A SMALL-ANGLE NEUTRON-SCATTERING STUDY ON THE SMALL AGGREGATES OF BACTERIOCHLOROPHYLLS IN SOLUTIONS

Small-angle neutron scattering measurements have been made to determin e the size of small aggregates of bacteriochlorophyll (BChl) a and c f ormed in non-polar solution. Both BChl a and c form dimers and oligome rs 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 obse rvations in the very small angle region suggest that the oligomers are of considerable dimension, approximately as large as a hundred angstr oms for the radius of gyration. The SANS results can be well correlate d with the electronic absorption and NMR spectra and are in good agree ment with other spectroscopic behavior of the small aggregates reporte d previously.