One of the early publications about surfactant protein D (Ed Crouch, et al, circa 1994) that had wonderful pictures of this particular protein did not, however, have the best descriptions of measurements and this causes problems in comparing those images (results) with current measurements. All this in an attempt to help define the molecular modeling of the collagen-like domain of SP-D.  The techniques were inventive for the time (lettraset grayscale screens — I used lettraset rub on letters for many publications from that era). Photoshop would be used today of course with comments in the materials and methods stating such.

The description of arm measurement goes like this “..tip of the globule on one arm to the tip of the globule on the apposing arm”. Notice the word apposing…. which might really need to be opposing?  The apposing arm of an SP-D dodecamer would be a line drawn down an arm from the CRD with an acute angle coming at the N termini junction, and a line drawn back up the “same side of the molecule” arm.  This would make a V shape, and a sharp angle in what has to be arbitrarily chosen as the center of the N termini junction.  My guess is they did not measure that way, but went across the N termini to the opposing (not apposing) arm.

I have to assume that their measurements went “straight” across, and did not curve to accommodate the increase in arm length that adding that few nm of distance would provide, but again, it is not clear.

There is no really accurate way (not that bar micron markers are accurate for they notoriously are not) to remeasure the dodecamer arm lengths, either as individual trimers or as hexamers.

One approach, howver, might be to compare arm lengths they measured against their measurements for the diameters of the CRD themselves.