Magnetic resonance water proton relaxation in protein solutions and tissue: T(1rho) dispersion characterization

Abstract

Background: Image contrast in clinical MRI is often determined by differences in tissue water proton relaxation behavior. However, many aspects of water proton relaxation in complex biological media, such as protein solutions and tissue are not well understood, perhaps due to the limited empirical data.

Principal findings: Water proton T(1), T(2), and T(1rho) of protein solutions and tissue were measured systematically under multiple conditions. Crosslinking or aggregation of protein decreased T(2) and T(1rho), but did not change high-field T(1). T(1rho) dispersion profiles were similar for crosslinked protein solutions, myocardial tissue, and cartilage, and exhibited power law behavior with T(1rho)(0) values that closely approximated T(2). The T(1rho) dispersion of mobile protein solutions was flat above 5 kHz, but showed a steep curve below 5 kHz that was sensitive to changes in pH. The T(1rho) dispersion of crosslinked BSA and cartilage in DMSO solvent closely resembled that of water solvent above 5 kHz but showed decreased dispersion below 5 kHz.

Conclusions: Proton exchange is a minor pathway for tissue T(1) and T(1rho) relaxation above 5 kHz. Potential models for relaxation are discussed, however the same molecular mechanism appears to be responsible across 5 decades of frequencies from T(1rho) to T(1).

Figure 1. Analysis of bovine serum albumin (BSA) samples by polyacrylamide gel electrophoresis (PAGE).

Gels are 7.5% polyacrylamide (PA) with stacking gel of 2% PA and stained with Coomassie Blue. (a) SDS-PAGE: Samples were incubated at 95° C in denaturing sample buffer (containing β-mercaptoethanol and SDS), and each lane was loaded with 10 mg of protein and run with SDS in the buffer. Lane 1: BSA monomer; lane 2: BSA dimer; lane 3: methylated BSA (Sigma Chemicals); lanes 4, 5, 7, 8, 9, 10 contain 10% BSA monomers crosslinked with increasing [GA]: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6% GA respectively; lane 6: molecular weight marker (229,126,80,48 kDa). (b) Non-denaturing PAGE. Lane 1: molecular weight marker (85, 50, 35 kDa); lane 2,3,4 are fraction V BSA with 0.0%, 0.4%, and 0.5% GA respectively. (c) Non-denaturing PAGE: Lanes 1–3: methylated BSA (Sigma Chemicals) with decreasing amounts (5, 2.5 and 1.25 ug) of loaded protein; lane 4: molecular weight marker (200, 116, 97, 66, 55 kDa); lanes 5–7: 10% fraction V BSA reacted with 0.4% GA in decreasing amounts (5, 2.5, and 1.25 ug) of protein loaded. See text for details.

Figure 2. T_1ρ dispersion characteristics of various protein solutions.

(a) T_1ρ of water protons in solutions of 10% BSA (fraction V) versus B₁ field strength at various glutaraldehyde (GA) concentrations. Samples were evaluated at 2T. Uncrosslinked BSA samples were studied at pH 5.5 and 7.0. The data points for 80 and 100 mM GA were fitted to the relaxation-time power law of Eq. [1]. (b) T₂ measurements were incorporated into the data of panel a to show T₂/T_1ρ ratios as a function of B₁ field strength. See text for details.

Figure 3. T_1ρ dispersion characteristics of BSA solutions derived from purified monomers.

T_1ρ of water protons in solutions of 10% BSA (purified monomers) versus B₁ field strength at various glutaraldehyde (GA) concentrations evaluated at 2T. Data from uncrosslinked purified BSA dimers are also shown. The solid lines represent the fit to the relaxation-time power law of Eq. [1].

Figure 4. T₂/T₁ ratio of BSA solutions reacted with various concentrations of glutaraldehyde.

All samples contain 10% BSA (fraction V), except for one sample (*) with 20% BSA. Measurements were performed at both 2 and 4.7 T. See text for details.

Figure 5. Normalized T_1ρ dispersion plots of methylated BSA solutions.

Samples were evaluated at 2T. Plots of native and crosslinked BSA (fraction V) are also shown for comparison. Note the similarity of T_1ρ dispersion of methylated BSA with crosslinked BSA above 5 kHz, and the similarity with native BSA below 5 kHz.

Figure 6. T₂/T_1ρ ratio as a function of B₁ field strength (a) and normalized T_1ρ dispersion plots (b) of various tissue samples.

Calf patella cartilage at various pH and rat myocardium samples were evaluated at 2T. Plots of cartilage and crosslinked BSA (fraction V) in DMSO solvent are also shown for comparison in panel b. See text for details.

Figure 7. Values of T_1ρ ² are plotted against B₁ field strength for various samples.

(a) Crosslinked BSA (fraction V) in H₂O. (b) Native BSA in H₂O. (c) Rabbit myocardial tissue. (d) Cartilage in DMSO solvent. See text for details.

Figure 8. Plots of T_1ρ vs. B₁ field strength are shown for two tissue and two crosslinked BSA samples (fraction V).

The lines drawn were fitted using Eqs. [1–3]. Note that relaxation-time power law (Eq. [1]) appears to best fit the data. See text for details.