Blue Emission in Proteins

Recent literatures reported blue-green emission from amyloid fibril as exclusive signature of fibril formation. This unusual visible luminescence is regularly used to monitor fibril growth. Blue-green emission has also been observed in crystalline protein and in solution. However, the origin of this emission is not known exactly. Our spectroscopic study of serum proteins reveals that the blue-green emission is a property of protein monomer. Evidences suggest that semiconductorlike band structure of proteins with the optical band-gap in the visible region is possibly the origin of this phenomenon. We show here that the band structure of proteins is primarily the result of electron delocalization through the peptide chain, rather than through the hydrogen bond network in secondary structure. Unrestrained protein association may result in deleterious consequences like amyloid plaque deposition culminating in numerous neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases. 1,2 Therefore, extensive spectroscopic studies have been carried out for the development and quantitative application of fluorescent assays to monitor the proteins self-assembly processes. 3 Recently, it has been reported that the amyloid fibrils formed from protein as well as small peptides exhibit luminescence in blue and green region, although the peptide or protein is devoid of chromophores which can emit in the visible region. 4-6 Guptasarma and co-workers reported about the blue fluorescence in protein crystal as well as in solution of γ-B-crystallin protein for the first time, and they attributed this unusual intrinsic fluorescence to the delocalization of electron through hydrogen bonding. 7,8 Smith et al. suggested that the blue emission is related to the formation of extensive Jaggregates that arise from π-stacking of aromatic moieties. However, Chan et al. argued that the intrinsic fluorescence observed in human peptides amyloid-β(1-42), lysozyme and Tau is independent of the presence of aromatic side chain residues; rather electronic delocalization via hydrogen bonds in β-sheet structure can cause this phenomenon. 9,10 This view has been supported by Sharpe et al. 11 Anand and Mukherjee ascribed the visible fluorescence of amyloid fibril obtained from peptide to the excitonic transition. 4 Very recently, Chatterjee et al. reported concentration dependent emission study of Bovine Serum Albumin (BSA), and attributed the blue fluorescence to protein aggregation. 12 Evidently, the origin of this ‘unusual visible luminescence’ is highly debated. Yet it is believed that blue-green fluorescence is a generic property of amyloid structures, and efforts are made to use this fluorescence to monitor the fibril growth. 3 Herein, we report a systematic study on intrinsic fluorescence property of serum proteins, copiously present in blood serum of vertebrates, to find the origin of this blue fluorescence. Our study proves that blue-green fluorescence of protein can be