A small-angle neutron scattering study on the small aggregates of bacteriochlorophylls in solutions

Small-angle neutron scattering measurements have been made to determine the size of small aggregates of bacteriochlorophyll (BChl) a and c formed in non-polar solution. Both BChl a and c form dimers and oligomers in equilibrium with each other at high concentrations in benzene-d(6), solutions. The size and molecular weight for each dimer have been quantitatively evaluated based on Guinier approximation. The radii of gyration of the dimers are determined to be 17.0 +/- 0.5 Angstrom and 16.5 +/- 0.5 Angstrom for the BChl a and c, respectively. Limited observations in the very small angle region suggest that the oligomers are of considerable dimension, approximately as large as a hundred angstroms for the radius of gyration. The SANS results can be well correlated with the electronic absorption and NMR spectra and are in good agreement with other spectroscopic behavior of the small aggregates reported previously.