Supplementary material
Assessment of Chemical Exchange in Tryptophan-Albumin solution through 19F Multicomponent TransverseRelaxation Dispersion Analysis
Ping-Chang Lin
Department of Radiology, College of Medicine
Howard University, Washington, DC 20060, USA
*to whom the correspondence should be addressed
Supporting information
Sample preparation. 6-fluoro-DL-tryptophan (6F-TRP) (Gold Biotechnology, Inc., St. Louise, MO) was dissolved in 0.05M HCl at room temperature to make a 45-mM stock solution. Bovine serum albumin (BSA) (Amresco, LLC, Solon, OH) was then dissolved in the 6F-TRP stock to prepare a 1.13-mM BSA/ 45-mM 6F-TRP complex solution for characterization of chemical exchange process between the free and BSA-bound states of 6F-TRP. To mimic a condition that macromolecules cannot penetratecellular barriers such as cytoplasmic membranes, the semi-permeable dialysis membrane of 8-10 kD MWCO wasimplemented to separate the BSA-6F-TRP complex solution from the 6F-TRP solution. All the samples were separately placed in 5-mm NMR tubes for 19F transverse relaxation experiments.
NMR measurements. 19F NMR experiments were acquired at a 9.4T BrukerAvance NMR spectrometer (BrukerBiospin, GmbH, Rheinstetten, Germany) equipped with a 5-mm 1H/ multinuclear broadbandprobe at 20 ± 1 °C. 19F T2 relaxation data were collected using a spectroscopic CPMG pulse sequence for each sample prepared. The CPMG experiments were acquired with a sufficient number of transients (either 128 or 256), repetition time of 10 s, and echo-spacing varying from 0.2 to 25 ms in 19 steps, referring to the number of echoes from 20480 down to 192, respectively (detailed in Table S1). Signals acquired at individual echoes in each CPMG sequencesetting were then collected to generate a T2 decay curve for the NNLS analysis (Figures 2 and S1). For the 19F relaxation data acquired, the signal-to-noise ratio ranged from 16 to 442.
Fitting of multicomponent T2 relaxation data. The NNLS method has been adopted for multicomponent T2 relaxation analysis(Reiter et al. 2009).By incorporating regulations into the linear equation system of describing multi-exponential decays, theNNLS approach is easily changed to construct a continuous spectrum(Graham et al. 1996; Reiter et al. 2009; Whittall and MacKay 1989). Briefly, a set of linear equations, , illustrate the multiple T2 exponential decays in discrete form through using M-1relaxation components over N echoes generated by the CPMG pulse train. As described in the text, the vector includes N echo amplitudes;the matrix ,consistingof N x (M-1) kernels,, profiles a set of (M-1) T2 relaxation components at N different echo times and N x 1 elements of value 1 in the M-th column for baseline offset adjustment;and the array consists of M unknown amplitudes associated with the M-1 T2 components and a baseline offset in this linear equation system. The NNLS approach makes no a priori assumptions about the number of relaxation components present. A minimum energy constraint,i.e. a Tikhonov regularization of second kind in our study,is imposed into the function to lessen the impact of noise on the curve fitting and to permit generation of a continuous T2 distribution (Equation S1)(Graham et al. 1996; Reiter et al. 2009; Whittall and MacKay 1989).
,[S1]
Given the 2 misfit defined as
[S2]
, which is the sum of variancesof the prediction errors divided by the standard deviation of , a nonnegative set of was obtained by performing regularization of NNLS fits.An appropriate value of the regularizer μ was selected for an optimal condition that the 2 misfit value from the regularized fit was 100.5% of the non-regularized 2based on the strategy of “least squared-based constraints”, which evenly regularizes all datasets on a percentage basis across the study (Graham et al. 1996; Reiter et al. 2009). The T2 distribution, which was constructed by the T2 values and the associated component fractions , was interpreted in terms of matrix composition. The T2 distribution consisting of one or two T2 relaxation components was then fitted using a 4- or 7-parameter lognormal model. All fitting routines were implemented using MATLAB (MathWorks, Natick, MA, USA).
Lognormal curve fitting. The T2 distributions resulting from the NNLS fits were further fitted into the probability density function of a lognormal distribution:
, for single component T2 distribution, [S3]
or
, for double component T2 distribution. [S4]
The geometric mean (or ) and multiplicative standard deviation ( or ) were calculated from the corresponding fitting outcome.
echo spacing (ms) / CPMG (Hz) / number of echoes0.2 / 1250 / 20480
0.333 / 750 / 13312
0.4 / 625 / 11264
0.5 / 500 / 9216
0.8 / 313 / 6000
0.9 / 278 / 5120
1 / 250 / 4608
1.25 / 200 / 3584
1.429 / 175 / 3328
1.667 / 150 / 2816
2 / 125 / 2304
2.5 / 100 / 1792
3.333 / 75 / 1400
5 / 50 / 960
6.667 / 37 / 720
10 / 25 / 492
13.33 / 19 / 360
20 / 13 / 240
25 / 10 / 192
Figure S1
19F T2 decay curvesand the corresponding fitting analyses in(A) the 6F-Trp solution, (B) the BSA-6F-Trp complex solution, and (C) the two-compartmental 6F-Trpsystem. The echo spacing present in the CPMG pulse sequence was 2 ms for all the solution systems. In the TE vs. signal intensity plotforeach solution system, green circles represent T2 decay signals acquired at the respective echoes; blue fitting curveexhibits the NNLS fit of the T2 decay data;and red curve represents the mono-exponential fit without restraints. Additional two T2 decay analyses through use of bi- and tri-exponential fittings without regularization are included in (C), which are barely distinct from the fitting curve genereated from the NNLS analysis. As expected, the arithmetic means of T2 in the mono-exponential fits were close to the geometric means of the T2 dispersions yielded from the NNLS analysisin (A) and (B), but not in (C) (Table S2).On the other hand, the variances of fits in the bi- and tri-exponential analyses were compatible with that in the NNLS analysis in (C), evidenced in the2 statistics and in the residual plot (blue circle: residuals of the NNLS fit; red circle: residuals of the mono-exponential fit; purple cross: residuals of the bi-exponential fit; and green triangle: residuals of the tri-exponential fit). Regarding the bi-exponential and tri-exponential analyses, the residual plot in (C) does not show any substantial difference between the fits although the number of exponential components and their decay rates differ quiet significantly. In fact, the variances of fits cannot be evaluated with an F-test because the residuals of the fits are not independent of each other, which isdue to the consequence of non-orthogonality of exponentials (Istratov and Vyvenko 1999; Johnson 2008). Therefore, there is no compelling evidence supporting either the bi-exponential or tri-exponential model in the two-compartmental 6F-Trp system.
In addition, the T2curve fitting outcomes are shown in the T2 vs. intensity plots of (A), (B) and (C), respectively, with the discrete T2 values,resulting from the mono-, bi- or tri-exponential fitting,presented by the line segments, accompanied with the corresponding T2 distributions resulting from the NNLS analysis. For all the curve fittings, the 2 statisticswereapplied to goodness-of -fit tests, with the calculated 2shown in Table S2.
Table S2 Fitting analysis of T2 relaxation curves acquired at 2 ms
Method / 2 (p value) / Estimated T2 (ms) / Weight fractionFig S1A / NNLS w/ regularization / 2317 (p = 0.41) / 1912 (1.31) / -
Mono-exponential fitting / 2322 (p = 0.38) / 1829 ± 14 / -
Fig S1B / NNLS w/ regularization / 2370 (p = 0.16) / 150 (1.52) / -
Mono-exponential fitting / 2362 (p = 0.19) / 132 ± 2 / -
Fig S1C / NNLS w/ regularization / 2206 (p = 0.92) / 237 (1.54)
1851 (1.51) / 0.28
0.72
Mono-exponential fitting / 6146 (p ~ 0) / 1315 ± 6 / -
bi-exponential fitting / 2217(p = 0.89) / 257 ± 8
1651 ± 11 / 0.32
0.68
2243(p = 0.80) / 234 ± 13
1603 ± 14 / 0.31
0.69
tri-exponential fitting / 2196(p = 0.93) / 211 ± 17
1023 ± 393
1944 ± 317 / 0.27
0.27
0.46
* For 2 goodness-of-fit test,df = 2303 in the NNLS analysis, df = 2301 in the mono-exponential fitting, df = 2299 in the bi-exponential fitting, and df = 2297 in the tri-exponential fitting.
** Estimated T2 is presented as geometric mean (multiplicative standard deviation) for the NNLS analysis and as arithmetic mean ± standard deviation for the mono-, bi- or tri-exponential fitting.
References
Graham SJ, Stanchev PL, Bronskill MJ (1996) Criteria for analysis of multicomponent tissue T2 relaxation data Magn Reson Med 35:370-378
Istratov AA, Vyvenko OF (1999) Exponential analysis in physical phenomena Rev Sci Instrum 70:1233-1257 doi:10.1063/1.1149581
Johnson ML (2008) Nonlinear least-squares fitting methods Method Cell Biol 84:781-805 doi: 10.1016/S0091-679x(07)84024-6
Reiter DA, Lin PC, Fishbein KW, Spencer RG (2009) Multicomponent T-2 Relaxation Analysis in Cartilage Magnetic Resonance in Medicine 61:803-809 doi:10.1002/mrm.21926
Whittall KP, MacKay AL (1989) Quantitative interpretation of NMR relaxation data Journal of Magnetic Resonance (1969) 84:134-152 doi:10.1016/0022-2364(89)90011-5
S1