Housaindokht M R, Chamani J, Moosavi-Movahedi A A
Department of Chemistry, Faculty of Science, Islamic Azad University of Mashhad, Mashhad, Iran.
Int J Biol Macromol. 2005 Aug;36(3):169-75. doi: 10.1016/j.ijbiomac.2005.05.005.
Bovine alpha-lactalbumin (alpha-LA) has been studied by differential scanning calorimetry (DSC), fluorescence spectroscopy and viscometry with various concentrations of Cu2+ and DTAB to elucidate the effect of these ligands on its thermal properties. The DSC profile of dialyzed form of alpha-lactalbumin (m-alpha-LA) contrary to the undialyzed form (holo-form, h-alpha-LA) shows two temperature induced heat absorption peaks. The m-alpha-LA is not a new form of alpha-LA. It contains mixture of the apo (a-alpha-LA) and holo (h-alpha-LA) forms of alpha-LA at low and high temperatures, respectively. Therefore, these two states of alpha-LA (apo and holo) are equilibrating with together after dialyze experiment. The Cu2+ as a metal ion and DTAB as a non metal ion alter the two heat-absorption peaks, in such a manner that, the addition of Cu2+ to the m-alpha-LA increases partial molar heat capacity and enthalpy change values of the h-alpha-LA form at high temperature because the molecular population of the a-alpha-LA form changes into the h-like-alpha-LA. On the contrary, the interaction between the DTAB and the m-alpha-LA increases these thermodynamic values for the a-alpha-LA at low temperature. However, DTAB bound to m-alpha-LA prevents from Ca2+ binding to protein, because there are positive charges repulsion between them. The high temperature peak occurs at the same temperature as the unfolding of the h-alpha-LA, while the low temperature peak lies within the temperature range associated with the unfolding of the a-alpha-LA. The R(s) values of m-alpha-LA, h-alpha-LA and a-alpha-LA forms confirmed the folding and unfolding of the m-alpha-LA during the addition of Cu2+ and DTAB at different concentration, respectively.
通过差示扫描量热法（DSC）、荧光光谱法和粘度测定法，在不同浓度的Cu2+和十二烷基三甲基溴化铵（DTAB）存在的情况下，对牛α-乳白蛋白（α-LA）进行了研究，以阐明这些配体对其热性质的影响。与未透析形式（全蛋白形式，h-α-LA）相反，透析形式的α-乳白蛋白（m-α-LA）的DSC曲线显示出两个温度诱导的吸热峰。m-α-LA不是α-LA的新形式。它分别在低温和高温下包含脱辅基（a-α-LA）和全蛋白（h-α-LA）形式的α-LA混合物。因此，在透析实验后，α-LA的这两种状态（脱辅基和全蛋白）相互平衡。作为金属离子的Cu2+和作为非金属离子的DTAB改变了这两个吸热峰，具体方式为，向m-α-LA中添加Cu2+会增加高温下h-α-LA形式的偏摩尔热容和焓变值，因为a-α-LA形式的分子群体转变为h-类-α-LA。相反，DTAB与m-α-LA之间的相互作用增加了低温下a-α-LA的这些热力学值。然而，与m-α-LA结合的DTAB会阻止Ca2+与蛋白质结合，因为它们之间存在正电荷排斥。高温峰出现在与h-α-LA展开相同的温度，而低温峰位于与a-α-LA展开相关的温度范围内。m-α-LA、h-α-LA和a-α-LA形式的R(s)值分别证实了在添加不同浓度的Cu2+和DTAB期间m-α-LA的折叠和展开情况。
