Organic & Biomolecular Chemistry PAPER Cite this: Org. Biomol. Chem., 2013, 11, 8348 Received 28th September 2013, Accepted 15th October 2013 DOI: 10.1039/c3ob41967c www.rsc.org/obc Ester vs. amide on folding: a case study with a 2-residue synthetic peptide† Kuruppanthara N. Vijayadas, a Roshna V. Nair, a Rupesh L. Gawade, b Amol S. Kotmale, c Panchami Prabhakaran, a,d Rajesh G. Gonnade, b Vedavadi G. Puranik, b Pattuparambil R. Rajamohanan c and Gangadhar J. Sanjayan* a Although known for their inferiority as hydrogen-bonding acceptors when compared to amides, esters are often found at the C-terminus of peptides and synthetic oligomers (foldamers), presumably due to the synthetic readiness with which they are obtained using protected peptide coupling, deploying amino acid esters at the C-terminus. When the H-bonding interactions deviate from regularity at the termini, peptide chains tend to “fray apart” . However, the individual contributions of C-terminal esters in causing peptide chain end-fraying goes often unnoticed, particularly due to diverse competing effects emanating from large peptide chains. Herein, we describe a striking case of a comparison of the individual contri- butions of C-terminal ester vs. amide carbonyl as a H-bonding acceptor in the folding of a peptide. A simple two-residue peptide fold has been used as a testing case to demonstrate that amide carbonyl is far superior to ester carbonyl in promoting peptide folding, alienating end-fraying. This finding would have a bearing on the fundamental understanding of the individual contributions of stabilizing/destabi- lizing non-covalent interactions in peptide folding. Introduction Hydrogen-bonding interactions play a central role in the stabil- ization of secondary structures – the local conformations that impart a specific shape to the peptide backbone. There are innumerable examples of peptide-based systems known in the literature wherein the C-terminal esters, as clearly evident from their crystal structures, are seen not participating in intramolecular hydrogen-bonding that would have further reinforced the stability of the secondary structure. 1–4 In fact, in most of the peptide hairpin crystal structures, the terminal hydrogen bond that connects the N- and the C-termini of the peptide is absent as a consequence of strand fraying. 1,2 Once “frayed”, the exposed termini may interact with solvent mole- cules for hydrogen-bonding and when the hydrogen bonding solvents are not available, peptides may aggregate by inter- molecular hydrogen-bonding interactions. 2 In large peptides, including the synthetic oligomers called foldamers, 5–7 it would often be a challenging task to pinpoint the individual contri- butions of the C-terminal esters in causing end-fraying. This is particularly due to the competing diverse effects originating at the peptide termini such as: π–π interactions, 2a NH–π inter- actions, 2b,c a similar chirality array of amino acids in the hairpin region, 4f,g temperature effects 2d and bulkiness of groups at the end terminals 4h etc., which are all well known to cause peptide chain end-fraying. Using a simple two-residue reverse-turn mimic, 8–19 herein we demonstrate the conse- quences of swapping C-terminal amides with esters on the overall folding ability of a synthetic peptide. Crystallographic studies of several C-terminal esters and amides unequivocally reveal the high propensity of the amide carbonyls to promote a closed hydrogen-bonded network adopting a folded confor- mation, whilst the C-terminal esters suffer end-fraying leaving the termini open-ended without intramolecular hydrogen- bonding. Employing the ready-to-understand 2-residue reverse turn motif with least interference from other effects, as noted above, 20 we also exemplify how tiny structural changes at the peptide termini can have dramatic consequences for the folding characteristics. 20b,21,22 † Electronic supplementary information (ESI) available: 1 H, 13 C, DEPT-135 NMR, 2D study spectra, ESI mass spectra and theoretical study of new compounds are included. CCDC 874039, 699500, 874040, 874041, 874042, 874043, 679454, 874044, 874045, 874046 and 874047. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ob41967c a Division of Organic Chemistry, National Chemical Laboratory, Dr Homi Bhabha Road, Pune 411 008, India. E-mail: gj.sanjayan@ncl.res.in; Fax: +91-020-2590-2629; Tel: +91-020-2590-2082 b Centre for Materials Characterisation, National Chemical Laboratory, Dr Homi Bhabha Road, Pune 411 008, India c Central NMR Facility, National Chemical Laboratory, Dr Homi Bhabha Road, Pune 411 008, India d Department of Chemistry & Biology, University of Leeds, Leeds, LS2 9JT, UK 8348 | Org. Biomol. Chem., 2013, 11, 8348–8356 This journal is © The Royal Society of Chemistry 2013 Published on 15 October 2013. Downloaded by National Chemical Laboratory, Pune on 20/11/2013 09:13:41. View Article Online View Journal | View Issue