title: Entropy and biological systems: experimentally-investigated entropy-driven stacking of plant photosynthetic membranes.
abstract: According to the Second Law of Thermodynamics, an overall increase of entropy contributes to the driving force for any physicochemical process, but entropy has seldom been investigated in biological systems. Here, for the first time, we apply Isothermal Titration Calorimetry (ITC) to investigate the Mg(2+)-induced spontaneous stacking of photosynthetic membranes isolated from spinach leaves. After subtracting a large endothermic interaction of MgCl₂ with membranes, unrelated to stacking, we demonstrate that the enthalpy change (heat change at constant pressure) is zero or marginally positive or negative. This first direct experimental evidence strongly suggests that an entropy increase significantly drives membrane stacking in this ordered biological structure. Possible mechanisms for the entropy increase include: (i) the attraction between discrete oppositely-charged areas, releasing counterions; (ii) the release of loosely-bound water molecules from the inter-membrane gap; (iii) the increased orientational freedom of previously-aligned water dipoles; and (iv) the lateral rearrangement of membrane components.
authors.affiliation: 1] Division of Plant Science, R. N. Robertson Building (46), Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia [2].
authors.forename: Husen
authors.initials: H
authors.lastname: Jia
authors.affiliation: 1] Division of Plant Science, R. N. Robertson Building (46), Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia [2].
authors.forename: John R
authors.initials: JR
authors.lastname: Liggins
authors.affiliation: Division of Plant Science, R. N. Robertson Building (46), Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia.
authors.forename: Wah Soon
authors.initials: WS
authors.lastname: Chow
mesh_terms: D002151:Calorimetry; D019277:Entropy; D015636:Magnesium Chloride / Q000494:pharmacology; D045332:Photosystem II Protein Complex / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D018724:Spinacia oleracea / Q000378:metabolism*; D020524:Thylakoids / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D015636:Magnesium Chloride