The maturation of RNAs includes site-specific post-transcriptional modifications that contribute significantly to hydrogen bond formation within RNA and between different RNAs, especially in formation of mismatch base pairs. Thus, an understanding of the geometry and strength of the base-pairing of modified ribonucleoside 5'-monophosphates, previously not defined, is applicable to investigations of RNA structure and function and of the design of novel RNAs. The geometry and free energies of base-pairings were calculated in aqueous solution under neutral conditions with AMBER force fields and molecular dynamics simulations (MDSs). For example, unmodified uridines were observed to bind to uridine and cytidine with significant stability, but the ribose C1'-C1' distances were far short ( approximately 8.9 A) of distances observed for canonical A-form RNA helices. In contrast, 5-oxyacetic acid uridine, known to bind adenosine, wobble to guanosine, and form mismatch base pairs with uridine and cytidine, bound adenosine and guanosine with geometries and energies comparable to an unmodified uridine. However, the 5-oxyacetic acid uridine base paired to uridine and cytidine with a C1'-C1' distance comparable to that of an A-form helix, approximately 11 A, when a H(2)O molecule migrated between and stably hydrogen bonded to both bases. Even in formation of canonical base pairs, intermediate structures with a second energy minimum consisted of transient H(2)O molecules forming hydrogen bonded bridges between the two bases. Thus, MDS is predictive of the effects of modifications, H(2)O molecule intervention in the formation of base-pair geometry, and energies that are important for native RNA structure and function.