Optical spectra (absorption, circular dichroism, linear dichroism, and triplet-minus-singlet), measured at 6 K, are presented for the Fenna-Matthews-Olson (FMO) complex from the green sulfur bacterium Chlorobium (C.) tepidum. Significant differences are observed in comparison with the corresponding spectra of Prosthecochloris (P.) aestuarii. The spectra of C. tepidum FMO were simulated using exciton calculations, based on the recently resolved structure of this complex [Li et al. J. Mol. Biol. 1997, 271 456]. These calculations apply the same basic assumptions as were earlier used for the FMO complex from P. aestuarii [Louwe et al. J. Phys. Chem. B 1997, 101, 11280], except that the site energies of all but one of the bacteriochlorophylls had to be changed in order to optimize the simulations. The good agreement that was obtained supports the assumptions in these model calculations, and the assignment of the site energies makes it possible to consider the spectra in relation to the structural differences between the two types of FMO complex.

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The specific heat (C) and entropy (q) in zero field are calculated for infinite chains of spins, coupled by a nearest-neighbor Heisenberg exchange. The data presented include all spin values S & 5/2 and cover ferromagnetic and antiferromagnetic exchange. Several techniques are used to obtain reliable estimates for the infinite chains, and much attention is given to the theory underlying these techniques. For high temperatures the series expansion of C is used for the estimates. Coefficients in the series are obtained from the energy spectra of different finite chains. This method is fully described, and attention is given to the estimation of further coefficients. For intermediate temperatures, around the region where C displays the characteristic broad maximum, the series expansions fail and here the estimates of the infinite chain are obtained from a suitable extrapolation of the data for finite chains. This method is shown to be correct for sufFiciently high temperatures. It is applicable to a wide temperature range. For temperatures near T = 0, C is described by a polynomial based on the spin-wave theory. The coefficients in this polynomial are equated such that a smooth fit in with the intermediate temperature specific heat is obtained and that at the same time the over-all entropy gain is correct. The results are, as far as possible, tabulated.

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