Nucleotides, complexes with cations

The differential absorption spectra obtained in the presence of these two nucleotides are indeed similar to those obtained after reduction electrolysis of the complex in the first reduction wave, and obtained by pulse radiolysis. The prerence of the deprotonated radical cation GMI —H) can also be detected by recording the transient absorption after reaction of the reduced complex with O2. 

For the alkoxy receptor 12, which can also form 1 1 complexes with guanine nucleotides, the influence of the cationic site/receptor ratio on the EMF selectivity was determined (Table 2). Electrodes based on 12 and 210 mol% 9 gave similar, Nemstian responses to both 5 -GMP and 5 -AMP, as expected for an excess of cationic sites. Due to the high concentration of cation 9, the concentration of uncomplexed nucleotides in these membrane is high and a selectivity typical for a... 

An additional method to determine the nature of the metal-nucleotide complex that is preferred by the enzyme is in the study of the effects of cation substitution on the stereoselectivity of chiral-specific thiophosphate nucleotides. Thiophosphate derivatives of cAMP, AMP, ADP, and ATP at the a-, /3-, and/or y-positions were initially introduced by Eckstein and co-workers (66, 67) and expanded to several other nucleotides. Substitution of a phosphate oxygen by sulfur at the a-position of cAMP, ADP, or ATP or at the /3-position of ATP gives rise to a new chiral center, Substitution in the a-position of AMP, j8-position of ADP, and the y-position of ATP changes an achiral center to a prochiral center. This substitution gives rise to a small decrease in p a for the phosphate, and with many enzymes these thiophosphate analogs have a decreased substrate ac-... 

Hodson and Azam (1977) have reported that up to 20% of the total ATP in seawater occurs in the free form. It is, however, difficult to say whether this finding was a product of sample manipulation. At least in bacterial cultures, it has been shown that nucleotides may be released into the medium (Chapmann et al., 1971). On the other hand, it is questionable whether ATP is stable enough to exist in solution over longer periods owing to its low stability constant or the possibility of complexation with divalent cations and fulvic acids (Tetas and Lowenstein, 1963 Hulett, 1970 Bulleid, 1978). 

Nucleosides and nucleotides can also be analyzed together. Lamb and Ye [149], for example, used a neutral divinylbenzene polymer (Dionex lonPac NSl), which they coated with an -decyl-substituted bicyclic polyether such as l,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane (D2.2.2). This coating results in a chromatographic matrix, on which both ionic and hydro-phobic interactions can occur. The so-called cryptands [150] (like their mono-cyclic analogs, bicyclic polyethers) form rather stable complexes with alkali metals. These metals are included in the cavity of the ring system their presence results in weak covalent bonds between the cation and the two N atoms. 

---