Table 1.

Relative translational diffusion coefficients, D_trans, determined by ¹⁵N-filtered X-STE NMR diffusion experiments

Protein	Relative Dtrans, %*	Molecular mass, kDa	Apparent molecular mass, kDa†
Human lysozyme	100 (1)	14	14
Ig2 NTD	107 (3)	11	11
Ig2	76 (4)	21	32‡
Ig2/70S	63 (2)		32§
Ig2 RNC	18 (>10)		>1,300

*Relative D_trans was defined relative to a ¹⁵N-labeled human lysozyme sample reference (error estimates are indicated in parentheses), as described elsewhere (28). The relative D_trans of the Ig2 RNC is estimated from the intensity ratio of the X-STE spectra with 10% and 70% Gz_max integrated over the region from 7.6 to 9.3 ppm in the ¹H dimension (see Fig. 4A). Only two gradient points were employed for the Ig2 RNC because of the limited sample concentrations.

^†The apparent molecular mass (MM) was derived by using the Stokes–Einstein relationship, MM ∝ (r)³ ∝ (1/D_trans)³. This estimation assumes a spherical species (of radius r) with a density similar to that of the lysozyme.

^‡The apparent MM of Ig2 in aqueous solution is significantly higher than that of a monomeric Ig2 due to its tendency to dimerize (unpublished data).

^§The presence, in a sample of isolated Ig2, of a concentration of 70S ribosomes similar to that present in the Ig2 RNC sample (12 μM) reduces the relative D_trans as a result of the increased sample viscosity. Under the assumption that the change in sample viscosity does not affect the monomer–dimer equilibrium of Ig2, and that the sample viscosity is dominated by the concentration of 70S ribosomes, the apparent MM of Ig2 derived from aqueous solution (32 kDa) was then used to determine the apparent MM of the Ig2 RNC. When human lysozyme is used as a reference without correcting for sample viscosity, the apparent MM of Ig2 RNC is found to be 2.4 MDa.