The Conformationally Constrained N-Methanocarba-dT Analogue Adopts an Unexpected C4′-exo Sugar Pucker in the Structure of a DNA Hairpin

Incorporation of a bicyclo[3.1.0]hexane scaffold into the nucleoside sugar was devised to lock the embedded cyclopentane ring in conformations that mimic the furanose North and South sugar puckers. To analyze the effects of North-methanocarba-2'-deoxythymidine (N-MCdT) on the B-form DNA, we crystallized d(CGCGAA[mcTmcT]CGCG) with two N-MCdTs. Instead of a duplex, the 12mer forms a tetraloop hairpin, whereby loop N-MCdTs adopt the C4'-exo pucker (NE; P = 50 degrees). Thus, the bicyclic framework does not limit the pucker to the anticipated C2'-exo range (NNW; P = 18 degrees).