H. Szmant
Jan 1, 1975
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Journal
Annals of the New York Academy of Sciences
Abstract
The unique capability of dimethyl sulfoxide (DMSO) to penetrate living tissues without causing significant damage is most probably related to its relatively polar nature, its capacity to accept hydrogen bonds, and its relatively small and compact structure. This combination of properties results in the ability of DMSO to associate with water, proteins, carbohydrates, nucleic acid, ionic substances, and other constituents of living systems. Of foremost importance to our understanding of the possible functions of DMSO in biological systems is its ability to replace some of the water molecules associated with the cellular constituents, or to affect the structure of the omnipresent water. The work discussed in this paper addresses itself t o the latter point, and is based on the study' of the liquid system water-DMSO by means of the spin-lattice relaxation and chemical shift behavior of both the water and DMSO protons. The binary system was investigated over a relatively large temperature range, and the relaxation times ( t l ) and chemical shifts were determined as a function of the concentration of the two components and of their deuterated analogues. The proton spin-lattice relaxation time is a measure of the energy decay of an array of excited hydrogen nuclei. The predominant mechanism of the dissipation of energy involves magnetic dipole-dipole interactions between the nuclei of neighboring protons. Since this principal relaxation path is very sensitive t o the distance that separates the interacting protons (a sixth-power relationship applies), the experimentally observed changes in the t l values reflect the dynamic changes that occur in the stereochemical environment of the protons under consideration as one varies the composition and the temperature of the system. Changes in the structure of the liquid can be inferred to have occurred from the experimentally determined t I values, since the reciprocal of t l is directly proportional to the molecular correlation time (rc), which in turn represents the time required for a complete rotation of the molecular moiety that contains the protons under consideration. Thus maxima in the plots of l / t l versus composition, for example, reveal an increased structuring of the system, which may involve attractions between the molecular moieties that contain the interacting protons, hence prolonging their correlation times. I t is of interest to note that the relaxation of deuteron nuclei results from the interaction not of nuclear dipoles, but rather of nuclear quadrupoles, and consequently the deuteron relaxation mechanism involves only intramolecular interactions. One can take advantage of this difference in the behavior of protons and deuterons by examining mixtures of the analogous isotopic substances; in this way one can separate the intraand intermolecular contributions to the relaxation process. It stands to reason that a decrease in the concentration of the protonated species dissolved in its deuterated counterpart gradually eliminates the intermolecular contributions to the total relaxation time, and that extrapolation of the relaxation times t o an infinitely dilute solution of the protonated species, dissolved in the magnetically relatively inert counterpart, allows one to evaluate the intramolecular 1 , or 7,values.