Peptoids are analogues of α-peptides where the backbone is achiral due to the proteinogenic side chains being relocated from the α-C to the amide nitrogen. In the peptoids discussed here, chiral centres are present in the side chains thus making them suitable for chiroptical analysis. The presence of tertiary amides complicates NMR interpretation which means that circular dichroism (CD) becomes an invaluable tool for the study of peptoid conformation. Understanding of the spatial orientation of this novel α,β-peptoid backbone is vital for its biological application. Due to the novel nature of this peptidomimetic backbone, it is essential to establish the distinctive spectral features to enable these to be related to peptoid conformation. Preliminary SRCD data has dramatically improved our ability to study the conformational preference of these systems.
|Figure 1: Peptide and peptoid architectures.|
A range of distinctive CD spectra have been identified within the initial set of compounds studied. In particular, the 170-200 nm region was found to contain several characteristic transitions. The use of different solvent environments and temperature has enabled us to establish that the cyclic peptoids have greater conformational order than the linear peptoids and this can be readily disrupted. However, the linear peptoids are relatively unperturbed by temperature suggesting that they have an intrinsic conformational flexibility. Extended exposure to synchrotron radiation was not found to affect the CD spectra acquired for these peptoid systems.
A larger compound library is available and will enable us to explore the conformational space sampled by peptoids. SRCD has shown itself to be an essential tool to further our studies.
The ability of proteins and peptides to fold into unique, stable, biologically active conformations has promoted increased interest in the development of peptidomimetic backbones which resemble their active conformations  and offer therapeutic advantages, such as increased specificity and improved bioavailability .
Synthetic modifications to the natural α-peptide architecture can involve alteration of the backbone itself and/or the proteinogenic side chains (Fig.1). Peptoids represent synthetic polyamides which are structurally related to α-peptides i.e. the side chain is “moved” from the α-C to the amide nitrogen. As a result, the peptoid backbone is inherently achiral and deprived of amide protons and typically has reduced hydrogen bonding potential. Non-hydrogen bonded and extended conformations are notoriously difficult to characterise and circular dichroism (CD) is an extremely sensitive tool to assess the conformational preference as it can easily distinguish between ordered and disordered conformations. It is the distinction between an ordered (fixed) or disordered (dynamic) conformation that is commonly misinterpreted when the conformation adopted is not compact and/or hydrogen bonded. Despite this, hydrogen bonded conformations have been reported in literature for α-peptoids .
The peptidomimetic backbone studied is a hybrid of α- and β-peptoids (where the latter is a CH2 homologue of α-peptoids). The α- and β-peptoid residues alternate in the backbone and are thus known as α,β-peptoids. The presence of a tertiary amide decreases the energy for cis/trans isomerisation around the amide backbone  which complicates NMR spectra, due to the simultaneous presence of all conformational isomers . This means that circular dichroism (CD) becomes an invaluable tool for the study of peptoid conformation when chirality is introduced into the side chain(s). A shift of the CD bands is observed in a tertiary amide (peptoid) in comparison to the corresponding transitions in an α-peptide. Therefore, it is essential to examine the extended far UV region which can be achieved by CD at a synchrotron light source. This novel backbone will be used as a peptidomimetic platform by a collaborator for the presentation of bioactive carbohydrate ligands which will be tested for anticancer immunotherapy applications.
|Figure 2: Variable temperature study of a cyclic α,β-peptoid.|
CD analysis, using a commercial instrument, of the first family of α,β-peptoids enabled study in the 190-260 nm wavelength region . This demonstrated that the novel α, β-peptoid backbone can adopt ordered conformation(s), which can be perturbed by different solvent environments and/or cyclization. The linear peptoids were found to have the same ordered conformational preference regardless of the chain length. By contrast, their cyclic counterparts were found to adopt more than one ordered conformation and a chain length (ring size) dependence was observed. Closely related linear β-peptoids had similar findings . Further interpretation of conformation is restricted by the limited reference CD spectra and complexity of the NMR spectra.
Experimental conditions at the circular dichroism beamline (B23) were optimised for the Module B instrument to enable peptoid spectra to be obtained in organic solvents to ~170 nm; trifluoroethanol, acetonitrile and hexafluoroisopropanol. SRCD data was obtained for a family of linear octameric peptoids with differing side chain combinations (sequence patterns) and also the corresponding cyclic derivatives. This enabled the contribution from the side chains to the peptoid backbone conformation to be assessed. This was further probed by manipulation of the peptoid environment by using variable temperature (Fig.2) and also a range of organic solvents.
 D.J. Hill, M.J. Mio, R.B. Prince, et al., J. S. Chemical Reviews, 101, 3893-4011 (2001).
 S.M. Miller, R.J. Simon, S. Ng, et al., Drug Development Research, 35, 20-32 (1995).
 T.C. Tan, P. Yu, Y.U. Kwon, and T. Kodadek, Bioorganic & Medicinal Chemistry, 16, 5853-5861 (2008).
 K. Huang, et al. Journal of the American Chemical Society, 128, 1733-1738 (2006).
 Q. Sui, D. Borchardt, D.L. Rabenstein, Journal of the American Chemical Society, 129, 12042-12048 (2007).
 P. Armand, et al. , Proceedings of the National Academy of Sciences of the United States of America 95, 4309-4314 (1998).
 T. Hjelmgaard, et al., Organic Letters, 11,4100-4103 (2009).
 A.S. Norgren, S.D. Zhang, P.I. Arvidsson, Organic Letters, 8, 4533-4536 (2006).
Principal Publications and Authors:
T. Hjelmgaard, S. Faure; C. Caumes, E. De Santis, A.A. Edwards, and C. Taillefumier, Organic Letters, 11,4100-4103, (2009).
The Medway School of Pharmacy, Universities of Kent and Greenwich at Medway.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2018 Diamond Light Source
Diamond Light Source Ltd
Harwell Science & Innovation Campus