The back photoreaction of the M intermediate in the photocycle of bacteriorhodopsin: mechanism and evidence for two M species

The back photoreaction of the M intermediate in the photocycle of bacteriorhodopsin is investigated both for the native pigment and its D96N mutant. The experimental setup is based on creating the M intermediate by a first pulse, followed by a (blue) laser pulse which drives the back photoreaction of M. Experiments are carried out varying the delay between the two pulses, as well as the temperature over the -25 degrees C-20 degrees C range. It is found that the kinetic patterns of the M back photoreaction change with time after the generation of this intermediate. The data provide independent evidence for the suggestion of a photocycle mechanism based on two distinct M intermediates. They are thus in keeping with the consecutive model of Varo and Lanyi (Biochemistry 30, 5016-5022; 1991), although they cannot exclude other models such as those based on branched or parallel cycles. More generally, we offer a "photochemical" approach to discriminating between intermediate stages in the photocycle which does not depend on spectroscopic and/or kinetic data. While markedly affecting the rate of the M --> N transition in the photocycle, the rate of the thermal step in back photoreaction of M, at both room and low temperatures, is not significantly affected by the D96N mutation. It is proposed that while Asp 96 is the Schiff-base protonating moiety in the M --> N transition, another residue (most probably Asp 85) reprotonates the Schiff base following light absorption by M.     

Structural hierarchies and interactions in the tendon composite

Tendon functions by transmitting tensile loads from muscle to bone. Morphologically, it can be described as a macromolecular multicomposite material, basically consisting of collagen fibrils held together by a soft, hydrated matrix material. Recently, tendon has been deformed beyond the "in vivo" elastic limit and by cyclical loading systematically damaged. Using high-resolution electron microscopy, decomposition of the collagen fibril into subfibrils (15 nm diameter) and microfibrils (3.5 nm diameter) has been noted. The interfacial adhesion between such units is strongly dependent on age, and is probably related with crosslinking phenomena observed by biochemical methods. In addition, tendon collagen contains a considerable amount of water throughout the entire structure which strongly affects its overall mechanical behavior. The various bound states of water have been identified using primarily dynamic mechanical spectroscopy coupled with more conventional methods of structural characterization.Published in Mekhanika Polimerov, No. 4, pp. 693–701, July–August, 1976.     

Cup and cap products in intersection (co)homology

We construct cup and cap products in intersection (co)homology with field coefficients. The existence of the cap product allows us to give a new proof of Poincaré duality in intersection (co)homology which is similar in spirit to the usual proof for ordinary (co)homology of manifolds.