Synthetic spectroscopic models related to coenzymes and base pairs. VII. Stacking interactions in tRNA; the "bend" at dimethylguanosine.

We have examined the stacking interactions of N(2)-dimethyl-guanosine with the nucleosides, e.g., adenosine and cytidine, found adjacent to it in certain tRNA's, by the use of model compounds in which the trimethylene bridge was substituted for the ribose-phosphate-ribose linkage. From the hypochromism exhibited by synthetic 9-[3-(aden-9-yl)propyl]-2-dimethylaminopurine-6-one (IV) and by 9-[3-(cytos-1-yl)propyl]2-dimethylaminopurin-6-one in aqueous solution (VI) it is appearent that the interaction is at least as great between the N(2)-dimethylguanine moiety and adenine or cytosine as between guanine and these two bases. The fluorescence and phosphorescence emission spectra were obtained in ethylene glycol-water glass at 80 degrees K. The exciplex fluorescence observed for both bi-molecules (IV and VI) containing the N(2)-dimethylguanine unit provides further evidence for stacked chromophores.