Magnetic Resonance and Picosecond Laser Spectroscopy of Light-Harvesting Chlorophyll A/B-Protein

Abstract

Chlorophyll (Ch1) a/b — protein 2, a light-harvesting protein, isolated in the monomeric form from barley, has unusual spectroscopic properties. The fluorescence detected magnetic resonance spectrum at 4.2 K shows a narrow (< 10 MHz) intense 2E line, which is lost on partial denaturation, whilst fluorescence fading experiments at 4.2 K indicate triplet state spin level decay rate constants which are much slower than for either Ch1 a or Ch1 b in vitro. At 293 K the Ch1 a fluorescence decay cannot be described by a single exponential and is best described by two exponential components (2.5 and 4.7 ns). These observations have led to a model for chlorophyll interactions within the protein in which one or more Ch1 a is closely associated with a Ch1 b, whilst remaining Ch1 a shows much weaker Chl-Ch1 interactions. Fluorescence spectra of Ch1 a/b-protein 2 at 293 K and 4.2 K support this hypothesis of a uniquely strong interaction between Ch1 a and Ch1 b molecules in Ch1 a/b-protein 2.