AbstractTwotrans‐syncyclobutane photodimers of thymidylyl (3′–5′) deoxyuridine were formed by deamination of the correspondingtrans‐syncyclobutane photodimers of thymidylyl(3′–5′) deoxycytidine and were examined by1H−,13C−, and31P‐nmr spectroscopy. Correlation spectroscopy, nuclear Overhauser enhancement spectroscopy, and one‐dimensional heterodecoupling experiments allowed a more complete assignment of the1H spectra, compared with previous reports by Koning et al. [ (1991)European Journal of Biochemistry, Vol. 195, pp. 29–40] and Liu and Yang [(1978)Biochemistry, Vol. 17, pp. 4865–4876]. Deoxyribose ring conformations were calculated from1H coupling constants by pseudorotational analysis, and rotamer distributions of exocyclic bonds were calculated from the observed homonuclear and heteronuclear coupling constants. The cyclobutane ring configuration (CB) of each isomer was identified, using arguments based upon observed scalar and dipolar couplings. Glycosidic bond conformation was ascertained from nuclear Overhauser enhancements observed between base and deoxyribose protons. Isomer I (S‐type class; CB−; SYN‐ANTI) and isomer II (N‐type class; CB+; ANTI‐SYN) exhibit markedly different conformational features.31P chemical shifts and exocyclic bond rotamer distributions indicate diminished backbone flexibility for both photoproducts relative to parent thymidylyl (3′–5′) deoxyuridine. Isomer I (SYN‐ANTI) is particularly rigid, while isomer II (ANTI‐S YN) maintains some flexibility. Also,13C spectra were acquired and assigned unequivocally with the aid of short‐ and long‐range two‐dimensional heteronuclear shift