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. Author manuscript; available in PMC: 2011 Dec 1.
Published in final edited form as: Structure. 2004 Jun;12(6):1099–1108. doi: 10.1016/j.str.2004.03.019

Figure 6.

Figure 6

Small angle X-ray scattering of trpR: pair-distance distribution functions (p(r)). P(r) is obtained by Fourier transformation directly from scattering curves and it describes the distribution of distances r between pairs of scattering centers within particles present in solution. The function provides a measure of particle dimensions and shape. (a) and (b): P(r) of experimental scattering profiles presented in Fig. 5. (c): P(r) calculated for models derived from crystal structures. DNA was excluded from all calculations. The “1-dimer” and “2-dimer” models (Rg = 21 Å and 32 Å, respectively) are as illustrated in Fig. 1 panels a and b. The “16-dimer” model corresponds to 4-turns of the left-handed (43) superhelix that forms around DNA in tandem trpR/DNA crystals (Rg = 125 Å, length ≈ 440 Å, model not shown) (Lawson and Carey, 1993). A similar superhelix is formed by trpR dimers without DNA (A. Chin, B.B., and C.L.L., manuscript in preparation). The p(r) curve for the 16-dimer model (dotted line) shows the characteristic sharp rise followed by linearly decreasing slope that is typical for rod-shaped particles (Glatter and Kratky, 1982). For ease of comparison, p(r) curves are shown with their peak values set to a common, arbitrary y value.