Uncovering the physical origin of the difference between aliphatic chain and aromatic ring in the "hydrophobic" effects on partial molar volume.

The partial molar volume changes in the transfer of several hydrophobic molecules, which are composed of aromatic rings and an aliphatic chain of different lengths, from carbon tetrachloride to water (DeltaV(hyd)) are calculated using the three-dimensional interaction site model theory of molecular solvation. The theory reproduces recent experimental observations: the addition of a methyl group decreases DeltaV(hyd); in contrast, the addition of an aromatic ring increases DeltaV(hyd). The discrepancy is found to originate from the difference between chain and ring structures rather than that between aliphaticity and aromaticity. Furthermore, a general rule of the variation in DeltaV(hyd) due to the addition of a hydrocarbon is found through the theoretical analysis. An outward addition at the trans position, which is to form chain structure, decreases DeltaV(hyd), while an inward addition at the cis position, which is to form ring structure, increases DeltaV(hyd). This is explained in terms of solvent packing rather than the so-called hydrophobic hydration. The present findings argue against the traditional idea that the hydrophobic hydration can be represented by the observed values of DeltaV(hyd).

Pressure-induced red shift and broadening of the Qy absorption of main light-harvesting antennae chlorosomes from green photosynthetic bacteria and their dependency upon alkyl substituents of the composite bacteriochlorophylls.

When pressure was applied to the main light-harvesting apparatus (chlorosomes) isolated from several green photosynthetic bacteria (up to 128 MPa), the Qy-absorption band in an aqueous solution was shifted to longer wavelengths. The shift, deltav, was completely reversible for (de)compression and also showed a linear relation as a function of the applied pressure. The pressure-sensitivity in the deltav was dependent upon the bacterial species. The pressure coefficient, deltav/deltaP, was -565 to -535 cm(-1) GPa(-1) for the chlorosomes from several green sulfur bacteria (Chlorobium species), which have several bacteriochlorophyll(BChl) homologues at the 8- and 12-positions as the antenna pigments. In contrast, a smaller value (-445 cm(-1) GPa(-1)) was estimated for the chlorosomes from the green nonsulfur bacterium (Chloroflexus aurantiacus), which has a single homologue with 8-ethyl and 12-methyl groups. These results were confirmed by the similar pressure dependency of in vitro self-aggregates of isolated BChls-c having various alkyl substituents at the 8- and 12-positions. The present pressurization study enables us to discuss a physiological meaning of a variety of antenna pigments in green photosynthetic bacteria.