Table 1. Particle density calculation.

Fraction #	Rh (nm)	Length (nm)	PrP monomers per PrPSc particle	PrP normalized intensity	Relative number of PrPSc particles	Max. possible PrPSc particle density (particles/mL)
24	11.6	39.1	78	0.08316	0.001065	3.08E+09
26	15.5	57.8	116	0.102213	0.000884	2.56E+09
28	19.6	80.0	160	0.148777	0.000093	2.69E+09
30	22.9	106.6	213	0.179116	0.000084	2.43E+09
32	26.9	142.1	284	0.224708	0.000079	2.29E+09
34	32.9	175.1	350	0.348884	0.000996	2.88E+09
36	38.7	199.4	399	0.370763	0.000930	2.69E+09
38	46.0	223.5	447	0.322006	0.000720	2.09E+09
40	49.2	253.5	507	0.301998	0.000596	1.72E+09
42	57.6	361.2	722	0.290421	0.000402	1.16E+09
44	69.3	524.2	1048	0.299313	0.000286	8.26E+08
46	77.9	704.0	1408	0.309167	0.000220	6.36E+08 (*)
48	105.6	1621.9	3244	0.394646	0.000122	3.52E+08 (*)

The R_h of particles was calculated from DLS measurements. The length of the particles was calculated fitting the MALS measurements to a rod-shape model. The number of PrP monomers per PrP^Sc particle was estimated from the PrP^Sc particle length, assuming a contribution of 0.49 nm in length per PrP monomer based on the recent parallel in-register intermolecular beta sheets (PIRIBS) model for the 263K strain. The PrP normalized intensity was determined by immunoblotting using Sha31 antibody. The relative number of PrP^Sc particles was calculated by dividing the PrP normalized intensity by number of PrP monomers per PrP^Sc particle. The maximum possible PrP^Sc particle density was calculated with ASTRA software (see methods) for fractions 46 and 48, and extrapolated to the rest of the fractions based on the relative number of PrP^Sc particles. (*) these values of particle density were calculated from MALS measurements directly, fitting the data to a rod-shape model, and assuming a rod radius of 4.25 nm. This calculation was performed for all samples; the specific values obtained for sample HY-E are shown in the table.