Figure 4: The chemical basis of observed base-pairing interactions of S34 with the third codon adenosine or guanosine.

(a) Equilibrium between two forms of mnm5s2U41. (b) Interactions with adenosine of the neutral form of mnm5s2U. (c) Theoretically predicted base pair with guanosine of zwitterionic form of mnm5s2U carrying negative charge on sulfur atom. (d,e) Two alternative pairing interactions with guanosine and enol tautomeric forms of mnm5s2U. (f) Base-pairing interactions with guanosine of a possible zwitterionic form of mnm5s2U.