2-Naphthol-phosphatidylethanolamine: A fluorescent phospholipid analogue for excited-state proton transfer studies in membranes

A 1-Hydroxy-1H-imidazole ESIPT Emitter Demonstrating anti-Kasha Fluorescence and Direct Excitation of a Tautomeric Form

The ability of 1-hydroxy-1H-imidazoles to exist in the form of two prototropic tautomers, the N-hydroxy and the N-oxide forms, can be utilized in the design of new types of ESIPT-fluorophores (ESIPT=excited state intramolecular proton transfer). Here we report the first example of 1-hydroxy-1H-imidazole-based ESIPT-fluorophores, 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL), featuring a short intramolecular hydrogen bond O-H⋅⋅⋅N (O⋅⋅⋅N 2.56 Å) as a pre-requisite for ESIPT. The emission of HL originates from the anti-Kasha S₂ →S₀ fluorescence in the N-oxide form as a result of a large S₂ -S₁ energy gap slowing down the S₂ →S₁ internal conversion. Due to an energy barrier between the N-hydroxy and N-oxide forms in the ground state, the HL molecules can be trapped and photoexcited in the N-oxide form leading to the Stokes shift of ca. 60 nm which is the smallest among known ESIPT-fluorophores.

Investigating 2,2‘-Bipyridine-3,3‘-diol as a Microenvironment-Sensitive Probe: Its Binding to Cyclodextrins and Human Serum Albumin

The 2,2'-bipyridine-3,3'-diol molecule (BP(OH)2) was investigated as a potential photophysical probe in inclusion and biological studies. Binding of BP(OH)2 to cyclodextrins (CDs) and human serum albumin (HSA) was studied by following the changes in its absorption and fluorescence spectra. The stoichiometric ratios and binding constants of the complexes were deduced by fitting the changes in the spectral intensity to binding isotherms. The stoichiometric ratio in the BP(OH)2/(alpha-CD) complex is dominated by 1:2, whereas in all other CDs and in HSA this ratio is 1:1. The structure of the BP(OH)2:(alpha-CD)2 complex, calculated using ab initio methods, indicates that the inclusion of the BP(OH)2 molecule is axial and centered between the two cavities of alpha-CD with van der Waals and electrostatic interactions dominating the binding. Analysis of these results along with the inclusion results of BP(OH)2 in beta-CD, methyl-beta-CD, 2,6-di-O-methyl-beta-CD, and gamma-CD shows that absorption and fluorescence of BP(OH)2 are very sensitive to the change in the cavity size of CD and its hydrophobicity. This change is reflected in the form of a decrease in the intensity of the absorption peaks of the BP(OH)2/water complex in the region 400-450 nm and a red shift in the fluorescence peak as the cavity size decreases and its hydrophobicity increases. Binding of BP(OH)2 as a probe ligand to HSA, a prototype protein, reflects the hydrophobic interior of HSA in a similar manner. The spectral changes indicate that BP(OH)2 binds in the hydrophobic cavity of HSA's subdomain IIA. The results presented here show that BP(OH)2 can be used in binding sites and biological systems as a microenvironment-sensitive probe.

A coupled proton-transfer and twisting-motion fluorescence probe for lipid bilayers

A new and sensitive molecular probe, 2-(2'-hydroxyphenyl)imidazo[1, 2-a]pyridine (HPIP), for monitoring structural changes in lipid bilayers is presented. Migration of HPIP from water into vesicles involves rupture of hydrogen (H) bonds with water and formation of an internal H bond once the probe is inside the vesicle. These structural changes of the dye allow the occurrence of a photoinduced intramolecular proton-transfer reaction and a subsequent twisting/rotational process upon electronic excitation of the probe. The resulting large Stokes-shifted fluorescence band depends on the twisting motion of the zwitterionic phototautomer and is characterized in vesicles of dimyristoyl-phosphatidylcholine and in dipalmitoyl-phosphatidylcholine at the temperature range of interest and in the presence of cholesterol. Because the fluorescence of aqueous HPIP does not interfere in the emission of the probe within the vesicles, HPIP proton-transfer/twisting motion fluorescence directly allows us to monitor and quantify structural changes within bilayers. The static and dynamic fluorescence parameters are sensitive enough to such changes to suggest this photostable dye as a potential molecular probe of the physical properties of lipid bilayers.