title: The laser-induced blue state of bacteriorhodopsin: mechanistic and color regulatory roles of protein-protein interactions, protein-lipid interactions, and metal ions.
abstract: In this paper we characterize the mechanistic roles of the crystalline purple membrane (PM) lattice, the earliest bacteriorhodopsin (BR) photocycle intermediates, and divalent cations in the conversion of PM to laser-induced blue membrane (LIBM; lambda(max)= 605 nm) upon irradiation with intense 532 nm pulses by contrasting the photoconversion of PM with that of monomeric BR solubilized in reduced Triton X-100 detergent. Monomeric BR forms a previously unreported colorless monomer photoproduct which lacks a chromophore band in the visible region but manifests a new band centered near 360 nm similar to the 360 nm band in LIBM. The 360 nm band in both LIBM and colorless monomer originates from a Schiff base-reduced retinyl chromophore which remains covalently linked to bacterioopsin. Both the PM-->LIBM and monomer-->colorless monomer photoconversions are mediated by similar biphotonic mechanisms, indicating that the photochemistry is localized within single BR monomers and is not influenced by BR-BR interactions. The excessively large two-photon absorptivities (> or =10(6) cm(4) s molecule(-1) photon(-1)) of these photoconversions, the temporal and spectral characteristics of pulses which generate LIBM in high yield, and an action spectrum for the PM-->LIBM photoconversion all indicate that the PM-->LIBM and Mon-->CMon photoconversions are both mediated by a sequential biphotonic mechanism in which is the intermediate which absorbs the second photon. The purple-->blue color change results from subsequent conformational perturbations of the PM lattice which induce the removal of Ca(2+) and Mg(2+) ions from the PM surface.
authors.affiliation: Department of Chemistry, Murray State University, 456 Blackburn Science Building, Murray, Kentucky 42071-3346, USA. mark.masthay@murraystate.edu
authors.forename: Mark B
authors.initials: MB
authors.lastname: Masthay
authors.forename: David M
authors.initials: DM
authors.lastname: Sammeth
authors.forename: Merritt C
authors.initials: MC
authors.lastname: Helvenston
authors.forename: Charles B
authors.initials: CB
authors.lastname: Buckman
authors.forename: Wuyi
authors.initials: W
authors.lastname: Li
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Cde-Baca
authors.forename: John T
authors.initials: JT
authors.lastname: Kofron
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000502:physiology; D003116:Color; D007834:Lasers; D008563:Membrane Lipids / Q000737:chemistry* / Q000502:physiology; D010777:Photochemistry; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D018717:Purple Membrane / Q000737:chemistry / Q000502:physiology; D013056:Spectrophotometry, Ultraviolet
chemical_list: D008563:Membrane Lipids; D001436:Bacteriorhodopsins