A proton relaxation enhancement investigation of the binding of fatty acid spin labels to human serum albumin

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Abstract
Proton relaxation enhancement (PRE) values for fatty acid spin labels bound to human serum albumin have been investigated using the inversion‐recovery method at 24 MHz. At 0.1 mM protein concentration and a label‐to‐protein ratio of one‐to‐one, the PRE value for 12‐Doxylstearate‐albumin complex is 7.8 ± 2.3, whereas the PRE values for 5‐Doxylstearate and 16‐Doxylstearate‐albumin complexes are 1.5 ± 0.6 and 1.7 ± 0.7, respectively. Addition of 10‐fold excess of stearic acid reduced the PRE values nearly to 1, indicating that the strong enhancements arise from direct binding of fatty acid spin labels to human serum albumin. PRE values for all three labels exhibit maxima as a function of the label‐to‐protein ratio, suggesting multiple binding sites for fatty acid spin labels with labels in the tightest binding sites not resulting in the most effective relaxation. Based on the rates of reduction of ESR signal amplitudes by sodium ascorbate, the difference in PRE values for the three fatty acid spin labels bound to albumin is attributed to the difference in water accessibility of the nitroxide moieties at various positions along the acyl chain, being greater at the C‐12 position than at C‐5 or C‐16 position. The PRE value of 8 for 12‐Doxylstearate bound to human serum albumin indicates that this complex may be a suitable paramagnetic contrast agent for in vivo NMR imaging. © 1986 Academic Press, Inc.