Entropic Force in Drosophila Resilin: Molecular Dynamics Study Roman Petrenko * , Matthew B. Dickerson † , ‡ , Rajesh R. Naik ‡ , Soumya S. Patnaik § , ‡ , Thomas L. Beck ¶ and Jaroslaw Meller ‖ * Department of Physics University of Cincinnati, Cincinnati, OH 45221-0011 † Research Associateship Program National Research Council, National Academy of Sciences USA 500 Fifth Street, NW, Washington, DC 20001 ‡ Materials and Manufacturing Directorate Air Force Research Lab, WPAFB, Dayton, OH 45433 § Modeling Solutions, Inc., 6649 Switchback Lane, West Chester, OH 45069 ¶ Departments of Chemistry and Physics University of Cincinnati, Cincinnati, OH 45221-0172 ‖ Department of Environmental Health University of Cincinnati, Cincinnati, OH 45267-0056 Email: jmeller@cchmc.org Abstract—Resilin is a rubber-like elastomeric protein found in many insects, including D. melanogaster (fruit fly). Resilin is highly extensible and appears to have the highest resilience among all known elastic materials. We have performed molecular dynamics study of two repeat units from the N-terminal elas- tomeric domain in fruit fly resilin. A single repeat unit (sequence NGGRPSSSYGAPGGG) was derived as a consensus repeat from different fruit fly species. Our simulations show high flexibility of the peptide in water and yield secondary structure content that is consistent with a disordered ensemble supported by circular dichroism (CD) spectra. In agreement with previous studies, indicating the entropic origin of the elastomeric force in resilin, we find that configurational entropy is markedly decreased upon stretching of the simulated peptide. Index Terms—resilin, resilin-like peptides, elastomeric force, molecular dynamics, entropy I. I NTRODUCTION Elastomeric proteins possess a rubber-like elasticity and are capable of storing energy upon deformation 1 . Examples include: elastin, resilin, spider silk, abductin, gliadin, and gluten. Among these elastomers only resilin and elastin can be stretched more than twice of their original length and can recover more than 90% of the deformation energy once the stretching (compressing) force is removed. The latter property is called resilience, thus the name resilin. For comparison, resilience of the polybutadiene rubber is only 80% [1]. Elastin is found in aorta and skin of mammals, and resilin is found in insects and other arthropods. Particularly, resilin is present in the flight organs of dragonflies and locusts, in the jumping organs of fleas and froghoppers, and in the legs of cockroaches. While resilin and elastin have similar properties, elastin has 1 Comprehensive reviews of the current state in the elastomeric research can be found in [1], [2]. Fig. 1. Schematic representation of the domain structure of the gene product CG15920 from Drosophila species, which consists of three domains. From left to right these domains are: N-terminal elastic repeats (red), chitin-binding domain (black), and C-terminal elastic repeat units (blue). been studied more extensively. Resilin, which has around 5% higher resilience than elastin, to the best of our knowledge, has not been systematically studied using computational methods as yet. The purpose of this study is to report the first molecular dynamics simulation of two N-terminal repeats from fruit fly resilin and to address the origin of the elastomeric force. Resilin was first discovered by Weis-Fogh as a highly extensible protein found in the flight system of Schistocerca gregaria (desert locust) [3], [4]. It was shown that resilin is highly hydrated in water, covalently cross-linked and has a rubber-like elasticity. Elastic properties of resilin are preserved in a wide range of temperatures. Weis-Fogh had recognized [3] the presence of stable cross-linking covalent bonds which keep protein chains together and later Andersen [5] discovered the nature of crosslinks in the form of di- and trityrosine bonds. The importance of polar solvent in resilin elasticity was found by observing that the protein becomes glassy on dehydration and regains its elasticity in any polar solvent. The degree of swelling is increased with the increase of pH level of a solvent. The average number of residues between cross-links was estimated to be around 60 [6]. Recently, Ardell et al. identified gene product CG15920 as a tentative D. melanogaster resilin precursor [7], which has the following domain organization: N-terminal region (containing signal peptide and elastic repeats), chitin-binding domain