Proton hyperfine tensors based on ENDOR measurements at 4.2 K are reported for five distinct radical types in X-irradiated crystals of partially deuterated malonic acid. A total of 30 tensors are reported reflecting slightly different orientations of the damage products in the crystal: four tensors for RCHR; four for (RCH&; ten for ( R C H Z )~; four for the u radical, (RCH2C-O)u; six relatively isotropic proton tensors; and two d i k e tensors. R = -COOH throughout. The classic RCHR radical is found in two conformations at 4.2 K, separated by a f 1 2 O rotation of the >C-H direction with respect to the C-C-C plane of the room temperature neutron diffraction structure. It is hypothesized that the RCHR radical observed at room temperature is undergoing rapid librational motion between these potential energy minima (separated by 24O) thereby explaining the observed asymmetry of the dipolar hyperfine tensor. The proton hyperfine tensors for the RCH2 radicals whose plane is nearly parallel to the malonic acid C-C-C plane were measured to a precision of f50 kHz, allowing an accurate determination of the anisotropy difference between the two protons. From this difference, the spin density delocalization onto the carboxyl oxygens has been deduced and compared with the results of ab initio calculations. The set of radicals with relatively isotropic proton tensors are tentatively assigned to stabilized forms of a malonic acid reduction product, in which an extra electron is trapped on one of the carboxyl groups.

transition for pyrene on silica gel may also reflect the importance of an asymmetric interaction on the molecule, part of which is in an environment approaching the vapor or inert solvent. This aspect of the photophysics of adsorbed species invites theoretical attention. ConclusionThe picture that emerges from these results can be summarized as follows. The distribution of pyrene molecules on the surfaces in question is not random: there appear to be preferred sites. The result is an inhomogeneous distribution which yields a multicomponent decay. Strong evidence exists for the formation of a very weakly bound ground-state bimolecular association product, particularly on silica gel, and the data suggest associating this phenomenon with particular sites at which an interaction stabilizes this bimolecular association. The data also indicate that this phenomenon may be related to some peculiarity of the hydrogen-bonding interaction with the x system of the aromatic hydrocarbon. It is suggested that the static and highly asymmetric interaction may cause pyrene to behave in a manner different from that in hydroxylic solvents where the interactions are rapidly averaged and, on the time scale of the lifetime of pyrene, the interactions are symmetric over the whole of the x system.If the silanol groups are blocked by a long-chain alcohol and polyalcohol, the pyrene molecules appear forced to be adsorbed in areas of weaker interactions where the tendency to form the bimolecular ground-state arrangement is diminished. The mobility during the lifetime also appears to be greatly enhanced, and dynamic excimer formation is then possible.

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Optical detection of magnetic resonance (ODMR) has been employed to examine the homogeneity of the tryptophan environment, both of the isolated residue in solvent, and of tryptophan in glucagon and lysozyme and azurin B (Pseudomonas aeruginosa). From the shifts in the zero-field splittings, we can safely conclude that tryptophan in lysozyme, azurin B, or glucagon does not have the same type of solvent interaction as the free residue. However, by "burning holes" in the OSMR lines, it is evident that the lines in these cases are inhomogeneously broadened. From the relative line widths and hole widths, it appears that ODMR can be used to examine the relative diversity of interactions for a luminescent amino acid in a protein. We have followed the ODMR line characteristics in a progression from free N-acetyl-L-tryptophanamide, to tryptophan in lysozyme, to "denatured" lysozyme, and present evidence that the line widths narrow as the tryptophan residues become less solvent accessible.