Solution scattering experiments using both X-rays and neutrons are reported for human complement component C 3 and up to six other glycoprotein fragments that are derived from C3. The X-ray and neutron molecular masses and neutron matchpoints are in agreement with the known primary sequence of C 3. The X-ray radius of gyration RG of C3 is 5.2 nm and is similar for the related forms C~U , C3(a+ b) and C3 b. The X-ray cross-sectional radius of gyration Rxs of C3b is however less than that of C3, C3u and C3(a+b). The major fragments of C3b, namely C3c and C3dg, were studied. The Ro of C3c is 4.7 nm and for C3dg is 2.9 nm. C3c and C 3dg do not interact when they coexist in solution in equimolar amounts. When C 3u is cleaved into iC 3u, the RG of iC3u increases to 5.9 nm and its Rxs decreases, showing that C3c and C3dg behave as independent entities within the parent glycoprotein. Analyses of the neutron RG and Rxs values by contrast variation techniques confirm the X-ray analyses, and show no evidence for significant hydrophobic or hydrophilic domains within C3 or any of its fragments. Shape analyses show that C3, C3c and C3dg are elongated particles. Debye models were developed using the scattering curve out to Q = 1.6 nm-I . These show that C 3 and C 3c resemble oblate ellipsoids while C3dg resembles a prolate ellipsoid. C3dg lies on the long edge of C3c within C3. The dimensions of the modelsare 18 nmx 2 nm x 10 nmforC3,18 nmx 2 nm x 7 nmforC3cand 10 nm x 2 nm x 3 nm forC3dg. These models are compatible with analyses of the scattering curve RG and Rxs values, data from sedimentation coefficients, and images of C 3 and C 3c seen by electron microscopy.

A wealth of accurate amino acid compositions for proteins and glycoproteins is presently available due to modern, rapid means of sequencing proteins and nucleic acids. In principal, macromolecular physical properties are more accurately and easily determined from these compositions in comparison to the use of classical biochemical techniques. The calculation of partial relative molecular masses M , is an obvious example of this. The calculation of macromolecular volumes V and partial specific volumes V is required in a range of applications: scattering density, matchpoint and molecular mass control measurements in X-ray and neutron solution scattering; molecular mass determinations by ultracentrifugation; constraints for use in low-resolution modelling of macromolecular shapes; packing analyses of amino acid and carbohydrate residues by solution or crystallographic studies. Starting from the classical 1943 Cohn and Edsall publication, several tabulations of residue volumes for amino acid and carbohydrates have been reported [l -71. These volumes can be summed on the basis of accurate amino acid and carbohydrate compositions to give V and V. In the present study, new compilations of amino acids and carbohydrate residue volumes are derived from small-molecule crystal studies. These are critically compared with previous compilations [I -71. The ability of these residue volumes to predict partial specific volumes and neutron scattering densities for proteins and glycoproteins is in turn critically compared with experimental data, bearing in mind that the partial volume is the particle volume corrected for hydration, solute binding and electrostriction effects. This clarifies the application of these calculations at a phenomenological level with the use of accurate composition data for not only solution scattering studies, but also for more general biophysical and biochemical Correspondence to S. J. Perkins, Kennedy Institute, Bute Gardens, Hammersmith, London W6 7DW, England applications. It is to be noted that these calculated V values are only valid for use in two-component solutions; in multicomponent solutions, such as in the presence of high concentrations of denaturants or electrolytes, considerable changes due to interactions with the solvent or with ligands will occur, and in such cases the calculations will generally fail [8]. Finally, the calculations of V and matchpoint data are correlated with protein hydration concepts on the basis of observed protein-bound water molecules from macromolecular crystal studies. This enables the macromolecular volume to be interpreted in molecular terms of (a) apparent changes induced by protein-bound water in densitometric studies and (b) the 'dry' molecular volume as visualised by neutron scattering studies.Macromolecular concentrations are conveniently determined in a wide range of applications by absorbance measurements at 280 nm on the basis of the absorption coefficients A:& Cm. The ability to calculate A:& ' cm from accurate amino acid sequences would be more straig...

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