Low-temperature polymorphic phase transition in a crystalline tripeptide L-Ala-L-Pro-Gly·H2O revealed by adiabatic calorimetry

Betulin-3,28-diphosphate. Physico-Chemical Properties and In Vitro Biological Activity Experiments

Betulin-3,28-diphosphate (BDP) obtained by phosphorylation of betulin using POCl₃ has two main structural forms-BDP-1 and BDP-2-which differ in ethanol solubility, melting point, FTIR spectra, thermoanalytical characteristics and biological activity. Betulin-3,28-diphosphate and its sodium salt (Na-BDP) were characterized using 13C and 31P-NMR spectra, powder XRD experiments, as well as differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) methods. The exo-effects at 193 ± 8 °C for ethanol soluble BDP-1 samples (-19.7 ± 0.2 kJ∙mol-1) were about three times less than for ethanol insoluble BDP-2 samples f (-70.5 ± 0.7 kJ∙mol-1). The DSC curves of Na-BDP-1 and Na-BDP-2 characterized the endo-effects having a maximum at 95⁻112 °C. Water-soluble Na-BDP-1 was obtained as needle-like crystals, unlike poorly crystalline Na-BDP-2, whereas BDP-1 and BDP-2 aged with time and were isolated as amorphous substances. In vitro experiments on rats showed that compared to the control, Na-BDP-1 increased catalase and SOD activity and improved energy metabolism more effectively than Na-BDP-2.

Peptide and Protein Dynamics and Low-Temperature/DNP Magic Angle Spinning NMR

In DNP MAS NMR experiments at ∼80-110 K, the structurally important -¹³CH₃ and -¹⁵NH₃⁺ signals in MAS spectra of biological samples disappear due to the interference of the molecular motions with the ¹H decoupling. Here we investigate the effect of these dynamic processes on the NMR line shapes and signal intensities in several typical systems: (1) microcrystalline APG, (2) membrane protein bR, (3) amyloid fibrils PI3-SH3, (4) monomeric alanine-CD₃, and (5) the protonated and deuterated dipeptide N-Ac-VL over 78-300 K. In APG, the three-site hopping of the Ala-C_β peak disappears completely at 112 K, concomitant with the attenuation of CP signals from other ¹³C's and ¹⁵N's. Similarly, the ¹⁵N signal from Ala-NH₃⁺ disappears at ∼173 K, concurrent with the attenuation in CP experiments of other ¹⁵N's as well as ¹³C's. In bR and PI3-SH3, the methyl groups are attenuated at ∼95 K, while all other ¹³C's remain unaffected. However, both systems exhibit substantial losses of intensity at ∼243 K. Finally, with spectra of Ala and N-Ac-VL, we show that it is possible to extract site specific dynamic data from the temperature dependence of the intensity losses. Furthermore, ²H labeling can assist with recovering the spectral intensity. Thus, our study provides insight into the dynamic behavior of biological systems over a wide range of temperatures, and serves as a guide to optimizing the sensitivity and resolution of structural data in low temperature DNP MAS NMR spectra.