Low-Temperature Polymorphic Phase Transition in a Crystalline Tripeptide L-Ala-L-Pro-Gly·H2O Revealed by Adiabatic Calorimetry

We demonstrate application of precise adiabatic vacuum calorimetry to observation of phase transition in the tripeptide l-alanyl-l-prolyl-glycine monohydrate (APG) from 6 to 320 K and report the standard thermodynamic properties of the tripeptide in the entire range. Thus, the heat capacity of APG was measured by adiabatic vacuum calorimetry in the above temperature range. The tripeptide exhibits a reversible first-order solid-to-solid phase transition characterized by strong thermal hysteresis. We report the standard thermodynamic characteristics of this transition and show that differential scanning calorimetry can reliably characterize the observed phase transition with <5 mg of the sample. Additionally, the standard entropy of formation from the elemental substances and the standard entropy of hypothetical reaction of synthesis from the amino acids at 298.15 K were calculated for the studied tripeptide.National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-003151)National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-001960)National Institute of Biomedical Imaging and Bioengineering (U.S.) (EB-002026

Hydration of ds-DNA and ss-DNA by Neutron Quasielastic Scattering

Quasielastic neutron scattering measurements were performed in hydrated samples of ds-DNA and ss-DNA. The samples were hydrated in a high relative humidity atmosphere, and their final water content was 0.559 g H(2)O/g ds-DNA and 0.434 g H(2)O/g ss-DNA. The measurements were performed at 8 and 5.2 Å for the ds-DNA sample, and at 5.2 Å for the ss-DNA sample. The temperature was in both cases 298 K. Analysis of the obtained data indicates that in the ds-DNA sample we can distinguish two types of protons—those belonging to water molecules strongly attached to the ds-DNA surface and another fraction belonging to water that diffuses isotropically in a sphere of radius 2.8 Å, with a local diffusion coefficient of 2.2 × 10(−5) cm(2) s(−1). For ss-DNA, on the other hand, no indication was found of motionally restricted or confined water. Further, the fraction of protons strongly attached to the ds-DNA surface corresponds to 0.16 g H(2)O/g ds-DNA, which equals the amount of water that is released by ds-DNA upon thermal denaturation, as studied by one of us (G.M.) by differential scanning calorimetry. This value also equals the difference between the critical hydration values of ds-DNA and ss-DNA, also determined by DSC. These results represent, thus, a completely independent measurement of water characteristics and behavior in ds- and ss-DNA at critical hydration values, and therefore substantiate the previous suggestions/conclusions of the results obtained by calorimetry