Characteristics of light-induced H+ transport in spinach chloroplast at lower temperatures II. A possible role of structured water in H+ transport of chloroplast near 0{degrees}C

Light-induced H⁺ transport in spinach chloroplasts at lowertemperatures was investigated with a glass electrode. Half-decaytime of the H⁺ transport on turning off the illumination increasedwith lowering of the temperature. However, near the freezingpoint of bulk water, the H⁺ transport showed no dependency ontemperature. An Arrhenius plot of the apparent first-order rate constantof H⁺ transport showed a break at about 8°C. Below thatcritical temperature, activation energy was about 0 kcal/mole. The break temperature in the Arrhenius plot shifted by 2–3°Cto a higher temperature in the presence of 10% glycerol, aneffective water structure-enhancing agent. On the other hand,addition of 5 mM NaSCN, known as a potent water structure breaker,abolished the break and a constant activation energy was obtainedover the entire low temperature range. A similar effect wasobserved in the presence of 50 mM urea. On the basis of these results, we propose a mechanism for theH⁺ transport of chloroplasts at lower temperatures involvingstructured water, where rapid proton transfer between hydrogen-bondedstructured water molecules takes place. (Received September 11, 1975; )