Three-bond (3)J(C'C') and (3)J(HNH alpha) couplings in peptides and proteins are functions of the intervening backbone torsion angle phi. In well-ordered regions, (3)J(HNH alpha) is tightly correlated with (3)J(C'C'), but the presence of large phi angle fluctuations differentially affects the two types of couplings. Assuming the phi angles follow a Gaussian distribution, the width of this distribution can be extracted from (3)J(C'C') and (3)J(HNH alpha), as demonstrated for the folded proteins ubiquitin and GB3. In intrinsically disordered proteins, slow transverse relaxation permits measurement of (3)J(C'C')' and (3)J(HNH) couplings at very high precision, and impact of factors other than the intervening torsion angle on (3)J will be minimal, making these couplings exceptionally valuable structural reporters. Analysis of alpha-synuclein yields rather homogeneous widths of 69 +/- 6 degrees for the phi angle distributions and (3)J(C'C') values that agree well with those of a recent maximum entropy analysis of chemical shifts, J couplings, and H-1-H-1 NOEs. Data are consistent with a modest (<= 30%) population of the polyproline II region.