Intraprotein Formation of a Long Wavelength Absorbing Complex and Inhibition of Excited-State Deprotonation in a Chiral Hydroxybiphenyl

Abstract

The two enantiomers of 2-(2-hydroxybiphenyl-4-yl)propanoic acid ((S)- and (R)-BPOH) have been selected as probes for human serum albumin (HSA). Photophysical characterization in the absence of protein led to emission maxima, singlet energies, quantum yields, and fluorescence lifetime values of 332 nm, 91 kcal mol(-1), and 0.28 and 1.8 ns for BPOH or 414 nm, 79 kcal mol(-1), and 0.31 and 3.3 ns for the corresponding phenolate BPO(-); the pK(a)* was found to be 1.17. In the presence of HSA, a light-absorbing ground-state complex (S)-BPOH@HSA was detected (maximum at ca. 300 nm) whose intensity increased with increasing protein concentration. The fluorescence spectra of (S)-BPOH in PBS, after addition of HSA, revealed a progressive diminution of the phenolate band, indicating that excited-state deprotonation is disfavored within the hydrophobic protein cavities. A similar trend was observed for (R)-BPOH, but the extent of deprotonation was significantly lower for this enantiomer. Addition of increasing amounts of the site II displacement probe (S)-ibuprofen ((S)-IBP) to BPOH@HSA led to a significant decrease of the absorption maximum at ca. 300 nm and to a recovery of the phenolate emission band at ca. 410 nm, which were again configuration dependent. The transient absorption spectrum of (S)-BPOH consisted on a broad band centered at 380 nm, attributed to the first triplet excited state. A dramatic enhancement of the triplet lifetimes within HSA was observed (19.0 μs within protein versus 1.3 μs in bulk PBS), although no stereodifferentiation was noticed in this case.

Photoactive assemblies of organic compounds and biomolecules: drug–protein supramolecular systems

“…A dramatic enhancement of the triplet lifetimes within HSA has been observed (19.0 ms, compared with 1.3 ms in bulk PBS). 45 The study has also been performed with (R)-BOH, and no stereodifferentiation has been noticed in this case.…”

“…300 nm. 45 In neutral buffer, emission of BOH displays two maxima at 343 and 412 nm, attributed to 1 BOH* and to the corresponding excited phenolate, respectively. However, a progressive diminution of the 412 nm band is observed upon addition of protein, indicating that excited state deprotonation is disfavoured within the hydrophobic HSA cavities ( Figure 1A).…”

Triplet Excited States as a Source of Relevant (Bio)Chemical Information

“…The technique has also been successfully employed for other purposes, such as the drug photobinding to proteins in photoallergy, 32 determination of enantiomeric composition in mixtures of enantiomers 33 or inhibition of ligand deprotonation after protein encapsulation. 34 …”