Cytosine Dissociation

LCA toward amino acids and nucleic bases has also been measured. Wesdemiotis and Cerda measured the alkali metal ion affinities of nucleobases in the gas phase from the dissociation of metal ion-bound heterodimers [nucleobase + B]M+, in which B represents a reference base of known affinity and M is an alkali metal. By assessing the dimer decomposition for two different internal energies, entropy is deconvoluted from enthalpy and LCA values are obtained. For guanine, cytosine, adenine, thymine and uracil, the corresponding Li+-nucleobase bond energies are as follows 57.2, 55.5, 54.1,... 

Scheme 2 Rearrangement of hydrogen bonding induced by hole localization at the G-C Watson-Crick base pair, a Proton shift from the G radical cation to cytosine, b Dissociation of the N2 hydrogen bond in the G radical cation (adapted from [9] and [48])...

On the basis of the UV spectra of the neutral forms of cytosine and of the bathochromic and hypsochromic spectral shifts caused by protonation of cytosine and of its 1-methyl, 3-methyl, 5- and 6-halogeno derivatives, Wempen and Fox105 have concluded that 6-chloro and 6-fluorocytosine were best represented by the form 3 in which the dissociable proton was affixed to N-3. 5 - Ha I ogenocytosines by contrast occur mainly as form 2. 

A less ambiguous situation prevailed when cytosine and cytidine were compared. The former possesses a pK for the ammonium function (4.60) and an acidic pKa (12.16) for the 2-hydroxyl group. The nucleoside also shows two dissociations (4.22 and 12.3). Since the higher dissociation constant was stated109 to be due to the sugar moiety, it followed that the pyrimidine portion, itself, shows no acidic dissociations in this nucleoside. The authors109 concluded, therefore, that cytidine (and uridine) bear the sugar radical on position 1 rather than 6 of the base. 

Since coiled chains of proteins are known to uncurl because of ionic repulsions when ionization occurs, Reid (1957) suggested that excited state dissociation acts as a trigger in rapid biological processes. The 7-azaindole dimer, which undergoes photo-induced double proton transfer (see Section 4), has similarities to the adenine-thymine and guanine-cytosine base pairs of DNA. Its excited state proton transfers have been proposed as possible mechanisms of mutagenesis (Ingram and El-Bayoumi, 1974). 

Fig. 5-3. (a) The structure of acetic acid dimer which has an enthalpy of dissociation of 57.9 kJ mol-1 (A. D. H. Ciague and H. J. Bernstein, Spectrochim. Acta, 1959, 25A, 593). (b) Hydrogen bonding between base pairs such as adenine-thymine or cytosine-guanine. 

The nucleic acids obtained from nucleoproteins are of unknown constitution, but their dissociation-products are fairly well known. They yield on hydrolysis phosphoric acid, pentoses, and derivatives of pyrimidine and of purine (415). It is beyond the scope of this book to discuss adequately the structure of the derivatives of pyrimidine and purine which are obtained from the nucleic acids. The graphic formulas of the derivatives which have been isolated are given here for reference. Uracil, cytosine, and thymine are derivatives of pyrimidine —... 

---