Effect of Protonation on the Secondary Structure and Orientation of Plant Light-Harvesting Complex II Studied by PM-IRRAS

Abstract

The major light-harvesting pigment-protein complex of photosystem II, LHCII, has a crucial role in the distribution of the light energy between the two photosystems, the efficient light capturing and protection of the reaction centers and antennae from overexcitation. In this work direct structural information on the effect of LHCII protonation, which mimics the switch from light-harvesting to photoprotective state of the protein, was revealed by polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS on LHCII monolayers verified that the native helical structure of the protein is preserved in both partly deprotonated (pH 7.8, LHCII) and protonated (pH 5.2, p-LHCII) states. At low surface pressure, 10 mN/m, the orientation of the α-helices in these two LHCII states is different-tilted (θ ≈ 40°) in LHCII and nearly vertical (θ ≈ 90°) in p-LHCII monolayers; the partly deprotonated complex is more hydrophilic than the protonated one and exhibits stronger intertrimer interactions. At higher surface pressure, 30 mN/m, which is typical for biological membranes, the protonation affects neither the secondary structure nor the orientation of the transmembrane α-helices (tilted ∼45° relative to the membrane surface in both LHCII states) but weakens the intermolecular interactions within and/or between the trimers.

A Key Role of Xanthophylls That Are Not Embedded in Proteins in Regulation of the Photosynthetic Antenna Function in Plants, Revealed by Monomolecular Layer Studies

“…The monomolecular layer system has been shown to provide a suitable model for study of the molecular organization and structural features of LHCII, in which the complex retains its native structure and functional properties, manifested by efficient excitation energy transfer. 18 method developed by Krupa et al 22 The purity of the preparations has been controlled with application of spectroscopic and electrophoretic techniques. 23,24 All-trans violaxanthin and all-trans zeaxanthin were isolated from Narcissus jonquilla L. flowers and Lycium barbarum fruits, respectively, according to the procedures described previously.…”

“…A monomolecular layer approach, combined with several molecular spectroscopy and imaging techniques, was applied to get insight into the molecular and photophysical mechanisms underlying such a regulatory activity. The monomolecular layer system has been shown to provide a suitable model for study of the molecular organization and structural features of LHCII, in which the complex retains its native structure and functional properties, manifested by efficient excitation energy transfer. − …”

Secondary Structure‐Dependent Charge Transport in iLOV Protein Junctions

“…Amide I at ∼1670 cm −1 is derived from carbonyl stretching, and amide II at ∼1547 cm −1 is caused by N‐related groups’ stretching, vibration, and deformation. The amide II/I ratios are related with the conformation and orientation of proteins adsorbed on substrate in a certain degree [26] . Taken iLOV 3.0 as an example, the PM‐IRRAS of iLOV 3.0 SAMs display little difference for amide II/I ratio at various pH (Table S1).…”

“…The amide II/I ratios are related with the conformation and orientation of proteins adsorbed on substrate in a certain degree. [26] Taken iLOV 3.0 as an example, the PM-IRRAS of iLOV 3.0 SAMs display little difference for amide II/I ratio at various pH (Table S1). PM-IRRAS may not be sensitive enough to display the small changes in iLOV 3.0 by pH.…”