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A Brief Background
Published in Nathan Keighley, Miraculous Medicines and the Chemistry of Drug Design, 2020
Understanding electrons is essential to chemistry. In a reaction, chemical bonds must be broken: this may be a heterolytic cleavage, where two electrons in the bond move to one species to form ions, or a homolytic cleavage, where the pair of electrons are shared to produce free radicals. In organic chemistry, the movement of electrons is shown with curly arrows to produce organic reaction mechanisms, which will feature later in the text. Since reactivity is the movement of electrons to break weak bonds and make new, stronger bonds, it is possible to predict how an organic reaction mechanism will proceed. For two reacting molecules, identify where the electrons are coming from. This molecule is termed the nucleophile—a negatively charged ion, or neutral molecule with a lone pair of electrons which are donated to form a covalent bond. The electrons are received by the electron-deficient molecule called an electrophile. Whether a given molecule will react as a nucleophile or an electrophile depends on the functional groups that are present.
Chemistry of Essential Oils
Published in K. Hüsnü Can Başer, Gerhard Buchbauer, Handbook of Essential Oils, 2020
Geranyl pyrophosphate (68) is the precursor for the monoterpenoids. Heterolysis of its carbon–oxygen bond gives the geranyl carbocation (69). In natural systems, this and other carbocations discussed in this chapter do not exist as free ions but rather as incipient carbocations held in enzyme active sites and essentially prompted into cation reactions by the approach of a suitable reagent. For the sake of simplicity, they will be referred to here as carbocations. The reactions are described in chemical terms but all are under enzymic control, and the enzymes present in any given plant will determine the terpenoids it will produce. Thus, essential oil composition can give information about the genetic makeup of the plant. A selection of some of the key biosynthetic routes to monoterpenoids (Devon and Scott, 1972) is shown in Figure 6.15.
Affinity Modification — Organic Chemistry
Published in Dmitri G. Knorre, Valentin V. Vlassov, Affinity Modification of Biopolymers, 1989
Dmitri G. Knorre, Valentin V. Vlassov
Since affinity modification makes use of specific recognition and binding of molecules to biopolymers, this type of reaction should be performed under conditions where biopolymers can realize the potential of molecular recognition: in aqueous solutions at ambient temperature and pH values not far from neutral (so-called “physiological conditions”). The consequence is that only few of the reactions known to organic chemists can be used in affinity modification. These are predominantly heterolytic reactions. Homolytic reactions are met in affinity modification only as rare, although important, exceptions when reactive groups of reagents generate free radicals as does the EDTA•Fe(II) group or when biradicals carbenes and nitrenes are formed upon photolysis of reagents carrying diazo and azido groups. The great majority of affinity reagents are represented by electrophiles since reactive side-chain groups as well as some groups of the main chains of biopolymers are nucleophilic.
Next generation live-attenuated influenza vaccine platforms
Published in Expert Review of Vaccines, 2022
Another strategy to create a SciIAV candidate for vaccine development is to mutate specific viral components [100]. The HA sequence from the virulent strain was modified by introducing the TAG stop codon as a replacement for the ATG start codon that suppresses the signal sequence translation. The candidate virus induced a robust immune response and elicited heterolytic protection to the mice against highly pathogenic influenza virus challenge [119]. In another study using HA and NA from pH1N1 and six segments from PR8, the SciIAV candidate was developed by introducing reassortment between PR8 and pH1N1 viruses [120]. Immunized mice produced influenza-specific humoral antibodies (IgG and IgA) and T-cell immune responses, and the candidate vaccine provided protection against the highly pathogenic challenge.
Phenylalanine 4-monooxygenase: the “sulfoxidation polymorphism”
Published in Xenobiotica, 2020
Stephen C. Mitchell, Glyn B. Steventon
Phenylalanine 4-monoxygenase (PAH, phenylalanine hydroxylase, phenylalaninase, E.C. 1.14.16.1) is a non-haem iron-dependent enzyme that catalyses the hydroxylation of l-phenylalanine to l-tyrosine, a reaction which is the rate limiting step in phenylalanine catabolism. It proceeds via the heterolytic cleavage of molecular oxygen transferring one atom to the phenylalanine moiety whilst the other is reduced to water using tetrahydrobiopterin as the reductant. Inefficiency in the functioning of the enzyme may lead to hyperphenylalaninaemia and precipitate the clinical condition of phenylketonuria (PKU). However, owing to a copious built-in redundancy, as little as 10% of maximum activity is sufficient to maintain acceptable metabolic balance and prevent clinical consequence (Bartholomé et al., 1975; Friedman et al., 1972, 1973). Some even quote a value lower than this; “in classical PKU enzyme activity is <1% of normal” (Carr, 2009). Interestingly, it has been stated that the activity of rat liver phenylalanine 4-monoxygenase displayed circadian rhythmicity (Castells & Shirali, 1971; Kaufman & Fisher, 1974) and perhaps this phenomenon may be involved in diurnal variations seen in plasma phenylalanine levels in humans. However, without doubt many other factors in addition to food intake certainly contribute to this and may be the causation, such as protein catabolism predominating over protein anabolism during the fasting (night-time) period (Cleary et al., 2013).