Understanding the interaction between graphene oxide (GO) and the biomolecules is fundamentally essential, especially for disease- and drug-related peptides and proteins. In this study, GO was found to strongly interact with amino acids (tryptophan and tyrosine), peptides (Alzheimer's disease related amyloid beta 1-40 and type 2 diabetes related human islet amyloid polypeptide), and proteins (drug-related bovine and human serum albumin) by fluorescence quenching, indicating GO was a universal quencher for tryptophan or tyrosine related peptides and proteins. The quenching mechanism between GO and tryptophan (Trp) or tyrosine (Tyr) was determined as mainly static quenching, combined with dynamic quenching (Förster resonance energy transfer). Different quenching efficiency between GO and Trp or Tyr at different pHs indicated the importance of electrostatic interaction during quenching. Hydrophobic interaction also participated in quenching, which was proved by the presence of nonionic amphiphilic copolymer Pluronic F127 (PF127) in GO dispersion. The strong hydrophobic interaction between GO and PF127 efficiently blocked the hydrophobic interaction between GO and Trp or Tyr, lowering the quenching efficiency.

In the reaction center (RC) of Rhodobacter capsulatus, residue L212Glu is a component of the pathway for proton transfer to the reduced secondary quinone, QB. We isolated phenotypic revertants of the photosynthetically incompetent (PS-) L212Glu-->Gln mutant; all of them retain the L212Glu-->Gln substitution and carry a second-site mutation: L227Leu-->Phe, L228Gly-->Asp, L231Arg-->Cys, or M231Arg-->Cys. We also characterized the L212Ala strain, which is a phenotypic revertant of the PS- L212Glu-L213Asp-->Ala-Ala mutant. The activities of the RCs of these strains--all of which lack L212Glu--were studied by flash-induced absorption spectroscopy. At pH 7.5, the rate of second electron transfer in the L212Q mutant is comparable to the wild-type rate. However, this mutant shows a marked decrease in the rate of cytochrome oxidation under strong continuous illumination and a very slow phase (0.66 s-1) of the proton transfer kinetics following the second flash, indicating that transfer of the second proton to QB is slowed more than 1000-fold. The levels of recovery of the functional capabilities in the revertant RCs vary widely; their rates of cytochrome oxidation were intermediate between those of the wild-type and the L212Q mutant. The kinetics of proton transfer following the second flash show a significant recovery in the L212Q + M231C and L212A RCs (330-540 s-1), but the L212Q + L227F RCs recover this function only partially. Compensation for the lack of L212Glu in revertant RCs is discussed in terms of (i) conformational changes that could allow water molecules to approach closer to QB and/or (ii) the increase in the negative electrostatic environment and the resultant rise in the free energy level of QB- that is induced by the mutations. The stoichiometries of H+/QB- proton uptake below pH 7.5 in the L212Q mutant, the L212Q + M231C revertant, and the wild-type strains are essentially equivalent, suggesting that L212Glu is protonated at neutral pH in wild-type RCs. This is also supported by the P+QB- charge recombination data. Comparison of H+/QB- proton uptake data with those obtained previously for the stoichiometries of H+/QA- proton uptake [Miksovska, J., Maróti, P., Tandori, J., Schiffer, M., Hanson, D. K., Sebban, P. (1996) Biochemistry 35, 15411-15417] suggests that L212Glu is the key to the electrostatic and perhaps structural interaction between the two quinone sites.