NMR relaxation experiments often require site-specific isotopic enrichment schemes in order to allow for quantitative interpretation. Here we describe a new labeling scheme for site-specific C-13-H-1 enrichment of a single ortho position of aromatic amino acid side chains in an otherwise perdeuterated background by employing a combination of [4-C-13]erythrose and deuterated pyruvate during growth on deuterium oxide. This labeling scheme largely eliminates undesired contributions to C-13 relaxation and greatly simplifies the fitting of relaxation data using the Lipari-Szabo model-free formalism. This approach is illustrated with calcium-saturated vertebrate calmodulin and oxidized flavodoxin from Cyanobacterium anabaena. Analysis of C-13 relaxation in the aromatic groups of calcium-saturated calmodulin indicates a wide range of motion in the subnanosecond time regime.