Experimental support for tilt-dependent theory of biomembrane mechanics.

Recent simulations have indicated that the traditional model for topographical fluctuations in biomembranes should be enriched to include molecular tilt. Here we report the first experimental data supporting this enrichment. Utilizing a previously posited tilt-dependent model, a height-height correlation function was derived. The x-ray scattering from a liquid crystalline stack of oriented fluid phase lipid bilayers was calculated and compared with experiment. By fitting the measured scattering intensity, both the bending modulus K(c)=8.3±0.6×10⁻²° J and the tilt modulus K(θ)=95±7 mN/m were determined for DOPC lipid bilayers at 30 °C.

Note: Development of real-time epithermal neutron detector for boron neutron capture therapy.

The real-time detection of epithermal neutrons forms an important aspect of boron neutron capture therapy. In this context, we developed an epithermal neutron detector based on the combination of a small Eu:LiCaAlF6 scintillator and a quartz fiber in order to fulfill the irradiation-field requirements for boron neutron capture therapy. The irradiation test is performed with the use of a reactor-based neutron source. The thermal and epithermal neutron sensitivities of our epithermal neutron detector are estimated to be 9.52 × 10-8 ± 1.59 × 10-8 cm2 and 1.20 × 10-6 cm2 ± 8.96 × 10-9 cm2, respectively. We also subject the developed epithermal neutron detector to actual irradiation fields, and we confirm that the epithermal neutron flux can be measured in realtime.

The position of cytochrome b(559) relative to Q(A) in photosystem II studied by electron-electron double resonance (ELDOR).

The electron-electron double resonance (ELDOR) method was applied to measure the dipole interaction between cytochrome (Cyt) b(+)(559) and the primary acceptor quinone (Q(-)(A)), observed at g=2.0045 with the peak to peak width of about 9 G, in Photosystem II (PS II) in which the non-heme Fe(2+) was substituted by Zn(2+). The paramagnetic centers of Cyt b(+)(559)Y(D)Q(-)(A) were trapped by illumination at 273 K for 8 min, followed by dark adaptation for 3 min and freezing into 77 K. The distance between the pair Cyt b(+)(559)-Q(-)(A) was estimated from the dipole interaction constant fitted to the observed ELDOR time profile to be 40+/-1 A. In the membrane oriented PS II particles the angle between the vector from Q(A) to Cyt b(559) and the membrane normal was determined to be 80+/-5 degrees. The position of Cyt b(559) relative to Q(A) suggests that the heme plane is located on the stromal side of the thylakoid membrane. ELDOR was not observed for Cyt b(+)(559) Y(D) spin pair, suggesting the distance between them is more than 50 A.

Determination of distances from tyrosine D to QA and chlorophyllZ in photosystem II studied by '2+1' pulsed EPR

A '2+1' pulse sequence electron spin echo (ESE) method was applied to measure the dipole interactions between the tyrosine YD+ and QA- in Photosystem II (PS II). In a CN--treated PS II, QA- EPR signal was observed at g=2.0045 position, because the non-heme Fe(II) was converted into a low-spin (S=0) state. The radical pair of YD+QA- was trapped by illumination for 8 min at 273 K, followed by dark adaptation for 3 min and freezing into 77 K. By using a proton matrix ENDOR, these trapped radicals were confirmed to be YD+ and QA-, respectively. The distance between the radical pair was estimated from the dipole interaction constant fitted to the observed '2+1' ESE time profile. The distance of YD+-QA- is determined to be 38.8+/-1.1 A. The magnetic dipole interaction between YD+ and ChlZ+ was determined in a Tris-treated PS II in which ChlZ+ was generated by illumination at 200 K for 10 min. The YD+-ChlZ+ distance was estimated to be 29.4+/-0.5 A. Copyright 1998 Elsevier Science B.V.

The involvement of lipids in light-induced charge separation in the reaction center of photosystem II.

Extraction of PS II particles with 50 mM cholate and 1 M NaCl releases several proteins (33-, 23-, 17- and 13 kDa) and lipids from the thylakoid membrane which are essential for O2 evolution, dichlorophenolindophenol (DCIP) reduction and for stable charge separation between P680(+) and QA (-). This work correlates the results on the loss of steady-state rates for O2 evolution and PS II mediated DCIP photo-reduction with flash absorption changes directly monitoring the reaction center charge separation at 830 nm due to P680(+), the chlorophyll a donor. Reconstitution of the extracted lipids to the depleted membrane restores the ability to photo-oxidize P680 reversibly and to reduce DCIP, while stimulating O2 evolution minimally. Addition of the extracted proteins of masses 33-, 23- and 17- kDa produces no further stimulation of DCIP reduction in the presence of an exogenous donor like DPC, but does enhance this rate in the absence of exogenous donors while also stimulating O2 evolution. The proteins alone in the absence of lipids have little influence on charge separation in the reaction center. Thus lipids are essential for stable charge separation within the reaction center, involving formation of P680(+) and QA (-).