Edmonds P D
Bioelectromagnetics. 1982;3(1):157-65. doi: 10.1002/bem.2250030119.
This article reviews significant advances in understanding the basis for the magnitude of ultrasonic absorption in proteins and related biological materials. Carstensen and Schwan's accurate and extensive measurements on blood and hemoglobin solutions provided the initial experimental data; these were augmented by data from measurements on aqueous solutions of gelatin, bovine serum albumin, lysozyme, various polypeptides, and amino acids. The initial frequency range of 1-10 MHz was expanded to 0.035-1000 MHz; temperature and pH dependences of absorption were studied. Theoretical approaches included consideration of the relative motion of blood cells in plasma, perturbation of water structure around macromolecules, solvation of charged entities, proton-transfer reactions, and helix-coil transitions. Proton-transfer reactions between amino and carboxylic groups and water proved to be largely responsible for the observed peaks in pH dependence of absorption coefficient; the peaks occurred in the basic and acidic regions corresponding to the pKs for titration of these groups. Such reactions could not account for the magnitude of absorption at physiological pH because only histidine titrated in this range. Extensive analysis, using relaxation theory, and measurements have shown that the proton transfer reaction between the imidazole group of histidine and hydrogen phosphate ion (in solution) provides sufficient volume change for significant ultrasonic absorption at physiological pH. Excellent agreement between theory and experiment was found with the peptide bacitracin in phosphate buffer solutions. By generalizing these results to the case of a protein, Slutsky wt al estimated maximum values of frequency-dependent absorption coefficients for "typical tissue" and found them to be correct to order of magnitude, even exceeding observed values in soft tissues in some instances, instead of being far too small as was always the case in the past. Thus, in principle, adjustment of parameters, such as pK values, could bring theory and experiment into agreement for the first time.
本文综述了在理解蛋白质及相关生物材料中超声吸收量级基础方面取得的重大进展。卡斯滕森和施万对血液和血红蛋白溶液进行了精确且广泛的测量，提供了初始实验数据；对明胶、牛血清白蛋白、溶菌酶、各种多肽和氨基酸水溶液的测量数据进一步补充了这些结果。初始频率范围从1 - 10兆赫兹扩展到0.035 - 1000兆赫兹；研究了吸收的温度和pH依赖性。理论方法包括考虑血浆中血细胞的相对运动、大分子周围水结构的扰动、带电实体的溶剂化、质子转移反应以及螺旋 - 线圈转变。氨基和羧基与水之间的质子转移反应被证明在很大程度上导致了吸收系数pH依赖性中观察到的峰值；这些峰值出现在与这些基团滴定的pK值相对应的碱性和酸性区域。此类反应无法解释生理pH下的吸收量级，因为在此范围内只有组氨酸会发生滴定。运用弛豫理论进行的广泛分析和测量表明，组氨酸咪唑基团与磷酸氢根离子（在溶液中）之间的质子转移反应为生理pH下显著的超声吸收提供了足够的体积变化。在磷酸盐缓冲溶液中，肽杆菌肽的理论与实验结果达成了极佳的一致性。通过将这些结果推广到蛋白质的情况，斯卢茨基等人估算了“典型组织”频率依赖性吸收系数的最大值，发现其量级是正确的，甚至在某些情况下超过了软组织中的观测值，而不是像过去那样总是远小于观测值。因此，原则上，通过调整诸如pK值等参数，首次能够使理论与实验达成一致。
