THE 3‐METHYLINDOLE/n‐BUTANOL EXCIPLEXES: EVIDENCE FOR TWO EXCIPLEX SITES IN INDOLE COMPOUNDS

Abstract

The 1-butanol concentration dependence of fluorescence emission intensities and spectra from 3-methylindole/l-butanol exciplexes in 1-heptane reveals a 1 : 1 stoichimetry near the isoemissive point increasing to 1 :2 at higher concentrations. The large increase in stability of the 1 : 1 complex relative to indole itself is attributed to its relatively low dissociation rate with an activation energy of 36.8 kJ/mol. Each step of exciplex formation shifts the emission maximum about 15nm to the red. The stronger combining site is the negative C-3. The weaker site is the positive N-1. Stabilization due to charge transfer between excited indole and electrophilic or nucleophilic partners is small relative to dipole-dipole interactions. There is no indication of significant orbital-overlap. The detection of both positive and negative centers for exciplex formation on indole, I-methyl indole and electronically similar derivatives resolves some long-standing problems and extends the basis of knowledge necessary to use tryptophanyl-residue fluorescence as a quantitative probe for protein conformational character and its changes.

Mechanisms of Tryptophan Fluorescence Shifts in Proteins

“…It is rare to find a single water-induced red shift Ͼ5 nm for an exposed Trp when the solvation shell is in response to the ground state, but common to find shifts of 20 nm shortly after excitation. However, unlike the specifically localized exciplexes of Hershberger et al (1981), we find a wider locus of positions/orientations that can cause large red shifts. Our use of explicit solvent instead of a dielectric continuum has provided rich detail regarding close-range exciplex-type interaction that would be highly distorted in a continuum approach, wherein the results would be dependent upon an arbitrary cavity about the Trp and choices of the controversial protein dielectric constant, as well as some of the inherent problems associated with grid-based electrostatic approaches (e.g., Poisson-Boltzmann treatments).…”

“…For the -complex, wherein the HN donates a hydrogen bond to water, the computed red shift is ϳ10 nm. Because the interaction and shifts are much larger for the 1 L a state than for the ground state, they are reminiscent of the excited-state complexes ("exciplexes") postulated by the Lumry group (Hershberger et al, 1981;Walker et al, 1967). However, as will be discussed below, they have much different structures.…”

“…The idea of exciplexes grew from observations by Mataga et al (1964) and by Lumry and co-workers (Walker et al, 1967;Hershberger et al, 1981) that millimolar concentrations of alcohols, when added to a solution of indole or 3-methylindole in pure hydrocarbon, would cause considerable shifts in fluorescence while causing minute changes in the absorption. Although Mataga suggested that the interaction was similar to an Onsagertype reaction field, Hershberger et al (1981) advanced a rather detailed picture of two binding sites: an electrophilic one at C3(CG) and a nucleophilic one at N1. To date, however, all quantum mechanical treatments suggest that the considerable electron transfer to the benzene ring upon excitation to 1 L a would lead to an electrophilic site (or sites) on the benzene ring.…”

Electronic spectroscopy of aromatic Schiff bases. V. Specific interactions involved between some typical benzylideneaniline molecules and small polar molecules

“…Moreover, the entropy change is higher for the complex involving ethanol; this can be explained by the fact that complexes involving hydrogen bond interactions are much more ordered relative to those stabilized by dipolar interactions. Thermodynamics alone cannot differentiate the sites of the complexation, but recently Lumry et al (20) suggested that indole compounds possess two sites for complexation and that alcohols can serve as proton donor and also electron donor in the interactions with these type of compounds. Scheuer-Lamalle made some theoretical ab initio calculations of the electronic density on the substituted phenylimines (R1-CH=N-C6H4-R) and on the substituted benzaldimine molecules (R1-C6H4-CH=N-R) (21).…”

The Photophysics and Photochemistry of the Near‐uv Absorbing Amino Acids–i. Tryptophan and Its Simple Derivatives

“…1-Methylindole, 3, which has no NH group for possible ground state hydrogen bonding showed the same effect. 3-Methylindole, 4, forms exciplexes with ether, dioxane, acetronitrile, methanol, and butanol (Hershberger et al, 1981). For the latter case an isoemissive point at low alcohol concentrations indicated 1: 1 stoichiometry, whereas at higher concentrations further red shifts were attributed to formation of 4: alcohol exciplexes of 1:2 stoichiometry.…”