Excited State Dipole Moment of PRODAN as Determined from Transient Dielectric Loss Measurements

Abstract

Time-resolved changes in microwave dielectric absorption have been studied to determine the excited state dipole moment of 6-propanoyl(N,N-dimethylamino) naphthalene (PRODAN), a molecule extensively used as a fluorescence probe for the study of microheterogeneous environments of a wide variety of chemical and biological systems. This study, which provides by far the most reliable value of the excited-state dipole moment of PRODAN, shows that the change in the dipole moment of the molecule upon excitation (4.4−5.0 D) is far less than the commonly mentioned value (∼ 20 D). The results imply that solvatochromism of the fluorescence band of PRODAN, the property that makes this system an ideal probe, cannot be attributed to an extraordinarily large magnitude of Δμ. Other forces, such as a hydrogen-bonding interaction of the molecule with the solvent, must play a crucial role in determining the location of the fluorescent state of PRODAN. Furthermore, the results imply that internal rotation or twisting of the dimethylamino group in the excited state, if any occurs, does not influence the fluorescence properties of PRODAN and that the fluorescence originates from a locally excited state.

Novel Insights Into Protein Structure and Dynamics Utilizing the Red Edge Excitation Shift Approach

“…179 The dipole moment of PRODAN changes by ~5-8 D upon excitation. 180,181 A change in dipole moment of this magnitude, along with its hydrogen bonding capability, 181 makes PRODAN a suitable probe for monitoring REES effects to characterize hydrophobic binding sites in proteins. For example, PRODAN binds erythroid spectrin with a high affinity.…”

Organization and Dynamics of Tryptophan Residues in Brain Spectrin: Novel Insight into Conformational Flexibility

“…A large change in the dipole moment of the fluorophore upon excitation, which causes the solvent dipoles to reorient in an energetically favorable manner in response to the altered dipole moment in the excited state, is an important requirement for a fluorophore to be able to exhibit REES [43]. PRODAN fluorescence is very sensitive to the polarity of the environment and its dipole moment changes by~5-8 D upon excitation [62,63]. A change in dipole moment of this magnitude, coupled with its hydrogen bonding capability [63] makes PRODAN an ideal probe for REES measurements.…”

Spectrin Organization and Dynamics: New Insights

“…Prodan, introduced by Weber and Farris (1979), is an extrinsic fluorescent probe which has been shown to bind hydrophobic sites of proteins (Weber and Farris 1979;Mazumdar et al 1992). It is a naphthalene derivative characterized by both electron donor and electron acceptor substituents resulting in a large change in dipole moment upon excitation (Balter et al 1988;Samanta and Fessenden 2000), leading to extensive solvent polarity dependent shifts in the fluorescence emission maximum. Prodan and its derivatives have earlier been successfully used as hydrophobic markers to estimate the polarity of the heme-binding pocket in apomyoglobin (Macgregor and Weber 1986).…”

“…Utilizing this approach, it becomes possible to probe the mobility parameters of the environment itself (which is represented by the relaxing solvent molecules) using the fluorophore merely as a reporter group. Interestingly, a large change in dipole moment (Balter et al 1988;Samanta and Fessenden 2000), along with its hydrogen bonding capability (Samanta and Fessenden 2000), makes Prodan a suitable probe for REES effects in order to characterize the hydrophobic binding sites in spectrin. Prodan bound to spectrin was shown to display a significant REES of 9 nm .…”