实验技术,比如低温电子显微镜,需要在低温温度(T≈100K)下回收生物样品,同时水处于无定形冰态。然而,(大量)水在P<1GPa时可以存在于两个无定形冰中,低压下的低密度无定形(LDA)冰和高压下的高密度无定形冰(HDA);HDA比LDA密度约20-25%。当在1bar下快速/骤降冷却使样品进入LDA时,高压冷却(HPC),在足够高的压力下,产生HDA。HDA也可以通过在低温下等温压缩LDA来生产。这里,我们进行经典的分子动力学模拟来研究LDA的影响,HDA,和LDA-HDA转化对小肽的结构和水合作用,聚丙氨酸。我们遵循对应于(i)快速/骤降冷却1bar的热力学路径,(ii)P=400MPa时的HPC,和(iii)T=80K的压缩/减压循环。当过程(i)在系统中产生LDA时,路径(iii)产生HDA。有趣的是,在方法(ii)中产生的无定形冰是具有介于LDA和HDA之间的性质的中间无定形冰(IA)。值得注意的是,在所有研究条件下(0-2000MPa,80-300K)即使当水在低密度和高密度液态以及无定形固体LDA之间变化时,IA,和HDA。LDA玻璃化的聚丙氨酸水化的异同,IA,和HDA被描述。由于所研究的热力学路径适用于生物分子的低温保存,我们还研究了聚丙氨酸沿等压和等容加热路径的结构和水合作用,可以通过实验来回收冷冻保存的样品。加热时,聚丙氨酸的结构几乎保持不变。最后,我们简要讨论了(a)使用HDA和IA作为冷冻保护剂环境(相对于LDA)的实际优势,和(b)使用等压加热作为回收过程(与等压加热相反)。
Experimental techniques, such as cryo-electron microscopy, require biological samples to be recovered at cryogenic temperatures (T ≈ 100 K) with water being in an amorphous ice state. However, (bulk) water can exist in two amorphous ices at P < 1 GPa, low-density amorphous (LDA) ice at low pressures and high-density amorphous ice (HDA) at high pressures; HDA is ≈20-25% denser than LDA. While fast/plunge cooling at 1 bar brings the sample into LDA, high-pressure cooling (HPC), at sufficiently high pressure, produces HDA. HDA can also be produced by isothermal compression of LDA at cryogenic temperatures. Here, we perform classical molecular dynamics simulations to study the effects of LDA, HDA, and the LDA-HDA transformation on the structure and hydration of a small peptide, polyalanine. We follow thermodynamic paths corresponding to (i) fast/plunge cooling at 1 bar, (ii) HPC at P = 400 MPa, and (iii) compression/decompression cycles at T = 80 K. While process (i) produced LDA in the system, path (iii) produces HDA. Interestingly, the amorphous ice produced in process (ii) is an intermediate amorphous ice (IA) with properties that fall in-between those of LDA and HDA. Remarkably, the structural changes in polyalanine are negligible at all conditions studied (0-2000 MPa, 80-300 K) even when water changes among the low and high-density liquid states as well as the amorphous solids LDA, IA, and HDA. The similarities and differences in the hydration of polyalanine vitrified in LDA, IA, and HDA are described. Since the studied thermodynamic paths are suitable for the cryopreservation of biomolecules, we also study the structure and hydration of polyalanine along isobaric and isochoric heating paths, which can be followed experimentally for the recovery of cryopreserved samples. Upon heating, the structure of polyalanine remains practically unchanged. We conclude with a brief discussion of the practical advantages of (a) using HDA and IA as a cryoprotectant environment (as opposed to LDA), and (b) the use of isochoric heating as a recovery process (as opposed to isobaric heating).