Bende Attila, Bogár Ferenc, Beleznay Ferenc, Ladik János
Department of Molecular and Biomolecular Physics, National Institute for R and D of Isotopic and Molecular Technologies, Str. Donath 65-103, C.P. 700, Cluj Napoca RO-400293, Romania.
Phys Rev E Stat Nonlin Soft Matter Phys. 2008 Dec;78(6 Pt 1):061923. doi: 10.1103/PhysRevE.78.061923. Epub 2008 Dec 29.
Recent high resolution x-ray diffraction experiments have determined the structure of nucleosomes. In it 147 base pair long DNA B superhelix is wrapped around the eight nucleohistone proteins. They have found that there are many hydrogen-bonds (H-bonds) between the negative sites phosphate (PO4-) groups DNA, and first of all there is the positively charged lysine and arginine side chains of the histones. This means that there is a non-negligible charge transfer from DNA to the proteins causing a hole current in DNA and an electronic one in the proteins. If the relative positions of the two macromolecules change due to some external disturbances, the DNA moves away from the protein and can be read. If this happens simultaneously at several nucleosomes and at many places in chromatin (built up from the nucleosomes), undesired genetic information becomes readable. This final end can cause the occurrence of oncoproteins at an undesired time point which most probably disturbs the self-regulation of a differentiated cell. The connection of these chain of events with the initiation of cancer is obvious. To look into the details of these events we have used the detailed band structures of the four homopolynucleotides in the presence of water and natrium (Na+) ions calculated previously with the help of the ab initio Hartree-Fock crystal orbital method. We have found that in the case of three homopolynucleotides the width of their valence band is broad enough (approximately 10 times broader than the thermal energy at 300K) for the application of the simple deformation potential approximation for transport calculations. With the help of this we have determined the hole mobilities at 300K and 180K of poly(guanilic acid), poly(adenilic acid), and polythimidine (polycytidine has a too narrow valence band for the application of the deformation potential method). The obtained mobilities are large enough to allow Bloch-type conduction in these systems. At the end of the paper we discuss briefly the possible mechanism of charge transport in aperiodic DNA as a combination of Bloch-type conduction, hopping, and tunneling.
最近的高分辨率X射线衍射实验确定了核小体的结构。在该结构中，147个碱基对长的DNA B型超螺旋缠绕在八个核组蛋白上。他们发现，DNA的磷酸根（PO4-）基团的负性位点与组蛋白带正电荷的赖氨酸和精氨酸侧链之间存在许多氢键（H键）。这意味着存在从DNA到蛋白质的不可忽略的电荷转移，从而在DNA中产生空穴电流，在蛋白质中产生电子电流。如果这两种大分子的相对位置由于某些外部干扰而发生变化，DNA就会从蛋白质上移开并可以被读取。如果这种情况同时在几个核小体以及染色质（由核小体组成）的许多位置发生，不需要的遗传信息就会变得可读。这最终可能导致在不期望的时间点出现癌蛋白，这很可能会干扰分化细胞的自我调节。这些事件链与癌症发生之间的联系是显而易见的。为了深入研究这些事件的细节，我们利用了先前借助从头算Hartree-Fock晶体轨道方法计算得到的四种同聚核苷酸在水和钠离子（Na+）存在下的详细能带结构。我们发现，对于三种同聚核苷酸，其价带宽度足够宽（大约比300K时的热能宽10倍），可以应用简单的形变势近似进行输运计算。借助于此，我们确定了聚（鸟苷酸）、聚（腺苷酸）和聚胸腺嘧啶（聚胞苷酸的价带太窄，无法应用形变势方法）在300K和180K时的空穴迁移率。所获得的迁移率足够大，足以在这些系统中实现布洛赫型传导。在论文结尾，我们简要讨论了非周期性DNA中电荷输运的可能机制，它是布洛赫型传导、跳跃和隧穿的组合。
