Understanding correlations between secondary relaxations and thermal behaviour of biologically relevant molecules

• Main Author: Owusu-Ware, Samuel
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2013
• Subjects: Insulin; Investigations; Peptides; Proteins; Spectrum analysis

The research presented in this doctoral thesis examines the application of thermally stimulated depolarisation current (TSDC) spectroscopy as a probe of molecular mobility in the solid (powdered) state. TSDC spectroscopy has proved useful in understanding the molecular mobility of amorphous materials. However, few investigations have been undertaken in order to understand molecular mobility in crystalline and semi-crystalline solid-state (powder) materials and how mobility in such systems can be related to stability. Differential scanning calorimetry (DSC), including - where applicable - temperature modulated differential scanning calorimetry (TMDSC), and thermal gravimetric analyses (TGA) were employed to study the thermal behaviour/stability of the different molecular systems under scrutiny. Experiments on crystalline butyric acids, γ-aminobutyric acid (GABA), DL-α-aminobutyric acid (AABA) and DL-β-aminobutyric acid (BABA), were undertaken in order to examine the influence of positional isomerism on molecular mobility and stability. These molecules differ in the position of the amine group relative to the carboxyl functionality. GABA, DL-BABA and DL-AABA were found to undergo localised non-cooperative rotational motions at 77 ± 2°C and 114 ± 2°C, 104 ± 1°C and 109 ± 1°C, respectively, prior to the higher temperature thermal events detected via DSC and TGA. The carbon chain length between the amino and the carboxyl moiety is found to be a key contributor to solid-state molecular mobility. Molecular relaxation frequency was found to correlate to thermal stability of the materials investigated i.e. the lower the relaxation frequency the greater the thermal stability. The molecular mobility of a homologous series of semi-crystalline phosphatidylcholines: 1,2-dilauryl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with acyl chain lengths of 12, 14, 16 and 18, respectively has been examined at temperatures below the chain melting transition temperature (Tm). The results show that increasing the acyl chain length increases the temperature at which molecular mobility is observed i.e. the temperature maxima (Tmax) of the global relaxation processes occur at 41 ± 2, 47 ± 2, 57 ± 2 and 67± 3°C for DLPC, DMPC, DPPC and DSPC, respectively. The molecular mobilities of the phosphatidylcholines are hypothesized to involve both inter- and intra-molecular co-operation and originate from the segmental motion of the diacyl backbone. The relaxation times (21 ± 2, 19 ± 3, 25 ± 1 and 32 ± 4 s for DLPC, DMPC, DPPC and DSPC, respectively) are shown to correlate with the apparent activation energy (Ea) and Tm values of the phosphatidylcholines. Studies of lyophilized peptide/proteins namely insulin, lysozyme and myoglobin were investigated in the hydrated and dehydrated state in the temperature range -150 to 150°C. This body of work attempts to understand if and how moisture and/or parameters associated with structure influence molecular mobility of lyophilized peptide/proteins. The results show that the presence of low levels of moisture content (<10%) facilitates molecular mobility at sub-zero temperatures; no mobility is observed in the sub-zero temperature regions after dehydration. The temperature locations of the relaxation processes in the proteins containing moisture are -95 ± 3, -102 ± 3 and 111 ± 2°C for insulin, lysozyme and myoglobin, respectively. Whilst the temperature location of the relaxation processes in the dehydrated protein samples are 96 ± 3, 72 ± 4 and 82 ± 3°C for insulin, lysozyme and myoglobin, respectively. It was found that the temperature order in which molecular mobility occurs correlates with the temperature order at which denaturation-decomposition processes occur for the three peptide/protein systems investigated. The work presented herein demonstrates the capability of TSDC spectroscopy to detect and characterise low energy transitions (secondary relaxations) that are inaccessible by conventional DSC methods. In addition, the ability to relate molecular mobility to thermal stability of materials examined has been established.