A molecular model is proposed to explain water 1H NMR spin-lattice relaxation at different levels of hydration (NMR titration method) on collagen. A fast proton exchange model is used to identify and characterize protein hydration compartments at three distinct Gibbs free energy levels. The NMR titration method reveals a spectrum of water motions with three well-separated peaks in addition to bulk water that can be uniquely characterized by sequential dehydration. Categorical changes in water motion occur at critical hydration levels h (g water/g collagen) defined by integral multiples N = 1, 4 and 24 times the fundamental hydration value of one water bridge per every three amino acid residues as originally proposed by Ramachandran in 1968. Changes occur at (1) the Ramachandran single water bridge between a positive amide and negative carbonyl group at h1 = 0.0658 g/g, (2) the Berendsen single water chain per cleft at h2 = 0.264 g/g, and (3) full monolayer coverage with six water chains per cleft level at h3 = 1.584 g/g. The NMR titration method is verified by comparison of measured NMR relaxation compartments with molecular hydration compartments predicted from models of collagen structure. NMR titration studies of globular proteins using the hydration model may provide unique insight into the critical contributions of hydration to protein folding.

The orientation dependence of the free induction decay (FID) of 1 H NMR water signal in ex vivo bovine digital flexor tendon at the native level of hydration is reported. Residual dipolar coupling due to the overall tissue anisotropy produces a 6:1 change in the signal intensity as an angle between the long axis of a specimen and the external magnetic field is changed from the "magic angle" of 54.7°to 0°. The strength of residual dipolar interactions between water protons was estimated by orientational analysis of the signal intensity to be equal to 780 Hz. Apparent signal maxima are observed at orientations 8 -13°a way from 54.7°due to an inhomogeneous contribution to the decay. A small fraction of total water in tendon is detectable at all orientations and exhibits a shift in the precession frequency. It is hypothesized that this water fraction resides in the interconnecting gaps at the ends of collagen molecules. The gaps have a disordered environment that allows for a zero time average of dipolar interactions. Measured frequency and phase shifts are interpreted as signatures of the bulk magnetic susceptibility effect due to geometry of the cavity formed by adjacent gaps at the ends of the collagen molecules. The multiexponentiality of the FID decay is hypothesized to be due to the exchange between orientationally restricted water structured along the length of the collagen molecule and disordered water in the cavity. Key words: free induction decay; bulk magnetic susceptibility; residual dipolar coupling; anisotropy; collagen; tendon Water protons are a major source of signal in clinical MRI. The NMR water signal provides valuable information about tissues. It depends not only on water content but also on types of micro-and macroscopic order, the presence of barriers, the extent of participation in hydrogen bonds, and interactions with numerous macromolecules.No other macromolecule is as abundant in the human body as collagen. Collagen is present in different amounts, types, and spatial structures, and its water content varies throughout the body. In tendon, collagen accounts for up to 95% of the total protein content, or 80 -90% of its dry weight (1). Its triple helical structure is embedded into the extracellular matrix, which is composed of elastins, glycoproteins, and tendocytes. The composition and structure of tendon are optimized for the function of transmitting force created in the muscle to the bone, and making joint movement possible. Fibrils are assembled in an anatomically defined preferential direction from the nanometer scale and up (2). Packing of collagen in microfibrils, as revealed by scanning electron microscopy (SEM) (3), shows a pattern of alternating gaps and overlaps indicative of a staggered supramolecular assembly.The ordered hierarchy of collagen in fibrils precludes isotropic motion of almost all water present in tendon, which behaves quite differently from bulk water. In addition, the motion is slowed because water molecules are involved in the formation of hydrogen bonds and wate...

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A novel adaptive mesh technique is introduced for problems of image reconstruction in luminescence optical tomography. A dynamical adaptation of the three-dimensional scheme based on the finite-volume formulation reduces computational time and balances the ill-posed nature of the inverse problem. The arbitrary shape of the bounding surface is handled by an additional refinement of computational cells on the boundary. Dynamical shrinking of the search volume is introduced to improve computational performance and accuracy while locating the luminescence target. Light propagation in the medium is modeled by the telegraph equation, and the image-reconstruction algorithm is derived from the Fredholm integral equation of the first kind. Stability and computational efficiency of the introduced method are demonstrated for image reconstruction of one and two spherical luminescent objects embedded within a breastlike tissue phantom. Experimental measurements are simulated by the solution of the forward problem on a grid of 5x5 light guides attached to the surface of the phantom.

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