Pruton magnetic resonaiice data and conformational calcolatioiw of a series of model compounds containing a NH-C"H group substituted as in peptides have been used to generate a proton-proton coupling constant-dihedral angle relation for the peptide unit. For those substances used in which the dihedral angle about the N-C" bond is not fixed, the angle dist,ributiori was calculated from conformational theory. Using eight examples in which the number of theoretical assumpt,ions were least, the best values of the coefficients A, B, and C in the expression J ( e ) = A cosze + H case + C sinZB were found by a least-squares procedure to be 7.9, -1.55, and 1.35, respectively. This relation gives reasonable values for the dihedral angles 6 in cyclir oligopeptide structures for which the availability of both NMR data and other structural information allow comparison. When applied to i\r-acetylamino acid N-methylamides having side chains extending beyond CD, however, agreement with the calculated conformational distribution was found for Leri, Met,, and Trp, but observed values of J were larger t,han expected for Val, Ile, Phe, and Tyr. These disagreements are considered to be the result of interactions not yet taken into account in t.he usual conformational calculations. * We follow here the lat,est peptide coiiforni:it.iotial riomenc.lnt.tire in which a11 dihedrd nrigles :ire Z(WJ for t,he cis c.oiiforin:ii ion of t.he peptide iiiiit .* t (;ild)otis el, al.' have rsttniiiied ~t few observed (.oiipliiig (*oiistiiiils :iii(I s l i o w ~i Llitit, (hey are i i i rettso~td~k ttgreeriiet11, with the 13yslrovG foriuiihi. ' h ~i i j : el. :11.~ have deducwl valiies o f A , H, L', w h i d i c.cirrespoiitl IO 8.7, -3.3, :tiid -O.:<, respeot.ively, if elec.lrciitega-1,ivit.y c:orrec.t,ioiis :ire ni:ttltr 1.0 :tgrcte wiih 1 he svl, 1)ropose(I i i i this ptper For the? NH-CH fragment.