Monosubstituted γ(4)-residues (γ(4)Leu, γ(4)Ile, and γ(4)Val) form helices even in short homooligomeric sequences. C14 helix formation is established by X-ray diffraction in homooligomeric (γ)n tetra-, hexa- and decapeptide sequences demonstrating the high propensity of γ residues, with proteinogenic side chains, to adopt locally folded conformations.

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Crystals of Boc-γ(4)(R)Val-Val-OH undergo a reversible first-order single crystal to single crystal phase transition at Tc ≈ 205 K from the orthorhombic space group P22121 (Z' = 1) to the monoclinic space group P21 (Z' = 2) with a hysteresis of ∼2.1 K. The low-temperature monoclinic form is best described as a nonmerohedral twin with ∼50% contributions from its two components. The thermal behavior of the dipeptide crystals was characterized by differential scanning calorimetry experiments. Visual changes in birefringence of the sample during heating and cooling cycles on a hot-stage microscope with polarized light supported the phase transition. Variable-temperature unit cell check measurements from 300 to 100 K showed discontinuity in the volume and cell parameters near the transition temperature, supporting the first-order behavior. A detailed comparison of the room-temperature orthorhombic form with the low-temperature (100 K) monoclinic form revealed that the strong hydrogen-bonding motif is retained in both crystal systems, whereas the non-covalent interactions involving side chains of the dipeptide differ significantly, leading to a small change in molecular conformation in the monoclinic form as well as a small reorientation of the molecules along the ac plane. A rigid-body thermal motion analysis (translation, libration, screw; correlation of translation and libration) was performed to study the crystal entropy. The reversible nature of the phase transition is probably the result of an interplay between enthalpy and entropy: the low-temperature monoclinic form is enthalpically favored, whereas the room-temperature orthorhombic form is entropically favored.

The drug-DNA interaction studies provide vital insights for designing and developing effective chemotherapeutic agents used in treatment of different cancers. Thus, the interaction of potential anticancer agent pentoxifylline and calf thymus DNA was analysed by various spectroscopic and electrochemical methods under physiological condition. The absorption spectra of pentoxifylline with calf thymus DNA revealed the formation of complex by hyperchromic effect with a binding constant of 1.461 × 10 4 M À 1 . Fluorescence studies, further showed that pentoxifylline binds to calf thymus DNA and form a stable complex with a static quenching mechanism. Thermodynamic analysis further confirms that the interaction process is spontaneous with Vander Waals force or hydrogen bonding playing a major role. The electrochemical studies and circular dichroism spectra signify that pentoxifylline interacts by groove binding mode. Based on electrochemical and spectroscopic results, the mode of pentoxifylline binding to calf thymus DNA is mainly established through groove binding.