Absorption spectral change of peripheral-light harvesting complexes 2 induced by magnesium protoporphyrin IX monomethyl ester association

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2015-02, Vol.137, p.1153-1157
• Main Authors: Yue, Huiying, Zhao, Chungui, Li, Kai, Yang, Suping
• Format: Article
• Language: English
• Subjects: Bacteriochlorophyll; Carotenoid; LH2; Magnesium protoporphyrin monomethyl IX ester; Rhodobacter azotoformans
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2014.08.132

[Display omitted] •MPE was additionally contained in the LH2 with ∼423nm absorption peak.•The absorption spectral change of LH2 was ascribed to MPE association.•MPE binded to LH2 in vitro, but did not bind to the BChl a binding sites in LH2.•MPE accumulation affected the conformation of integral LH2. Several spectrally different types of peripheral light harvesting complexes (LH) have been reported in anoxygenic phototrophic bacteria in response to environmental changes. In this study, two spectral forms of LH2 (T-LH2 and U-LH2) were isolated from Rhodobacter azotoformans. The absorption of T-LH2 was extremely similar to the LH2 isolated from Rhodobacter sphaeroides. U-LH2 showed an extra peak at ∼423nm in the carotenoid region. To explore the spectral origin of this absorption peak, the difference in pigment compositions of two LH2 was analyzed. Spheroidene and bacteriochlorophyll aP were both contained in the two LH2. And magnesium protoporphyrin IX monomethyl ester (MPE) was only contained in U-LH2. It is known that spheroidene and bacteriochlorophyll aP do not produce ∼423nm absorption peak either in vivo or in vitro. Whether MPE accumulation was mainly responsible for the formation of the ∼423nm peak? The interactions between MPE and different proteins were further studied. The results showed that the maximum absorption of MPE was red-shifted from ∼415nm to ∼423nm when it was mixed with T-LH2 and its apoproteins, nevertheless, the Qy transitions of the bound bacteriochlorophylls in LH2 were almost unaffected, which indicated that the formation of the ∼423nm peak was related to MPE-LH2 protein interaction. MPE did not bind to sites involved in the spectral tuning of BChls, but the conformation of integral LH2 was affected by MPE association, the alkaline stability of U-LH2 was lower than T-LH2, and the fluorescence intensity at 860nm was decreased after MPE combination.