Conformational Heterogeneity of a Leucine Enkephalin Analogue in Aqueous Solution and Sodium Dodecyl Sulfate Micelles: Comparison of Time-Resolved FRET and Molecular Dynamics Simulations

Abstract

We have undertaken time-resolved Förster resonance energy transfer (FRET) and molecular dynamics simulations to analyze conformations and conformational heterogeneity of an analog of leucine enkephalin in solution and in the presence of SDS micelles. Enkephalins are opioid pentapeptides that interact with opioid receptors in the central nervous system. We used time-correlated single-photon counting to detect energy transfer between the N-terminal tyrosine and a tryptophan residue substituted for phenylalanine at the 4 position. FRET from Tyr to Trp was measured over a temperature range from 5°C to 55°C in aqueous solution. By taking into account Tyr rotamer interconversion rates measured previously, we determined average distances between Tyr and Trp for the two populated rotameric conformations of Tyr. Molecular dynamics simulations (100 ns) support this analysis and indicate extensive conformational heterogeneity. The simulations also predict that the FRET orientational factor is correlated with the Tyr-Trp separation. Failure to account for the correlation between orientation and distance results in errors that appear to be largely offset in YGGWL by a weighting bias inherent in the R−6 dependence of the energy-transfer rate. The Tyr lifetimes decrease upon titration of the peptides with SDS, indicating formation of compact conformations of the peptide in the micelle environment. This result is consistent with the conjecture that the lipid environment may induce formation of bioactive conformations of the peptide.

Structural effects of chirally mutated Enkephalin neurotransmitters: An argument for biological homochirality

“…Enkephalins were reported to lack a well-defined secondary structure in aqueous solution [45,46], in agreement with the results presented here. Determining a quantitative structure-activity relationship has so far remained insurmountable [45,46]. shows that the vas deferens activity of Ala(2)-Leu-Enkephalins [28] increases exponentially with the tendency to adopt compact conformations, suggesting that folding into a compact conformation determines opioid activity.…”

“…The structural basis of Enkephalin opioid activity has long been investigated [28,40,45]. Enkephalins were reported to lack a well-defined secondary structure in aqueous solution [45,46], in agreement with the results presented here. Determining a quantitative structure-activity relationship has so far remained insurmountable [45,46].…”

Electronic energy transfer in biomacromolecules

“…Several groups have reported more elaborated approaches to describe EET in the intermediate averaging regime, in which Markov chain sampling techniques are used to model D (and eventually A) fluorescence decays in addition to FRET efficiencies from electronic coupling trajectories. 22,24,25,28,29,38 In Scheme 1, we illustrate as an example the approach proposed by Grubmüller and coworkers. 25 In this scheme, first a random donor excitation instance is chosen from the trajectory.…”

Using Molecular Dynamics and Quantum Mechanics Calculations To Model Fluorescence Observables

“…One exception is a recent paper from Unruh et al 46 in which a Markov chain sampling technique was used with the MD simulation results to generate modeled D fluorescence decays. These decays were fit and compared to fits of the experimental fluorescence decays.…”