The soliition-statc behavior o f two tr ir iboni ic l~~ot i~~~~s, udenj~lj*1(3'-5') adenj~Iy1(3'-5') cyt idinr [ r (AAC)] and adi~nyl~ l (3 ' -5 ' )c~t id~l .~ l (3 ' -5 ' )c~t id inr [ r(AC'C)], NUS studied with spectroscopic and moleciilar modding methods. Melting tctnperutiirrr 01'299 und 294 K,fOr r (AAC) and r(AC'C) , respectively, were obtainedj-om iiltraviolet ubsorption ( U V ) and circular dichroism (CD) temperatrivr pryfiles qf the order-disorder transif ion. Ti7e bclhuvior qfthe Ratnan tnurker modcs is consistent Mith greatu stabilitj, of r(AAC) iwnpurcd to that qf r(ACC) Niicleur magnetic resonance (nmr) reluxation data (homomiclear cross-relaxation rates, proton selective and nonselective longitudinal reluxat ion times, und carbon longitiidinal relaxation times) were measiired at 283, 296, and 318 K,for both trirners. In parallel, the major t j y s of conformations were explored with Metropolis Monte Curlo ( M M C ) and molmilar d.vnumics ( M D ) simiilations to obtain representations ofboth slow and,fust events. Fitting ofexperimental data .showed that althoiigh the MMC coi?ji,rmations do not represent an exhaiistive l ist yfcon,fi)rmers in solution, the cunonicul helicalJiirm (ARNA type) shoiild coocxist at low temperatiire with sign$cunt popdutions oj'oth1.v less classical conformers siich as half~stacked ( H S ) I biilged (BU) , and reverse-stacked ( R S ) . Fitting ofthe e,qxrimental rcluxation data ensemble at 283 K led t o very dffi.rent representutions,for the ~ M ' O trimers. r(AAC) IVUS shown to have a,fairly compact, rigid striictiire (ungulur order parameter, St,,,? 0.9, correlation time ,fiw internal motion, T, 0. I ns) , which iindergoes,fairl.v rupid overall tiimbling characterized bj, the correlation time T , 0.6 ns, whereas r(ACC) exhibits much more flexibility (S:,,, 0.7, T , 0. I ns) and slobver moleciilar reorientation ( T ~ 1 .O ns). The vuliirs of St,,,? tcnded to confirm thut