key hydroxyl groups on more

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than one sugar unit . It is also noteworthy that, in order to effect complementarity, stereoelectronically well stabilised water molecules are often occluded within the complex®®’®®. It was estimated that the cost in entropy for the imprisonment of a single water

i90 molecule can be as high as 2 kcal mol'

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Recently, Braga and co-workers stated that water molecules can play an important role in stabilisation of organometallic solids by participating in both OH-O and CH-O interactions. They also suggested that hydrogen bonds, both of conventional OH-O and controversial CH-O types, afford a pattern of interactions in common between organics, organometallics and water that can be utilised to engineer crystalline materials on the basis of the complementarity between donors and acceptors. The study recently reported by Abe and co-workers suggested that water molecules reduce the number of ionic contacts between cationic and anionic species and

Introduction

It should be stressed that the hydrogen-bond structure of water and its Interactions with the surroundings still represent a key issue in the study of highly hydrated systems®^. Additional information on water binding can be of some importance in the study of biological macromolecules^®.

1.4 Electrochemistry

Electrochemistry is primarily concerned with charge transfer at the boundary between an electronically conducting or semi-conducting phase and an ionically conducting phase, such as a liquid, molten or solid electrolyte. By extension, the subject has traditionally included the study of ionic equilibria and dynamic processes taking place within ionic electrolytes, particularly from the perspective of those processes determining the concentration of electroactive species at or near the electrode surface.

The overall electrochemical oxidation and reduction reactions of organic and organometallic molecules often comprise complex sequences of electrochemical and chemical steps. In the shorthand notation now universally used, an electrochemical step is designated with the identifier E and is defined as a step involving loss or gain of an electron at the electrode­ solution interface. Chemical steps, designated with the letter 0 , can be surface chemical reactions involving reactants. Most investigators have concentrated on the chemical steps. The types of chemical reactions that are encountered as steps in organic and organometallic electrode reactions are extremely diverse. A given reaction can be a protonation or deprotonation, bond cleavage, complexation or décomplexation, ligand exchange, nucleophilic or electrophilic attack, polymerisation, isomérisation, or conformational change. For example, the reduction of a quinone to the hydroquinone in a proton-donating medium requires two E steps and two C steps, the latter being protonations. Designating the quinone as a Q, one can write a substantial number of possible reaction sequences. However, for benzoquinone, it has been shown that only three reaction sequences are of importance, a CECE process in acidic media (pH < 2), an ECEC sequence at pH > 7, and an ECCE reaction at intermediate pH®®’®^.

Q — ► H Q ^ ► HQ* Q Q * ‘ HQ‘ Q Q * ' --- ► HQ* 0 ^ H j Q * ^

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c

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