Organization of the pigment molecules in the chlorophyll a/b/c containing alga Mantoniella squamata (Prasinophyceae) studied by means of absorption, circular and linear dichroism spectroscopy

In order to obtain information on the organization of the pigment molecules in chlorophyll (Chl) a/b/c-containing organisms, we have carried out circu lar dichroism (CD), linear dichroism (LD) and absorption spectroscopic meas urements on intact cells, isolated thylakoids and purified light-harvesting complexes (LHCs) of the prasinophycean alga Mantoniella squamata. The CD s pectra of the intact cells and isolated thylakoids were predominated by the excitonic bands of the Chi a/b/c LHC. However, some anomalous bands indica ted the existence of chiral macrodomains. which could be correlated with th e multilayered membrane system in the intact cells. In the red. the thylako id membranes and the LHC exhibited a well-discernible CD band originating f rom Chi c, but otherwise the CD spectra were similar to that of non-aggrega ted LHC II, the main Chi alb LHC in higher plants. In the Soret region, how ever, an unusually intense (+) 441 nm band was observed, which was accompan ied by negative bands between 465 and 510 nm, It is proposed that these ban ds originate from intense excitonic interactions between Chi a and caroteno id molecules. LD measurements revealed that the Q(Y) dipoles of Chl a in Ma ntoniella thylakoids are preferentially oriented in the plane of the membra ne, with orientation angles tilting out more at shorter than at longer wave lengths (9 degrees at 677 nm, 20 degrees at 670 nm and 26 degrees at 662 nm ); the Q(Y) dipole of Chi c was found to be oriented at 29 degrees with res pect to the membrane plane. These data and the LD spectrum of the LHC, apar t from the presence of Chl c, suggest an orientation pattern of dipoles sim ilar to those of higher plant thylakoids and LHC II. However, the tendency of the Q(Y) dipoles of Chi b to lie preferentially in the plane of the memb rane (23 degrees at 653 nm and 30 degrees at 646 nm) is markedly different from the orientation pattern in higher plant membranes and LHC II. Hence, o ur CD and LD data show that the molecular organization of the Chi a/b/c LHC , despite evident similarities, differs significantly from that of LHC II. (C) 2000 Elsevier Science B.V. All rights reserved.