# How to determine the dimensions of a bixin molecule?

I need the dimension of a bixin molecule. I do not know where to look or how to calculate this information.

I understand that this molecule has several rotational axes and conformation, so there is no uniform size to be determined. But I am interested in the standard method for the determination of the molecule's dimensions.

I am doing a series of refractive and scattering experiments in a concentrated bixin medium. I need to determine the most probable cross section area normal to its principal axes.

• There are crystal structures of cis- and trans-bixin published (D. R. Kelly, A. A. Edwards et al. J. Chem. Res., 446, 1996, p. 2640), you can roughly estimate the dimensions from CIF-files, I guess. But I have no clue which conformation dominates in solution. If it helps, I can provide the dimensions of both molecules packed in crystal in an answer. – andselisk Jul 13 '17 at 19:30
• If you need to know the dimensions of a conformationally flexible molecule in solution, the easiest way is to measure it (somehow). Predicting which conformation is likely is very hard to do. The best you could achieve would be to build a 3-D model (either on computer or using modelling kits) and measure the dimensions of the extreme conformations (stretched out or folded up). This would bound possible values but not tell you which is most likely in solution. – matt_black Jul 14 '17 at 12:31
• @andselisk Thank you for the edits. Much better . – Jose Enrique Calderon Jul 15 '17 at 5:03

There is a paper on bolaamphiphiles (1) where it is stated that measured surface area corresponding to the area of a bixin molecule lying on the water surface with most methyl groups pointing outside is approx. $\pu{1.0 nm^2/molecule}$ at zero pressure. If the methyl groups were parallel to the water surface, a molecular area of approx. $\pu{1.7 nm^2/molecule}$ would be occupied.

Also, there is a linear size of a single molecule stated as the following:

Bixin therefore constitutes a valuable starting material for various bolaamphiphiles which can be tied within lipid membranes. Its molecular length is about $\pu{3.1 nm}$ in the all-trans configuration.

I think it might be worth for OP to have a look at the full-text (1) paper as it also discusses various methods of characterization and altering of bolaamphiphiles.

A few years later a crystal structure has been published (2). There is not much to add, as it seems like there is no contradiction with the data obtained with reflection spectroscopy (1). Here are the notable linear dimensions of cis-bixin and overall geometry in 2 projections:

(1) Fuhrhop, J. H.; Krull, M.; Schulz, A.; Moebius, D. Langmuir 1990, 6 (2), 497–505. DOI: 10.1021/la00092a034
(2) Kelly, D. R.; Edwards, A. A.; Parkinson, J. A.; Olovsson, G.; Trotter, J.; Jones, S.; Malik, K. M. A.; Hursthouse, M. B.; Hibbs D. E. 1996, J. Chem. Res., 446, 2640

• What are the units of this dimensions? – Jose Enrique Calderon Jul 15 '17 at 5:07
• @JoseECalderon Ångströms [Å]. – andselisk Jul 15 '17 at 5:10