Just a quick set of measurements from White et al shadowed dodecamer (TEM) and Arroyo et al AFM (many dodecamers measured) there is not a significant difference in the diameter or segmented line measurements of the SP-D arms. I will add other measurements. Even though the White et al is ratSP-D, and Arroyo et al is human SP-D, the former with a small deletion in C1 of SP-Ddel29–37 (according to White et al). What this deletion does to change the absolute arm length of rSPD is not known. Objective is to find a common diameter of SP-D, and arm length for each species and mutation set.