Z-L-phenylalanine methyl ester

The structures of two pseudopolymorphs of N-acetyl-L-phenylalanine methyl ester, L-AcFOMe, were determined at both 293 (2) and 150 (2) K. At room temperature, the orthorhombic phase C(12)H(15)NO(3) (I), with space group P2(1)2(1)2(1), converts into the tetragonal phase C(12)H(15)NO(3).0.5H(2)O (II), with space group P4(1)2(1)2, in the presence of water. In the structures of both pseudopolymorphs, alternating layers of hydrophilic and hydrophobic intermolecular interaction can be distinguished. In the hydrophilic layers the structures are stabilized by moderate hydrogen bonds of the type N-H...O for the anhydrous L-AcFOMe and of types N-H...O and O-H...O for the hemihydrate. Weak C-H...pi interactions are observed within the hydrophobic layers: for (I) they are of type III [Malone et al. (1997). J. Chem. Soc. Faraday Trans. 93, 3429-3436], whereas typical type I edge-to-face interactions are present for (II). The differences between the hydrogen-bonding networks of (I) and (II) are discussed in terms of graph-set analysis.

The heat capacities of tripeptides N-formyl-l-methionyl-l-leucyl-l-phenylalaninol (N-f-MLF-OH) and N-formyl-l-methionyl-l-leucyl-l-phenylalanine methyl ester (N-f-MLF-OMe) were measured by precision adiabatic vacuum calorimetry over the temperature range from T = (6 to 350) K. The tripeptides were stable over this temperature range, and no phase change, transformation, association, or thermal decomposition was observed. The standard thermodynamic functions: molar heat capacity Cp,m, enthalpy H(T) - H(0), entropy S(T), and Gibbs energy G(T) - H(0) of peptides were calculated over the range from T = (0 to 350) K. The low-temperature (T ≤ 50 K) heat capacities dependencies were analyzed using the Debye's and the multifractal theories. The standard entropies of formation of peptides at T = 298.15 K were calculated.