The CD spectra were recorded on a Jasco 810 dichrograph using 1-mm thick quartz cells in 10 mM sodium phosphate (pH 7) at 20 C. Structural variations of N TAIL proteins were measured as a function of changes in the initial CD spectrum upon addition of either increasing concentrations of TFE (Fluka), or different amounts of XD or lysozyme (Sigma). CD spectra were measured between 185 and 260 nm, at 0.2 nm/min and were averaged from three independent acquisitions. Mean ellipticity values per residue ([Q]) were calculated as [Q] = 3300 m DA/(l c n), where l (path length) = 0.1 cm, n = number of residues, m = molecular mass in daltons, and c = protein concentration expressed in mg/mL. Number of residues (n) are 140 for N TAILHNFC , 131 for N TAILD3 , 103 for N TAILD2,3 , 120 for N TAILD1 , 132 for N TAIL W518, 56 for XD, and 129 for lysozyme, while m values are 15,626 Da for N TAILHNFC , 14,523 Da for N TAILD3 , 11,539 Da for N TAILD2,3 , 13,440 Da for N TAILD1 , 6690 Da for XD, and 14,300 Da for lysozyme. Protein concentrations of 0.1 mg/mL were used when recording spectra of both individual and protein mixtures. In the case of protein mixtures, mean ellipticity values per residue ([Q]) were calculated as [Q] = 3300 DA/{[(c 1 n 1 / m 1 ) + (c 2 n 2 /m 2 )] l}, where l (path length) = 0.1 cm, n 1 or n 2 = number of residues, m 1 or m 2 = molecular mass in daltons, and c 1 or c 2 = protein concentration expressed in mg/mL for each of the two proteins in the mixture. The theoretical average ellipticity values per residue ([Q] Ave ), assuming that neither unstructured-to-structured transitions nor secondary structure rearrangements occur, were calculated as follows: [Q] Ave = [([Q] 1 n 1 ) + ([Q] 2 n 2 R)]/(n 1 + n 2 R), where [Q] 1 and [Q] 2 correspond to the measured mean ellipticity values per residue, n 1 and n 2 to the number of residues for each of the two proteins, and R to the excess molar ratio of protein 2. The a-helical content was derived from the ellipticity at 220 nm as described in Morris et al. (1999) .