The Conformation of Nucleosides in Solution

The reliability of this method of determining pK was tested with methionine, the pK value of which is well known. The pK of 2.2 0.1 obtained by spectrophoto-metric titration is in good agreement with 2.28 (Emerson et al., 1931). 

The adenine frequencies of S-adenosylmethionine and S-adenosylhomocysteine in acid solution are identical with one another and with those for adenosine under similar conditions (Tsuboi et al., 1962). The absorption bands due to the COO and GOOD of these compounds are also very close to one another. The small shift in the GOO band may reflect a slight perturbation by a relatively far removed sulfonium center. Glosely spaced groups lead to a larger change in frequency. 

The pK and infrared data serve to rule out a number of possible conformations of S-adenosylmethionine which might have been proposed (Klee and Mudd, 1967). For example, there are ample possibilities for hydrogen bond formation between the 

The following picture of the conformation of S-adenosylmethionine evolves from the data a reasonably, but not completely, extended molecule which is highly hydrated due to many charged and other polar groups. The adenine ring may be anti more often than syn (see adenosine LXIII) (Klee and Mudd, 1967), but not for as much of the time as is the adenine of S-adenosylhomocysteine, which molecule has more opportunities for intramolecular bonding due to the absence of a charged sulfur atom. 

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