ULTRAVIOLET RESONANCE RAMAN-STUDY ON PURPLE AND BLUE MEMBRANES OF HALOBACTERIUM-HALOBIUM

Two hundred and forty and 213 nm excited resonance Raman spectra of purple membrane (PM) and blue membrane (BM) of Halobacterium halobium were studied. Generally intense Trp scattering and a strong relative intensity of the W3 band at 1553 cm-1 in the 240 nm spectrum of PM indicate red-shifted Bb absorptions of some Trp sidechains. A high intensity ratio of Trp doublet at 1360 and 1340 cm-1 suggests interactions with highly hydrophobic Trp environments. These Trp are not strongly H-bonded and their N1 sites are located in positions easily reached by solvent water molecules. Tyrosines are also in very hydrophobic environments and H-bonded. The mainchain consists of normal and distorted alpha-helices whose amide NH are hardly deuterated in D2O suspension, and some NH exchangeable irregular segments on the membrane surface. Upon acidification, the ratio of Trp doublet with 240 nm excitation decreases concomitant with increase in retinal absorbance at 600 nm, and the W3 relative intensity and overall Trp scattering also decrease. These observations strongly indicate that the counterpart of Trp interactions in PM is the retinal and that the interactions partly diminish upon acidification. The Tyr environment also changes with the color. Although the 240 nm amide I intensity is greater in acid BM than in PM, the change is not related to the color change because the amide I intensity of deionized BM is practically the same as that of PM. The amide I intensity increase in acid BM is ascribable to a structural change of the surface peptides due to acid induced aggregation.