An important prerequisite for efficient Förster resonance energy transfer (FRET) from human serum albumin to alkyl gallate

Abstract

Förster resonance energy transfer (FRET), originally described by Förster in the 1940s, results from long-range dipole–dipole interactions between a donor and an acceptor molecule. In this work, human serum albumin (HSA) was selected as donor and the alkyl gallates were selected as acceptors. In the presence of HSA, the alkyl gallates yielded different levels of fluorescence enhancement. The corresponding energy transfer efficiency at a molar ratio of 1 : 1 were 0.7% (gallic acid), 0.2% (methyl gallate), 2.4% (ethyl gallate), 20.9% (propyl gallate), and 33.1% (butyl gallate). The alkyl gallates shared the same main binding site (located in Sudlow site II) in HSA. The formation of a stable complex by fixing alkyl gallate to tryptophan of HSA at an appropriate orientation and distance was an important prerequisite for efficient FRET. Mant factors, including the factors influencing the complex formation, can hinder energy transfer. Energy transfer was seriously hindered when fatty acids preexisted in HSA. During energy transfer, some energy were dissipated as heat. Energy transfer did not happen when the donor was egg white lysozyme rather than HSA. FRET does not exhibit specificity, but the specific structure of HSA helped provide the selectivity of alkyl gallate.