Job/Unit: I42837 /KAP1 Date: 26-11-14 13:41:52 Pages: 13 FULL PAPER DOI:10.1002/ejic.201402837 Synthesis and Conformational Study of Bioconjugates 1 Derived from 1-Acetyl-1'-aminoferrocene and α-Amino Acids Mojca C ˇ akic ´ Semenc ˇic ´,* [a] Veronika Kovac ˇ, [a] Ivan Kodrin,* [b] Lidija Baris ˇic ´, [a] and Vladimir Rapic ´ [a] Keywords: Conformation analysis / Density functional calculations / Bioinorganic chemistry / Peptidomimetics / Hydrogen 6 bonds / Circular dichroism 1,1'-Disubstituted ferrocene conjugates present useful and efficient bioorganometallic constraint design to reduce the conformational flexibility of small peptides. In this study we present the first systematic conformational analysis of non- 11 symmetric ferrocene peptidomimetics (Boc-AA-NH-Fn- COMe; Boc = tert-butoxycarbonyl; AA = Gly, L-Ala, L-Val; Fn = 1,1'-ferrocenylene) and their monosubstituted analogues (Boc-AA-NH-Fc; Fc = ferrocenyl; AA = Gly, L-Ala, L-Val). The spectroscopic data (IR, NMR and CD) were corroborated by 16 Introduction The design of secondary structure mimetics of short 26 peptides is of vital importance not only for understanding the fundamentals of protein folding, but also for the devel- opment of functional peptidic compounds. 1,1'-Disubsti- tuted ferrocenes are recognized as molecular scaffolds for peptide mimetic models because the almost free-rotating 31 cyclopentadiene (Cp) rings are separated by about 3.3 Å, which is ideal for interstrand hydrogen-bonding interaction in the conjugates with natural amino acids or peptides. [1,2] If the peptide groups directly attached to ferrocene are taken into account, then three major classes of bioconju- 36 gates need to be considered: peptidomimetics derived from ferrocene-1,1'-dicarboxylic acid (Fcd, I), [3–6] 1'-aminoferro- cene-1-carboxylic acid (Fca, II), [7–11] and 1,1'-diaminofer- rocene (Fcda, III). [12] Extensive spectroscopic studies of conjugates I–III revealed the presence of two strong intra- 41 molecular hydrogen bonds supporting the helically chiral arrangement of the ferrocene subunit in symmetrically sub- stituted derivatives (Scheme 1, n = m = 1) both in solution [a] Department of Organic Chemistry, University of Zagreb, Faculty of Food Technology and Biotechnology, Pierottijeva 6, 10000 Zagreb, Croatia E-mail: mcakic@pbf.hr http://www.pbf.unizg.hr [b] Department of Chemistry, University of Zagreb, Faculty of Science, Horvatovac 102a, 10000 Zagreb, Croatia Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402837. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 DFT calculations and indicated the engagement of the NH group closest to the ferrocene unit in intrachain hydrogen bonds. This medium-strength bond is not disrupted by the introduction of a hydrogen-bonding acceptor on the other ferrocene ring, but rather is accompanied by an additional 21 interchain hydrogen bond, which causes the restricted rota- tion of ferrocene rings and gives rise to a chiral arrangement of the ferrocene core in a P helical manner. and in the solid state. We focused our research on type II derivatives, because in comparison to types I and III, only 46 conjugates derived from Fca maintain an antiparallel orien- tation of peptide chains and thus can be truly considered as peptide turn mimetics. In contrast to conjugates bearing the identical podand peptide chains (n = m = 1, 2, ...), nonsymmetric bioorgano- 51 metallics II (n ≠ m) do not show a tendency to adopt a single conformer in solution. [13–16] Detailed structural analysis of peptides derived by C-terminal elongation of Fca [II, n = 0, m = 1, R = H, CH 3 , CH(CH 3 ) 2 ] [8,10] revealed the presence of multiple conformational isomers, stabilized 56 by single intramolecular hydrogen bond, in the solution. Some conformers are destabilized when the steric demand of the amino acid side chain increases, and even changing the solvent can promote some conformers over others. In the solid state, dipeptide Boc-Fca--Ala-OMe is stabilized 61 by an eight-membered interchain NHAla···OCBoc hydrogen-bonded ring (numeration of rings is shown in Fig- ure 12), whereas achiral Ac-Fca-Gly-OMe (Ac = acetyl) features a nine-membered hydrogen-bonded ring spanning NH Fc and the carbonyl group of glycine. N-Terminal cou- 66 pling of Fca was used to obtain the second class of type II analogues (Y-AA-Fca-OMe; Y = Boc, Ac; AA = -Ala, - Ala), which are formally constitutional isomers of peptides derived by C-elongation of Fca. Combined spectroscopic and theoretical approach indicated the presence of several 71 conformations in solution stabilized by intramolecular hydrogen bonds. As a consequence of imposed interchain