Two-dimensional J-resolved 1H NMR spectroscopy was used to measure the vicinal spin-spin coupling constants 3JH alpha H beta for numerous, previously individually assigned amino acid residues in the basic pancreatic trypsin inhibitor at various temperatures between 30 and 85 degrees JC. An analysis of this data is proposed which enables one to compare the spatial arrangements of individual amino acid side chains in solution and in single crystals of the protein, and which also provides information on the mobility of the side chains in the solution conformation. As a rule, the amino acid side chains in the interior of the protein were found to be locked into unique spatial orientations, with the mobility restricted to rapid rotational fluctuations about this unique value for the dihedral angle chi 1. In most, but not all, instances the data for the interior amino acids indicate identical average conformations for the amino acid side chains in single crystals and in solution. For residues on the protein surface structural rearrangements between crystal and solution appear to be common, and the mobility in the solution conformation may include rapid averaging between two or several distinct, preferentially populated values of chi 1, analogous to the gauche-trans-gauche isomerization in isolated amino acids.