Taylor J W, Greenfield N J, Wu B, Privalov P L
Department of Chemistry, Rutgers University, Piscataway, NJ, 08855, USA.
J Mol Biol. 1999 Aug 27;291(4):965-76. doi: 10.1006/jmbi.1999.3025.
The thermal melting of a dicyclic 29-residue peptide, having helix-stabilizing side-chain to side-chain covalent links at each terminal, has been studied by circular dichroism spectropolarimetry (CD) and differential scanning calorimetry (DSC). The CD spectra for this dicyclic peptide indicate that it is monomeric, almost fully alpha-helical at -10 degrees C, and undergoes a reversible transition from the folded to the disordered state with increasing temperature. The temperature dependencies of the ellipticity at 222 nm and the excess heat capacity measured calorimetrically are well fit by a two-state model, which indicates a cooperative melting transition that is complete within the temperature ranges of these experiments (from -10 degrees C to 100 degrees C). This allows a complete analysis of the thermodynamics of helix formation. The helix unfolding is found to proceed with a small positive heat-capacity increment, consistent with the solvation of some non-polar groups upon helix unfolding. It follows that the hydrogen bonds are not the only factors responsible for the formation of the alpha-helix, and that hydrophobic interactions are also playing a role in its stabilization. At 30 degrees C, the calorimetric enthalpy and entropy values are estimated to be 650(+/-50) cal mol(-1)and 2.0(+/-0.2) cal K(-1)mole(-1), respectively, per residue of this peptide. Comparison with the thermodynamic characteristics obtained for the unfolding of double-stranded alpha-helical coiled-coils shows that at that temperature the enthalpic contribution of non-polar groups to the stabilization of the alpha-helix is insignificant and the estimated transition enthalpy can be assigned to the hydrogen bonds. With increasing temperature, the increasing magnitude of the negative enthalpy of hydration of the exposed polar groups should decrease the helix-stabilizing enthalpy of the backbone hydrogen bonds. However, the helix-stabilizing negative entropy of hydration of these groups should also increase in magnitude with increasing temperature, offsetting this effect.
通过圆二色光谱偏振法（CD）和差示扫描量热法（DSC）研究了一种双环29残基肽的热熔解情况，该肽在每个末端都具有稳定螺旋的侧链间共价连接。这种双环肽的CD光谱表明它是单体形式，在-10℃时几乎完全呈α螺旋结构，并且随着温度升高会经历从折叠态到无序态的可逆转变。在222nm处椭圆率的温度依赖性以及量热法测得的过量热容量，都能很好地用两态模型拟合，这表明在这些实验的温度范围内（从-10℃到100℃）发生了协同熔解转变。这使得能够对螺旋形成的热力学进行完整分析。发现螺旋解折叠过程伴随着较小的正热容量增量，这与螺旋解折叠时一些非极性基团的溶剂化作用一致。由此可见，氢键并非形成α螺旋的唯一因素，疏水相互作用在其稳定过程中也发挥着作用。在30℃时，该肽每个残基的量热焓和熵值估计分别为650（±50）cal mol⁻¹和2.0（±0.2）cal K⁻¹mol⁻¹。与双链α螺旋卷曲螺旋解折叠所获得的热力学特征进行比较表明，在该温度下，非极性基团对α螺旋稳定化的焓贡献微不足道，估计的转变焓可归因于氢键。随着温度升高，暴露的极性基团水化焓的负增量增大，这应会降低主链氢键的螺旋稳定焓。然而，这些基团水化的螺旋稳定负熵也会随着温度升高而增大，从而抵消这种效应。
