Febs Letters Volume 99, Number 1-2./Volume 100, Number 1-2. [antikvár]

Volume 107, number 565 FEBS LETTERS March 1979 Hypothesis MOLECULAR TUNNELLING IN CARBON MONOXIDE BINDING TO HEMOGLOBIN Joshua JORTNER and Jens ULSTRUP+ Depanment of Chemistry, Tel-Aviv University, Tel-Aviv, Israel and * Chemistry Department A, Building 207, The Technical University of Denmark, 2800 Lyngby, Denmark Received 1 December 1978 1. Introduction Electron transfer (ET) [ 1 -4] and atom or molecular group transfer (AT) [5-9] reactions constitute the most important elementary steps in a variety of biological processes. During the last decade the dynamics of several elementary biological processes were investigated over a broad temperature range from about 2 K up to room temperature. Notable examples are the liglit-induced oxidation of cytochrome c by optically excited bacteriochlorophyU [1-3], the recombination of CO and hemoglobin [9], and the production of prelumirhodopsin from electronically-excited rhodopsin [7]. In all three cases the unimolecular rate was found to be finite and nearly temperature independent at low temperatures, manifesting the effects of nuclear tunnelling, while with increasing temperature the rate changes within a narrow temperature interval into an activated rate exliibiting Arrhenius-type temperature dependence. In a microscopic theory for ET and AT processes two classes of reactions must be distinguished: (1) Adiabatic processes which proceed on a single potential energy surface. Most AT reactions probably belong to this category. (2) Nonadiabatic processes involving a transition between two potential energy surfaces which correspond to two (weakly-coupled) distinct zero-order electronic configurations. The theory of ET in biological systems as nonadiabatic multiphonon processes is now well developed [10-14] being isomorphous with the general quantum mechanical theory of homogeneous and heterogeneous ET processes [15-17]. On the other hand, the theoretical framework for AT processes [16,18,19] is far less comprehensive. In this note we advance a theory of nonadiabatic AT reactions in biological systems, with a specific application to the low-temperature recombination of CO with hemoglobin (hb) [9]. 2. The CO/hb recombination reaction The recombination of CO with /3-subunits of hb and derivatives of this compound has been studied over the temperature interval 2-300 K [9]. At high temperatures CO escapes into the outer solvent and passes several barriers on its way back to its coordination site. At temperatures below 180 K the system has only to overcome a single barrier. At these low temperatures the 'pocket' which surrounds the heme group is sealed off thus trapping the CO molecule. The experimental data furthermore showed [9]: (1) The rebinding kinetics does not exhibit exponential decay but foUows a power law. This effect originates from the freezing of different conformational states at the low temperatures. (2) The average rates (or rather T^^j^ where r^ refers to the time when the deoxy-hb concentration has dropped to 75% of its initial value) are pracfically temperature independent in the range 2-10 K. (3) The transition from the tunnelling range (2-10 K) to the temperature-activated range occurs in the region 10—20 K. Above this region Tg jj is temperature dependent corresponding to an apparent activation energy of = 0.045 eV. Consider the nuclear configurational changes Elsevier !North-Holland Biomedical Press 1