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1 Introduction

an enzyme, even that is important. Usually, the enzyme ensures that the substrate binds

correctly by offering corresponding functional groups creating the correct intermolec-

ular forces, such as a hydrogen donor opposite to a hydrogen acceptor.

Whether double bonds are cis or trans (or E and Z for more highly substituted dou-

ble bonds) makes a big difference, as can easily be seen in the structure of fatty acids.

Cis-isomers are bent and thus have a lower Tg than trans isomers, which are straight

chains and crystallize well (Figure 1.6). Using that fact, a membrane can control its stiff-

ness by choosing the correct composition of its lipids.

Stereoisomers that do not change the properties of the molecules but still are crucial

in binding and recognition events are compounds with chiral centers, i. e., carbons that

contain four different substituents. With one stereocenter or chiral carbon, these two

compounds are enantiomers, or mirror images from each other (like our hands, which

is why this property is also called “handedness”). The more stereocenters a molecule

has, the more compounds there are; if they are not fully mirror images anymore they

are either diastereomers or meso-compounds (Figure 1.26).

We looked at the properties and reactivities of organic and biomolecules. What reac-

tions can these molecules perform? There are two broad categories of reactions: radical

reactions or polar reactions, i. e., reactions moving one electron versus two electrons,

respectively. In Organic Chemistry, radical reactions mostly react with multiple bonds

(unless you work with very harsh reaction conditions). With radical reactions, you need

an initiating radical (often an initiator molecule split evenly into two), then the reaction

propagates itself and only ends when two radicals combine or the monomer runs out

(Figure 1.27). The majority of vinyl polymers are prepared in this fashion. In Biochem-

istry, some oxidation and reduction reactions are also radical reactions with the help of a

cofactor or catalyst that can feed or take one electron at a time. CytochromeP450 is such

an enzyme. CytP450 oxidizes foreign compounds in the body to make them hydrophilic

enough to be excreted in urine (i. e. water soluble). In most cases, these cofactors have

metals in their center that have several oxidation states and with that can remove or

donate electrons one at a time.

The large majority of Organic Chemistry or Biochemistry reactions, though, are po-

lar reactions where a nucleophile reacts with an electrophile. Organic Chemistry catego-

rizes these reactions by the types of mechanisms (e. g. electrophilic addition to multiple

bonds, nucleophilic substitution, carbonyl condensation reaction); biochemistry catego-

rizes reactions by the effect the reaction has on the carbon skeleton of the compound

(e. g. isomerization, group transfer reactions, condensation, hydrolysis). Regardless of

how the reactions are called, the mechanisms are the same. In Biochemistry, though,

most reactions occur via a catalyst, an enzyme. This allows the reaction to take place in

water at 37 °C, regardless of what the exact solubility and reactivity is under these condi-

tions. In fact, most biochemical reactions are faster and have less side products than they

would in Organic Chemistry because of the enzyme. The other advantage of the enzyme

is that it can be precisely regulated, either by binding different substrates differently or

not at all, or by binding other factors allosterically (at a different part of the enzyme)