RESEARCH ARTICLE Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 7, 2253–2259, 2007 Looking Beyond Carbon Nanotubes: Polypeptide Nanotubes as Alternatives? G. Praveena 1 , P. Kolandaivel 1 ∗ , N. Santhanamoorthi 1 , V. Renugopalakrishnan 2 3 , and Seeram Ramakrishna 3 1 Department of Physics, Bharathiar University, Coimbatore, Tamil Nadu 641046, India 2 Children’s Hospital, Harvard Medical School, Boston, MA 02115, USA 3 Division of Bioengineering and Department of Mechanical Engineering, NUS Nanotechnology Initiative (NUSNNI), National University of Singapore, Singapore-117576 Since the discovery of CNT by Iijima, Nature 354, 56 (1991). CNT’s have surged to the forefront as a versatile nanostructured material in nanoelectronic applications. Polypeptides nanotubular struc- tures with tunable properties offer a challenging alternative to CNT. Earlier experimental studies on L-Alanyl-L-Valine (AV) and L-Valyl-L-Alanine (VA) have demonstrated their potential as novel porous materials, which form channel-like structure (Soldatov et al., Angew. Chem. Int. Ed. 43, 6308 (2004)). In the study reported here, DFT calculations on two closely related cyclic dipeptides cyclo[L- alanyl-L-valine] 3 and cyclo[L-valyl-L-alanine] 3 and on their linear correlates, [L-alanyl-L-valine] 3 and [L-valyl-L-alanine] 3 have been performed. This paper presents the general structural and electronic properties of cyclic and linear correlates of the nanotubular oligomeric dipeptides constructs, AV, and VA. We have compared the energy gaps of these cyclic rings and their linear correlates with that of other nanotubular constructs. The calculated HOMO–LUMO gap of these isolated ring struc- tures is significantly larger than CNT’s. Further research is required to reduce the band gaps to be comparable to CNT’s and other inorganic tubular structures. Polypeptide design promises to be a major tool in engineering desirable band gap for the creation of novel nanostructured polypeptide nanotubes. Keywords: Cyclic Peptides, Nanostructures, Self-Assembly, Quantum Chemical Study, Band Gap, DFT. 1. INTRODUCTION Single wall carbon nanotubes (SWNTs) have unique elec- tronic, mechanical, and structural characteristics. Since its discovery by Iijima 1 promising applications of CNT’s as chemical sensors or nano-scale electronic devices have been exponentially growing. Structurally altered nanotubes with appropriate addends should facilitate utilization by improving solubility, processability, and ease of dispersion, as well as by providing sites for chemical attachment to CNT surfaces (Makala, Ramanath, Renugopalakrishnan et al., from our laboratories). Unfortunately CNT’s are chemically sluggish and are difficult for covalent attach- ment to proteins. Besides CNTs, boron nitride, 2 gallium selenide, 3 silicon, 4 MoS2, 5 boron carbonitride, 6 and tungsten disulfide 7 have also emerged as interesting nano- tubular structures. These quasi one dimensional nano- tubular constructs have opened an exciting field of research ∗ Author to whom correspondence should be addressed. because of their unique properties. The exceptional elec- tronic property, metallic versus semi conducting behaviour of carbon nanotubes depends on the diameter and the chirality i.e., on the way the graphene sheet is rolled, 8 whereas in contrast boron nitride nanotubes display a more uniform behaviour with a wide band gap ∼4 eV almost independent of diameter and chirality. 9 One of the biggest challenges in nanotechnology is the synthesis of pure, monodispersed nanotubes with identical structure and with tunable physical and functional prop- erties. Inorganic nanotubes partially fulfil this goal as it is difficult to synthesize them in a controlled manner to produce identical nanotubes in bulk. Therefore the need to design organic or biological nanotube structures has been raised in the literature. Peptide materials are non toxic and may be used in biological and medical context such as in chiral recognition, preservation, and storage of drugs. Recent studies have focused on polymeric lipid-based tubules, 10 carbohydrate based nanotubes, 11–13 and DNA J. Nanosci. Nanotechnol. 2007, Vol. 7, No. 7 1533-4880/2007/7/2253/007 doi:10.1166/jnn.2007.649 2253