Foldamers of β-peptides : conformational preference of peptides formed by rigid building blocks : The first MI-IR spectra of a triamide nanosystem

To determine local chirality driven conformational preferences of small aminocyclobutane-1-carboxylic acid derivatives, X-(ACBA) n -Y, their matrix-isolation IR spectra were recorded and analyzed. For the very first time model systems of this kind were deposited in a frozen (~10 K) noble gas matrix to reduce line width and thus, the recorded sharp vibrational lines were analyzed in details. For cis-(S,R)-1 monomer two “zigzag” conformers composed of either a six or an eight-membered H-bonded pseudo ring was identified. For trans-(S,S)-2 stereoisomer a zigzag of an eight-membered pseudo ring and a helical building unit were determined. Both findings are fully consistent with our computational results, even though the relative conformational ratios were found to vary with respect to measurements. For the dimers (S,R,S,S)-3 and (S,S,S,R)-4 as many as four different cis,trans and three different trans,cis conformers were localized in their matrix-isolation IR (MI-IR) spectra. These foldamers not only agree with the previous computational and NMR results, but also unambiguously show for the first time the presence of a structure made of a cis,trans conformer which links a “zigzag” and a helical foldamer via a bifurcated H-bond. The present work underlines the importance of MI-IR spectroscopy, applied for the first time for triamides to analyze the conformational pool of small biomolecules. We have shown that the local chirality of a β-amino acid can fully control its backbone folding preferences. Unlike proteogenic α-peptides, β- and especially (ACBA) n type oligopeptides could thus be used to rationally design and influence foldamer’s structural preferences

Carboxylic Group and Its Tetrazolyl Isostere in One Molecule. Matrix Isolation FTIR and DFT Studies on Thermal Decomposition and Photochemistry of (Tetrazol-5-yl)acetic Acid

The title compound (tetrazol-5-yl)­acetic acid (TA) is an interesting molecule that contains both a carboxylic group and its isostere tetrazolyl group. Out of nine theoretically predicted stable structures of TA, three appeared to be present in solid argon. Thermal decomposition of the species aided by water molecules was studied in detail both experimentally using FTIR matrix isolation technique and theoretically at the B3LYP/6-311++G­(2d,2p) level. Experimentally, it was found that the decarboxylation process appeared at the presence of water traces in the system. Theoretically, it was shown that the energy barrier of the water assisted process was lower by ca. 30 kJ mol^–1 comparing to the process without water participation. The UV photolysis of the TA/Ar system was studied using both broad-band and narrow-band sources. The main photoproducts appeared to be carbodiimidylacetic acid and (1<i>H</i>-diaziren-3-yl)­acetic acid. The progress of the reactions induced was followed by FTIR spectroscopy, whereas interpretation of the results was supported by quantum chemical calculations (DFT, TD-DFT)

Conformational Behavior and Tautomer Selective Photochemistry in Low Temperature Matrices: The Case of 5-(1H-Tetrazol-1-yl)-1,2,4-triazole

The conformational properties and the photolysis behavior of one of the simplest N-C bonded bicyclic azoles, 5-(1H-tetrazol-1-yl)-1,2,4-triazole (T), were studied in argon and xenon matrices by infrared spectroscopy. Analysis of the experimental results was supported by extensive theoretical calculations carried out at the B3LYP/6-311++G(2d,2p) level of approximation. Out of the eight T minima located on the potential energy surface, the three most stable species were detected in low temperature matrices, namely, 5-(1H-tetrazol-1-yl)-1H-1,2,4-triazole (T1) and two conformers of 5-(1H-tetrazol-1-yl)-2H-1,2,4-triazole (T2a and T2b). With increase of the substrate temperature either during deposition of the matrices or during annealing the T2b -> T2a conversion took place, in agreement with the predicted low energy barrier for this transformation (5.38 kJ mol(-1)). Both broad band and narrow band laser UV irradiations of T isolated in Xe and Ar matrices induce unimolecular decomposition involving cleavage of the tetrazole ring of T1 and T2a (T2b) that leads to the production of 1H-1,2,4-triazol-5-yl carbodiimide (P1) and 1H-1,2,4-triazol-3-yl carbodiimide (P2), respectively. When the laser is used, in addition to the main P1 and P2 photoproducts, several minor products could be successfully identified in the matrices: N-cyanocarbodiimide HNCNCN (detected for the first time) associated with nitrilimine HNNCH and HCN. An interesting phenomenon of tautomer-selective photochemistry was observed for the matrix-isolated compound. It could be explained by the different LUMO-HOMO energy gaps estimated for T1, T2a, and T2b, connected with different threshold energies necessary to start the photolysis of T1 and T2a (T2b)

Ultraviolet-Tunable Laser Induced Phototransformations of Matrix Isolated Isoeugenol and Eugenol

In situ photochemical transformations of monomers of 2-methoxy-4-(prop-1-enyl)­phenol (isoeugenol) and 2-methoxy-4-(prop-2-enyl)­phenol (eugenol) isolated in low temperature matrices were induced by tunable UV laser light, and the progress of the reactions was followed by FTIR spectroscopy. Conformer-selective <i>E</i> ↔ <i>Z</i> geometrical isomerizations could be successfully induced by irradiation at different wavelengths from the 310–298 nm range in the isoeugenol molecule, contains an asymmetrically substituted exocyclic CC bond. Photolysis of both studied compounds was also observed, with H-atom shift from the OH group and formation of two types of long-chain conjugated ketenes. The photoproduced ketenes were found to undergo subsequent photodecarbonylation. Interpretation of the observed photoprocesses was supported by quantum chemical calculations undertaken at different levels of theory (DFT, MP2, QCISD)

Polymorphism and conformational equilibrium of nitro-acetophenone in solid state and under matrix conditions

Conformational and polymorphic states in the nitro-derivative of o-hydroxy acetophenone have been studied by experimental and theoretical methods. The potential energy curves for the rotation of the nitro group and isomerization of the hydroxyl group have been calculated by density functional theory (DFT) to estimate the barriers of the conformational changes. Two polymorphic forms of the studied compound were obtained by the slow and fast evaporation of polar and non-polar solutions, respectively. Both of the polymorphs were investigated by Infrared-Red (IR) and Raman spectroscopy, Incoherent Inelastic Neutron Scattering (IINS), X-ray diffraction, nuclear quadrupole resonance spectroscopy (NQR), differential scanning calorimetry (DSC) and density functional theory (DFT) methods. In one of the polymorphs, the existence of a phase transition was shown. The position of the nitro group and its impact on the crystal cell of the studied compound were analyzed. The conformational equilibrium determined by the reorientation of the hydroxyl group was observed under argon matrix isolation. An analysis of vibrational spectra was achieved for the interpretation of conformational equilibrium. The infrared spectra were measured in a wide temperature range to reveal the spectral bands that were the most sensitive to the phase transition and conformational equilibrium. The results showed the interrelations between intramolecular processes and macroscopic phenomena in the studied compound