I have looked at these image for years, just now thinking that the bend that is sometimes seen on the N terminus side of the SP-D trimer has an impact on how those multiple N termini junctions might assemble to make multimers with different attributes at that junction: whether end to end, one top the other, or bending for form a ring as is seen more often in multimers with many trimers. So while the flexibility of the CRD regions of a trimer, which are easily seen in AFM image of SP-D are pretty obvious, finding a low spot, a reasonably “thin” part of the molecule seemingly just between the N and collagen-like domains, I have seen on hundreds of plots, however, that it has a “flexibility” is certainly something to think about (per image below).

Two trimers from the same AFM image are shown below, rings are drawn around the “bend”, arrows point to a very low point in the grayscale scan which is a very common feature of SP-D near the N term bright peak.

another image with an N term “foot” like bend

another trimer with an N term “foot” like bend

…and another one

I looked up all images in Arroyo 2018 and totalling up 50 trimers, only 5 had that little crooked hook at the junction of the N term domain and the collagen-like domain.