There is a section on high and low molecular weight surfactant protein D from an publication by Grith Sorensen in Frontiers in Medicine, 2018 which has the following excerpt. “High-molecular weight SP-D multimers are only partly dependent on disulfide crosslinking of the N-termini, and a proportion of SP-D subunits are non-covalently associated. This allows interconversion between HMW SP-D and LMW SP-D trimers, as demonstrated using size permeation chromatography (36) (Figure 1B). The HMW/LMW ratio depends on the concentration of the protein in solution, with low-protein concentrations favoring the decomposition of multimers into trimers. In addition, the HMW/LMW ratio increases with affinity purification of SP-D, suggesting that ligand-binding facilitates assembly of SP-D trimers into multimers (Reference to an earlier article by the same author).”
There is specific reference to the ratio of high to low molecular weight multimers of surfactant protein D in relation to protein concentration (in the laboratory setting), and to the methionine 11 to threonine 11 allelic variants on the ratio of high to low molecular weight multimers of SP-D in humans.
It seems almost legitimate to view the two different peak plot patterns foud in the N termini peaks, traced from actual images of SP-D dodecamers (traced as two arms, i.e. hexamers – arm 1, and arm 2) found in the N termini of SP-D dodecamers. This valley seen about half the time in the center of grayscale N termini peaks (LUT tables traced in ImageJ) from AFM images (Arroyo et al, 2018) might suggest that even among dodecamers there can be both close tie between N termini (covalent links between two trimers) and loose associations, as well as a single peak, or two peaks respectively). In addition, the trace depends also on “where the segmented line is drawing during the trace, and the brightness saturation of the image.