To assess our hypothesis, we first needed to identify a d-residue that would be capable of adopting the required conformation to form this predicted 13/11-helix. According to our minimization studies, we felt that cyclic d-residue 1 (Figure ) would satisfy these conditions (See SI). This residue could be accessed from the corresponding g-amino acid previously described by our group, as well as Gellman and others. Owing to the scalability of Gellman's method, we chose this for our studies to generate the d-subunit (R,S,R)-AChPA which we denote as 'X' in the sequences of Figure . Our first efforts involved the incorporation of this into 1:1-heteromeric oligomer sequences using either L-alanine, D-alanine, or Aib as the a-unit leading to sequences 2, 3 and 4 respectively (see SI for details of solution phase syntheses). Of these three sequences, only octamer 2 showed the characteristic downfield shift in multiple NH signals indicative of intramolecular H-bonding (IMHB, see SI), which were also confirmed with temperature coefficients from VT experiments in chloroform. The other sequences failed to show this, demonstrating the importance of the stereocenter in the a-residue for helix formation. To ascertain whether the observed hydrogen bonding of these NH signals resulted in a defined secondary structure, we first successfully obtained a single crystal diffraction of octamer 2 (Figure ). Remarkably this resulted in three interesting observations. First, it made clear why the stereogenicity of the a-amino acid is crucial, as if it is epimeric at that center, it points into the part of the helix where H-bonds occur, thus preventing their formation. Second, that there are two packing conformations within the unit cell. Both molecules show the characteristic hydrogen bonding pattern for a 13/11-helix, but differ in the placement of the terminal d-residue, which adopts either an (a,a) or (e, e) conformation (Figures 3B and C respectively). The underlying reason for this is that the terminal d-residue lacks a hydrogen bonding partner. Its conformational flexibility, therefore, increases and makes the two structural alternatives possible. According to crystallography, the diaxial conformer may additionally be stabilized by an apolar interaction between the ethyl group of the terminal d-residue and the cyclohexyl residue of the nearest d-residue at (i+2). Whilst this interaction is missing at the terminal (e,e)conformer in the crystal, it is prevalent between the other corresponding residues throughout both helices (Figure ) as we discuss in more detail later. The structure also confirms the arrangement of interior lying aresidues and exterior facing d-residues as represented by the wheel diagram (Figure ). Our shorthand depiction of α,δ-sequences used in this article The third point of interest arising from these structures is that the 13/11-helix realized in octamer 2 is not the most stable predicted by Hofmann for the unsubstituted backbone by theory (denoted H13/11 I ), but is actually the second most stable (denoted H13/11 II ) according to DFT calculations at the B3LYP/6-31G* level. We believe the aforementioned apolar interactions between the ethyl and cyclohexyl groups, are partially responsible for this change in helix preference, because although they are possible in both H13/11 I and H13/11 II , the stabilization conferred by it is greater for the latter. This conclusion comes from our investigation into the stability relationships of these helices, where we undertook in silico studies of the octamers of both helix types by the step-wise addition of the substituents of our d-amino acid constituents. Starting with unsubstituted d-amino acid constituents, and L-Ala as the a-residue, H13,11 I remains the preferred helix over H13,11 II by 21.8 kJ/mol as described in Hofmann's report. However, introduction of a b-ethyl substitution in the d-amino acid constituents changes the stability such that H13/11 II is now favored over H13/11 I by 26.4 kJ/mol. Introduction of the cyclohexane substituent without this bethyl group still favors H13/11 II , but only by 5.4 kJ/mol this time. It is only when we introduce both ethyl and cyclohexyl residues that the oligomer shows a huge preference for H13/11 II over H13/11 I by 60.2 kJ/mol indicating the stabilizing importance of these side chain interactions. i.e. if you remove one or other partner from this apolar interaction, the preference for the 13/11(II) helix becomes far less pronounced. Thus the preference of one helix type over the other is essentially a matter of balance between an intrinsic factor of helix stability, which seems to favor helix type-I, and the apolar side chain interactions which change the stability towards helix type-II (see SI). In order to understand the conformation in the solution state, we undertook further extensive modelling and NMR studies. To begin with, the observation of both (a,a) and (e,e)-conformations of the terminal d-residue that appear in the unit cell of the XRD was re-investigated. Unsurprisingly, the (e,e) was observed as the major state of 2 in solution, confirmed by a large 3 JHH-coupling (10.3 Hz) of the vicinal protons on the substituted cyclohexyl ring at the terminus. 20 Then, using unrestrained Monte-Carlo Multiple minimization (MCMM), followed by clustering and abundant DFT energy minimizations (ωB97M-D3BJ/def2-TZVP// ωB97XD/6-31G*) on hundreds of distinct conformations including both helical and non-helical scaffolds, 13 geometries were afforded as low energy conformers of 2 under solution states (15 kJ/mol threshold, Figure and SI). The computational ensembles of these 13 low-energy geometries in solution also suggested the (e,e)-helix are predominant whereas the (a,a)-helix were calculated to be less stable according to DFT energies (> 30 kJ/mol). We then validated the solution ensembles of these 13 lowenergy snapshots from unrestrained MCMM-DFT modelling with 37 H-H ROE restraints collected from ROESY (Rotating frame NOESY) experiment (Figure and SI). Their close alignment suggests that the core helix itself is highly stable as indicated from modelling. A satisfactory consistency was achieved between experimental restraints and DFT-calculated values, where mean absolute error (MAE) for all strong and medium ROEs (< 3.6 Å, Figure ) is below 8%. This includes the fitting of several cross-residue correlations under IMHBs for helicity. Moreover, we also observed two weak NOEs between the ethyl and cyclohexane moieties on different delta residues, attributed to the apolar interactions we described above (see SI). Meanwhile extractable 3 JHH coupling constants along the foldamer backbone were also reproduced using the Gauge-Independent Atomic Orbital (GIAO) method based on DFT minimized ensembles (Figure and SI), again supporting the distributions of the corresponding dihedral angles. No obvious violation of torsional regularity was observed for the core helix. Although the calculated 3 JHH for the vicinal protons on δ4 cyclohexane (11.0 Hz) approach the value observed from experiment (10.3 Hz), larger errors were identified for other 3 JHH on the terminal δ4 residue. We hypothesized a high flexibility of such region in solution, of which detailed dynamics can be hardly covered by modelling and NMR restraints.