Guanidinium ions

X-ray analysis of the complex [67] between 2,6-pyridino-17-crown-9 [9b] (n = 6) and a guanidinium ion showed that one hydrogen bond is formed between the lone pair of the pyridine nitrogen atom and a hydrogen atom of Gu". The other five hydrogen atoms are bound to ether oxygens in their vicinity. The association constants for the reaction of some macrocycles of type [9b] (n = 6-8) with Gu" were comparable (logX 1.18-1.44). 

The complex formed by thymine with the guanidinium ion (Figs. 6.7, 6.8) and the complex formed by thymine with the aminopyrolidinium ion (Figs. 6.9,6.10) were subjected to ab initio (Hartree-Fock) calculations using the 6-31G find the 3-21G basis sets, in order to... 

The binding energy of the guanidinium ion is found to be higher by 2.9 kcals/mole than the one of the aminopyrrolidium ion. This difference agrees with the fact that netropsin binds better than anthelvencin to DNA but it is too small to account for the experimental difference in binding. 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

The effects on the dynamics of photo-injected electrons where not systematically studied, despite scattered reports on the influence of amines, which induce surface deprotonation, and lower surface charge with a resulting negative shift in band edge position and an increase in the open circuit potential, Voc [103], The opposite effect is induced by Li+ ions, which intercalate in the oxide structure. Guanidinium ions increase Voc when used as counterions in place of Li+. Other adsorbing molecules that influence both Voc and short circuit current are polycar-boxylic acids, phosphonic acids, chenodeoxycholate and 4-guanidinobutyric acid. 

Infra-red studies [31] and molecular orbital calculations [33,34] have led to the description of the guanidinium ion as a tri-amino carbonium ion with the TT-electron charge distribution shown (IX) Most of the positive charge is located in the vicinity of the central carbon atom. The relevance of this description to the pharmacological properties of guanidinium ions will be discussed later. For typographical convenience, guanidines will be formulated in this review in the unprotonated form. 

The similarity of the guanidinium ion, in which the distance between the carbon and the hydrogen atoms is about 2.1 A, to the hydrated sodium ion [Na(OH2)3] in which the distance between the sodium and the hydrogen atoms is about 2.3A, has been pointed out [33, 36a], and the physiological... 

Guanidine forms salts with such relatively weak acids as nitromethane, phthalimide, phenol and carbonic acid [20], Interactions between carboxylate anions of proteins and added guanidinium ion are thought [19, 56] to be weaker than the interactions with ammonium ions the role of guanidinium-carboxylate interactions in stabilizing natural protein conformations has been discussed [36c]. A few reports of metal complex formation by guanidines [57-60], and aminoguanidines [61] have appeared. 

The above speculation [21] may be extended to include the related quaternary ammonium compounds such as xylocholine (XXXIX). It is probable that the volumes of the guanidinium ion and the trimethylammonium group are similar. The ionic radius of the guanidinium ion (IX) is about 3A the ionic radius of the tetramethylammonium ion has been estimated [300] to be 3-4A, although rather smaller values have also been proposed [301-303]. Crystallographic analyses of muscarine iodide [304], choline chloride [305] and acetylcholine bromide [306] have revealed that the carbon to nitrogen distance is about l-SA, and that a hydrogen bond (C-H-0 distance 2-87-3 07A) exists in the crystals of these compounds. 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

Reliable interatomic-distance data are not available for guanidine or the guanidinium ion. 

The side chains of aspartic and glutamic acids carry negatively charged carboxylate groups at pH 7 while those of lysine and arginine carry the positively charged -NH3+ and guanidinium ions, respectively. 

Figure 12-29 Drawing showing the hydrogen-bonding interactions between the guanidinium ions of arginines 35 and 87 of the micrococcal (staphylococcal) nuclease with the 5 -phosphate of the inhibitor thymidine 3, 5 -diphosphate in the complex of E + I + Ca2+. A possible mechanism is illustrated. A hydroxyl ion bound to Ca2+ carries out an in-line attack on the phosphorus. See Libson et al.S26...

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