Page - (000024) - in Biomedical Chemistry: Current Trends and Developments

difference, maintaining the same composition of atoms and bonds. 1.1.3.1 Cis/trans Isomerism Cis/trans stereoisomerism exists in alkenes (double bonds) and saturated rings (cycloalkanes), the latter because the π-bonds and σ– bonds of saturated rings cannot rotate freely. As a consequence, a group in one side of the double bond or saturated ring cannot change sides without breaking bonds. This causes different properties in compounds with opposite configurations. A double bond presents a cis- configuration if equal substituents are on the same side and a trans- configuration if they are on opposite sides (Fig. 1.1.2). This type of isomerism is particularly relevant for the physical properties of compounds, with differences being greater if polar groups are present. Reactivity is also affected, as cis compounds tend to have larger strain on the bonds making them easier to break. Although the cis/trans designation is more common, it is worth of mentioning that Z/E designation is also used in some cases. The general designation of Z/E isomerism for stereoisomers follows simple rules of priority established by Cahn, Ingold and Prelog which allow unequivocal classification of each isomer. In this nomenclature, substituents with equal priority on the same side of the double bond or saturated ring give the Z-isomer, whereas in opposite sides give the E- isomer. Nevertheless, it is important to highlight that in biological systems the vinylic systems present, almost always, hydrogen atoms attached to the vinylic carbons. Figure 1.1.2: Maleic and fumaric acid structures with their respective melting points, highlighting the importance of cis-trans isomerism for physical properties.