Ethyl p-(dimethylamino)cinnamate (EDAC) has been
used as a fluorescence probe for monitoring the
interaction
between a model water-soluble protein, bovine serum albumin (BSA), and
an anionic surfactant, sodium
dodecyl sulfate (SDS). The probe EDAC undergoes intramolecular
charge transfer (ICT) in the excited state
in water and other polar solvents. The emission from the ICT state
becomes more intense and blue-shifted
due to reduced polarity in the hydrophobic environments of BSA and SDS
micelles relative to that in pure
water. The intensity of the ICT emission from EDAC increases with
surfactant concentration and reaches a
maximum at the critical micelle concentration of SDS, which can be
employed as a simple technique for
following micellization. Analysis of the fluorescence spectra of
the probe provide evidences in favor of
surfactant-induced protein uncoiling due to massive binding of the SDS
molecules to BSA in the cooperative
binding region of the binding curve, describing protein
(BSA)−surfactant (SDS) interaction. The polarity of
the BSA−SDS aggregate formed is intermediate between that of
hydrophobic regions of BSA and SDS micelles
as sensed by the intramolecular charge-transfer (ICT) probe,
EDAC.
We report here on the steady-state and time-resolved fluorescence studies on proton-transfer (PT) reaction of 4-methyl 2,6-diformyl phenol (MFOH) in confined nanocavities in three solvents, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), and water. Though DMSO and DMF individually interact with MFOH in a similar fashion, their modes of interaction get significantly modified in the presence of cyclodextrin (CD) nanocages. In DMSO, in the ground state, the solvated molecular anion of MFOH forms 1:1 inclusion complex with beta- or gamma-CD and attains greater stability compared to the normal form. In DMF, the solvated molecular anion gets converted to the H-bonded complex within the CD cavity resulting in a 50-nm blue shift in the absorption spectra. In the excited state, the anionic species gets more stabilized in DMSO while in DMF it is significantly destabilized in the presence of CDs. However, in case of water, MFOH gets trapped inside the water cages so that the CDs fail to complex with it effectively. There are also no changes in the excited-state lifetimes in water in the presence of CDs, but in case of DMSO and DMF, because of restricted rotation of the formyl group within the CD cavity, the contribution of the shorter lifetime components reduce significantly increasing the larger components. Some theoretical calculations at the AM1 level of approximation have also been carried out to demonstrate how the dipolar nature of the solvent influences excited-state PT in confined media.
The “push–pull”
effects associated with heme enzymes manifest themselves through highly
evolved distal amino acid environments and axial ligands to the heme.
These conserved residues enhance their reactivities by orders of magnitude
relative to small molecules that mimic the primary coordination. An
instance of a mononuclear iron porphyrin with covalently attached
pendent phenanthroline groups is reported which exhibit reactivity
indicating a pH dependent “push” to “pull”
transition in the same molecule. The pendant phenanthroline residues
provide proton transfer pathways into the iron site, ensuring selective
4e–/4H+ reduction of O2 to
water. The protonation of these residues at lower pH mimics the pull
effect of peroxidases, and a coordination of an axial hydroxide ligand
at high pH emulates the push effect of P450 monooxygenases. Both effects
enhance the rate of O2 reduction by orders of magnitude
over its value at neutral pH while maintaining exclusive selectivity
for 4e–/4H+ oxygen reduction reaction.
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