SAXS experiments were carried out on beamline ID02 (Narayanan et al. 2001 ) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The wavelength was 1.0 Å and the sample-to-detector distance was 3.0 m and 1.0 m, leading to scattering vectors q ranging from 0.02 to 0.20 Å À1 and 0.05 to 0.40 Å À1 , respectively. The scattering vector is defined as q = 4 p/l sin, where 2 is the scattering angle. The detector was an X-ray image intensified optically coupled to an ESRF developed FReLoN CCD camera. Forty successive frames of 1.0 sec with a 5-sec pause between each frame were recorded for each sample. The protein solution was circulated through an evacuated quartz capillary between each frame. Thus, no protein solution was irradiated longer than 1.0 sec. Each frame was then carefully inspected to check for possible bubble formation or radiation-induced aggregation. No such effect was observed, and individual frames could then be averaged. Absolute calibration was made with a Lupolen sample. A series of measurements at different protein concentrations ranging from 1.8 to 10 mg/mL were performed for every protein (XD, N TAILHN , and the mixture N TAILHN -XD) to check for interparticle interaction. Background scattering was measured before or after each protein sample using the buffer solution and then subtracted from the protein scattering patterns after proper normalization and correction from detector response. All the experiments were carried out at 20 C. The data acquired at both sample-to-detector distances of 3 m and 1 m were merged and extrapolated to zero concentration for the calculations using the entire scattering spectrum.