Fast Dissolution Dynamic Nuclear Polarization NMRof 13C-Enriched 89Y-DOTA Complex: Experimentaland Theoretical Considerations

Lloyd Lumata • Matthew Merritt • Craig Malloy •

A. Dean Sherry • Zoltan Kovacs

Received: 28 February 2012 / Revised: 22 March 2012 / Published online: 27 April 2012

� Springer-Verlag 2012

Abstract The yttrium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tet-

ra(10-13C-acetic acid) [13C]DOTA was synthesized. Fast dissolution dynamic

nuclear polarization (DNP) nuclear magnetic resonance (NMR) studies revealed

that the 89Y, 13C, and 15N nuclei present in the complex could be co-polarized at

the same optimum microwave irradiation frequency. The liquid-state spin–lattice

relaxation time T1 of these nuclei were found to be reasonably long to preserve

some or most of the DNP-enhanced polarization after dissolution. The hyper-

polarized 13C and 89Y NMR signals were optimized in different glassing mix-

tures. The overall results are discussed in light of the thermal mixing model of

DNP.

1 Introduction

Nuclear magnetic resonance (NMR) of nuclei with low gyromagnetic ratio c at

low concentrations and ambient conditions is challenging and time-consuming

due to the inherent insensitivity arising from the small magnetic moment of the

nuclei [1]. Dynamic nuclear polarization (DNP) technology overcomes this

problem by creating a non-Boltzmann nuclear spin distribution between the

nuclear Zeeman energy levels [2, 3]. The process of NMR signal amplification

L. Lumata � M. Merritt (&) � C. Malloy � A. D. Sherry � Z. Kovacs (&)

Advanced Imaging Research Center, University of Texas Southwestern Medical Center,

5323 Harry Hines Boulevard, Dallas, TX 76390, USA

e-mail: matthew.merritt@utsouthwestern.edu

Z. Kovacs

e-mail: zoltan.kovacs@utsouthwestern.edu

A. D. Sherry

Department of Chemistry, University of Texas at Dallas, 800 West Campbell Road,

Richardson, TX 75080, USA

123

Appl Magn Reson (2012) 43:69–79

DOI 10.1007/s00723-012-0335-8

Applied

Magnetic Resonance

via DNP has been known since the 1950s and it has been mainly used in the

production of polarized targets for nuclear and particle physics experiments [2–

4]. DNP operates via microwave irradiation of samples doped with paramagnetic

electrons to transfer the high electron thermal polarization to the target nuclear

spins at low temperature and moderate magnetic field [2, 3]. The NMR

sensitivity enhancement provided by DNP became available for chemical and

biomedical NMR spectroscopy and imaging applications with the invention of

the fast dissolution method by Ardenkjaer-Larsen and co-workers in 2003 [5].

This process involves the rapid conversion of the hyperpolarized frozen sample

at cryogenic temperature into diluted hyperpolarized liquid at physiological

temperature via injection of pressurized superheated water or other solvents. The

hyperpolarized liquid can then be transported into an NMR tube for MR

spectroscopy or injected into a subject for in vivo MR spectroscopy or imaging

[5]. Fast dissolution DNP is especially useful for nuclei with extremely low csuch as 89Y (spin I = 1/2, c = 2.0864 MHz/T, 100 % natural abundance) [6–8].

In this case, the long spin–lattice 89Y relaxation time of diamagnetic Y(III)-

complexes, which are disadvantageous to conventional NMR, are beneficial to

DNP-NMR spectroscopy because the hyperpolarized state decays according to

the T1 of the target nucleus. For instance, 89Y-DOTA, one of the most stable

Y(III) complexes, has a T1 close to 500 s [6, 7], which is much longer than the

typical 13C T1. Since the 89Y NMR chemical shift is quite sensitive to the

coordination environment of the Y(III) ion, the long polarization lifetime of 89Y

combined with high 89Y DNP-NMR signal enhancements make hyperpolarized89Y(III) complexes attractive as design platforms for responsive magnetic

resonance spectroscopy and imaging probes (e.g., pH, protein binding) [8, 9].

In this work, we present the synthesis and fast dissolution DNP-NMR

optimization studies of the 89Y-[13C]DOTA complex in which the carboxylate

groups are 13C-enriched as shown in Fig. 1. This complex contains 89Y and 13C in

the same chemical entity and may serve as a model compound for heteronuclear co-

polarization studies. The presence of two or more long-lived hyperpolarized nuclear

species in the same molecule could provide an avenue for future liquid state

hyperpolarized cross polarization experiments.

N N

NN

13COO-

13COO-

-OO13C

-OO13C

Y3+Na+

Fig. 1 Structure of yttrium1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(10-13C-acetic acid)(Y-[13C]DOTA)

70 L. Lumata et al.

123

2 Experimental Details

2.1 Materials

The trityl OX063 free radical tris{8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)-

dithiole-4-yl}methyl sodium salt (OX063) was obtained from Oxford Instruments

Molecular Biotools. The solvents were obtained from commercial sources and used

without further purification.

2.2 Synthesis of [13C]DOTA

The ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(10-13C-acetic acid)

([13C]DOTA) was synthesized starting from 1,4,7,10-tetraazacyclododecane and

1-13C-bromoacetic acid following the procedure reported for the unlabeled

compound [10]. The product was isolated in the zwitterionic form at pH 3 and

recrystallized from water.1H NMR, 400 MHz, D2O, NH3, pH 10, d (ppm): 3.31 (s, br, 8H acetate), 2.91 (s,

br, 16H macrocyclic ring). 13C NMR, 100 MHz, D2O, NH3, pH 10, d (ppm):178.07

(br, CO), 58.38 (br, N–CH2–CO), 50.73 (macrocyclic CH2). ESI–MS (pos) (m/z):

409.27 (40 %) [M ? H]?, 431.33 (100 %) [M ? Na]?. Calculated:

[M ? H]? = 409.21, [M ? Na]? = 431.19.

2.3 Synthesis of Y-[13C]DOTA

The yttrium(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(10-13C-

acetic acid) was prepared by adding equivalent amount of YCl3 to [13C]DOTA and

maintaining the pH around 6 by the addition of NaOH solution. The mixture was

stirred at room temperature for 2 days and the pH was periodically adjusted to 6 as

necessary. The pH was then brought to 8, the solution was filtered to remove any

excess metal and the pH was readjusted to 7 by adding small amounts of solid 13C

DOTA. The solution tested negative for free metal by the xylenol orange test. The

complex was isolated by freeze drying the solution. 13C NMR, 100 MHz, H2O, d(ppm): 180.40 (CO). 15N NMR, 40.6 MHz, d (ppm): 45.36. 89Y NMR, 19.6 MHz,

H2O d (ppm) 109.88. ESI–MS (neg) (m/z): 493.13 (100 %) [M]-. Calculated:

[M]- = 493.08.

2.4 Fast Dissolution DNP of Y-[13C]DOTA

Y-[13C]DOTA was dissolved in the appropriate glassing agent to produce a saturated

solution and then doped with the optimum concentration of trityl OX063 free radical

(15 mM). Small aliquots (40 lL) of these samples were quickly frozen in liquid

nitrogen to ensure rapid cooling for optimum glass formation. The frozen sample was

rapidly inserted into the HyperSense commercial polarizer (3.35 T, 1.4 K) and then

irradiated with microwaves (100 mW) for 5–7 h at a frequency corresponding to the

positive polarization peak in the DNP spectrum of the trityl-doped sample [6]. For

dissolution, 4 mL of superheated water was injected into the sample holder and the

DNP Investigation of Y-[13C]DOTA 71

123

dissolution liquid was transferred into a 10 mm NMR tube in a 9.4-T Varian VNMR

high-resolution magnet via a Teflon tube. The transfer time, ttr, was 8 s prior to

collection of NMR spectra.

2.5 Measurement of the NMR Signal Enhancement e

Case 1: measurement of hyperpolarized and thermal NMR signals on the samesample. The NMR signal enhancement e was calculated by taking the ratio of the

integrated NMR area Ahp of the hyperpolarized signal over the spectral area in

thermal equilibrium Ath: e = Ahp/Ath [6]. The NMR enhancement can also be

written as e = Php/Pth where Php is the polarization in the hyperpolarized state and

Pth the thermal equilibrium polarization. For a spin-1/2 nuclear system, e = tanh

(�hxn/2kBTs)/tanh(�hxn/2kBTL) where �h is Planck’s constant divided by 2p, kB the

Boltzmann constant, Ts is the spin temperature, TL is the lattice temperature, and xn

is the nuclear Larmor frequency (92p).

Case 2: using a reference sample for measurement of the thermal equilibriumNMR signal. In the case of ultra-low c nuclei such as 89Y, obtaining a thermal NMR

signal in the low mM concentration range is impractical due to the insensitivity and

the long T1 of the nucleus. Thus, for practical purposes, a highly concentrated

reference sample is used to get a thermal signal with reasonably high signal to noise

ratio in one or few scans. Consequently, the NMR enhancement calculation must

incorporate a correction factor to compensate for the higher number of spins in the

thermal reference sample: e = (Ahp/Ath)(cth/chp) where cth and chp are the

concentration of the reference thermal and hyperpolarized samples, respectively.

In this work, we have used a 3 M YCl3 aqueous solution (doped with 50 mM

CuSO4) as the reference thermal NMR sample for 89Y enhancement measurements.

When the flip angles used to obtain the hyperpolarized and thermal NMR signals are

different, the correction factor sin (hth)/sin (hhp), is also incorporated in the NMR

enhancement calculation [6].

2.6 Measurement of the Liquid-State Spin–Lattice Relaxation Time T1

These measurements were performed on 1 mL of hyperpolarized dissolution liquid

in a 10-mm NMR tube to ensure that the total sample volume is enclosed within the

NMR probe coil. The decay of the hyperpolarized NMR signal was monitored by

applying a small rf excitation pulse (flip angle h) for each time interval TR at 298 K

in a 9.4-T high-resolution magnet. The decay curve was fitted with the equation

accounting for the loss of magnetization due to rf pulsing and T1 decay: [11]

Mz(t) = M0sin(h)[cos(h)]t/TRexp(-t/T1) where M0 is the original magnetization

prior to the application of rf pulse. The T1 values were obtained from these fits.

3 Results and Discussion

The 13C microwave DNP spectrum (a plot of solid-state 13C signal enhancement

versus microwave frequency) of Y-[13C]DOTA (100 lL, 0.325 M in 1:1 v/v

72 L. Lumata et al.

123

glycerol:water glassing matrix doped with 15 mM trityl OX063) (Fig. 2) shows that

there are two optimum microwave irradiation frequencies for DNP, namely the

positive and negative polarization peaks, which for a spin-1/2 nuclear Zeeman

system corresponds to more surplus spins populating the lower (positive spin

temperature) and the upper (negative spin temperature) Zeeman energy levels,

respectively. A comparison of the 13C DNP spectrum in Fig. 2 with the 89Y DNP

spectrum of 89YDOTA [6] reveals that the locations of the 13C polarization peaks in

Fig. 2 are nearly the same as those of the 89Y polarization peaks. This observation is

in compliance with the Borghini spin temperature or the thermal mixing model, [2,

3, 12], which predicts that the shape of the microwave DNP spectrum is dependent

only on the properties of the free radical polarizing agent. Considering the ESR

linewidth of the trityl OX063 free radical (full width at half maximum, 63 MHz)

[13] which is greater than the 89Y (6.989 MHz) or 13C (35.958 MHz) Larmor

frequency in the HyperSense polarizer (3.35 T and 1.4 K), the predominant DNP

mechanism for these samples is expected to be thermal mixing. In a previous DNP

study on samples doped with trityl OX063, it was shown that the optimum

polarization peaks obtained for 13C samples are the same for other low-c nuclei such15N, 2H, and 29Si [14]. In the thermal mixing regime, we can simultaneously

polarize two or more nuclear species at the same microwave irradiation frequency.

Based on these results, the Y-[13C]DOTA samples were polarized at the 13C DNP

positive polarization peak, 94.16 GHz, in the HyperSense commercial polarizer and

after dissolution, the hyperpolarized NMR signals of three spin-1/2 nuclear species

(89Y, 13C, 15N) present in the complex were detected (Fig. 3). The final

concentration of Y-[13C]DOTA in the liquid state was 3.7 mM after dissolution

94.10 94.15 94.20 94.25-15

-10

-5

0

5

10

15

3.35 T, 1.4 K

13C

NM

R In

tens

ity (

a.u.

)

DNP Microwave Frequency (GHz)

0.325 M 13C-YDOTA15 mM trityl OX0631:1 glycerol:water

Fig. 2 13C microwave DNP spectrum of Y-[13C]DOTA sample (100 lL, 0.325 M Y-[13C]DOTA in 1:1v/v glycerol:water glassing matrix doped with 15 mM trityl OX063 free radical) measured by taking theintegrated NMR intensity at 3.35 T and 1.4 K following a 3-min microwave irradiation of the sample atthe respective microwave frequencies. The up and down arrows denote the positive and negativepolarization peaks, respectively

DNP Investigation of Y-[13C]DOTA 73

123

with water. It is worth noting that even at this low concentration, the hyperpolarized15N NMR signal of the macrocyclic ring nitrogens (45.4 ppm, 0.36 % natural

abundance) could easily be detected in addition to the hyperpolarized 89Y

(109.88 ppm relative to 3 M YCl3, 100 % natural abundance) and 13C carboxyl

(180.4 ppm, 13C-enriched) NMR signals. The long T1s of the nuclei (89Y

T1 = 480 s, 13C T1 = 14 s, and 15N T1 = 27 s) made it possible to preserve some

or most of the DNP-enhanced nuclear polarization during the rapid dissolution of

the polarized frozen samples. The T1 decay curves in Fig. 3 illustrate the importance

of long T1 for experiments with hyperpolarized substances. After 60 s, the

remaining magnetization with respect to the original NMR signal immediately

N N

NN

13C O O -

13C O O -

-O O 13C

-O O 13C

Y 3+N N

NN

13C O O -

13C O O -

-O O 13C

-O O 13C

Y 3+N N

NN

1 3C O O -

1 3C O O -

-O O 13C

-O O 13C

Y 3+

(b)(a) (c)

60 50 40 3015

N chemical shift (ppm)

60 50 40 3015

N chemical shift (ppm)

1.0

0.8

0.6

0.4

0.2

0.015N

NM

R In

ten

sity

(a.

u.)

100806040200Time (s)

15N T1=27 s

HYPERPOLARIZED

THERMAL

130 120 110 100 9089

Y chemical shift (ppm)

130 120 110 100 9089

Y chemical shift (ppm)

HYPERPOLARIZED

THERMAL

1.0

0.8

0.6

0.4

0.2

0.089Y

NM

R In

ten

sity

(a.

u.)

150012009006003000

Time (s)

89Y T1=480 s

1.0

0.8

0.6

0.4

0.2

0.013C

NM

R In

ten

sity

(a.

u.)

6050403020100Time (s)

13C T1=14 s

190 185 180 175 17013

C chemical shift (ppm)

190 185 180 175 17013

C chemical shift (ppm)

HYPERPOLARIZED

THERMAL

Fig. 3 NMR characterization of 89Y-13CDOTA (3.7 mM final concentration in water) at 9.4 T and298 K via fast dissolution DNP: a Top: hyperpolarized 89Y (100 % natural abundance) NMR signaldetected by applying a 5-degree excitation pulse 8 s after dissolution of 40 lL of polarized frozen samplewith 4 mL of water. The calculated NMR enhancement is 5,700-fold using a 3-M YCl3 aqueous solutionas a reference. Middle: Thermal 89Y NMR spectrum of the same sample after four scans. Bottom: decayof the hyperpolarized 89Y NMR signal monitored by applying a 5-degree pulse every 30 s. The 89Y T1

extracted from these data is 480 s. b Top: hyperpolarized 13C NMR signal emanating from the 13C-enriched carboxylates of the complex detected by a 5-degree pulse. The calculated NMR enhancement is1,300-fold. Middle: the corresponding thermal 13C NMR spectrum after four scans. Bottom: decay curveof the hyperpolarized 13C NMR signal monitored by applying a 5-degree pulse every 2 s. The calculated13C T1 is 14 s. c Hyperpolarized 15N (0.36 % natural abundance, 1 scan) signal (top) from themacrocyclic ring nitrogen and the thermal NMR spectrum (middle) after four scans. The estimated NMRenhancement of hyperpolarized 15N signal is 3,500-fold with respect to a 15N-nitrobenzene referencesample. The decay curve of the 15N hyperpolarized signal (bottom) was monitored by applying a 5-degreerf pulse every 5 s. The calculated 15N T1 is 27 s

74 L. Lumata et al.

123

before rf pulsing is 87 % for 89Y, 9 % for 15N, and close to 0 % for 13C (see

hyperpolarized NMR signal decay in Fig. 3).

It is known that the nature of glassing matrix has a significant influence on the

efficiency of DNP process [6, 15]. The effect of the composition of the glassing

matrix (Table 1) on the hyperpolarized 89Y and 13C NMR enhancements of the

Y-[13C]DOTA was very similar to that observed for the unlabeled YDOTA complex

[6]. Viscous glass formers (e.g., ethylene glycol, glycerol, propanediol) appear to

yield higher 13C and 89Y NMR enhancements than the less viscous glassing agents

(methanol, DMSO), which likely reflect the fact that they are better glass formers.

The 13C NMR enhancements in Table 1 generally have lower values because of the

relatively short 13C T1 (14 s), which greatly contributes to the loss of nuclear

polarization during the dissolution transfer (ttr = 8 s).

Figure 4 shows the effect of varying content of glycerol in the aqueous glassing

matrix on the 13C and 89Y NMR enhancements. The 13C and 89Y e monotonically

increase with increasing glycerol:water volume ratio, but this improvement in the

NMR enhancement comes at the expense of decreasing solubility of the

Y-[13C]DOTA in the glassing matrix. The low solubility of Y-[13C]DOTA in the

glassing mixture adversely affects the 13C NMR signal buildup curves (Fig. 4c, d);

the 13C polarization builds up slower (longer buildup time constant sbu) which is

attributed to longer 13C intermolecular distance, thus slower 13C nuclear spin

diffusion [16]. The optimum glycerol content that would yield a balanced

combination of high NMR enhancement and a reasonable nuclear spin count (Y-

[13C]DOTA concentration) is around 50 %.

Simple theoretical considerations can reveal the maximum NMR enhancements

that one can expect for 89Y and 13C DNP. Figure 5 shows the simulated comparative

plots of DNP-enhanced nuclear polarizations of 89Y and 13C nuclear species where

the DNP mechanism proceeds either by thermal mixing or the solid effect. In the

case the solid effect [2], which occurs when the ESR linewidth D of the free radical

Table 1 A summary of liquid-state 13C and 89Y NMR enhancements (N = 3) e of hyperpolarized

Y-[13C]DOTA samples measured in a 9.4-T magnet at 298 K 8 s after dissolution

Glassing agent Y-[13C]DOTA (M) 13C e 89Y e

Ethylene glycol 0.238 3,320 8,040

Diethylene glycol 0.138 1,660 7,920

Triethylene glycol 0.109 1,940 5,100

Glycerol 0.325 1,340 5,720

Dimethyl sulfoxide 0.050 1,240 2,940

Propanediol 0.152 5,440 8,790

Methanol 0.175 380 1,640

Pure water 0.521 141 387

The samples were prepared by using the maximum soluble concentration of Y-13CDOTA in different

glass formers mixed with 50 % water and doped with 15 mM trityl OX063. Prior to dissolution with

4 mL of water, the samples (40 lL aliquots) were polarized for 5 h in the HyperSense at 3.35 T and 1.4 K

with a 100-mW microwave source. The standard deviation for NMR enhancements is *10 % for each

nucleus

DNP Investigation of Y-[13C]DOTA 75

123

is much smaller than the nuclear Larmor frequency xn, the nuclear polarization can

be written as a function of the electron thermal polarization: PI(x) = BI(x) where

x = �hxe/kBT (xe is the electron Larmor frequency) and BI(x) is the Brillouin

function given by BIðxÞ ¼ 2Iþ12I coth 2Iþ1

2I x� �

� 12I coth 1

2I x� �

[17]. For spin-1/2 nuclei

such as 89Y and 13C, P(89Y) = P(13C) = tanh(�hxe/kBT) = P(e-) (dashed line in

Fig. 5). Thus, the ideal solid effect, when the phonon bottleneck effect, nuclear

relaxation leakage and other factors are ignored, predicts that the maximum

polarization that a spin-1/2 nuclear spin system can achieve equals the electron

thermal polarization [16]. On the other hand, thermal mixing, [2, 3, 12, 17] which

dominates when the ESR linewidth D of the free radical polarizing agent is greater

than or comparable to the nuclear Larmor frequency, is an equal spin temperature

(EST) prediction, wherein the nuclear Zeeman system acquires the same spin

temperature Ts as the electron spin–spin interaction (eSSI) or dipolar system during

DNP. Unlike the solid effect where the target nuclei have distinct irradiation

frequencies (xe ± xn), two or more nuclei in the thermal mixing regime (D C xn)

0.6

0.5

0.4

0.3

0.2

0.1

0.0

13C

-YD

OT

A (

M)

100806040200

Glycerol (%)

15000

12000

9000

6000

3000

0

NM

R e

nhan

cem

ent

100806040200

Glycerol (%)

8000

6000

4000

2000

0

13C

bu

(s)

100806040200

Glycerol (%)

80

60

40

20

013C

NM

R In

tens

ity (

a.u.

)

150001000050000

Time (s)

Glycerol volume (%) 0 25 50 75

(a)

(b)

(c)

(d)89

Y

13C

Fig. 4 a Maximum soluble Y-[13C]DOTA concentration and b the 89Y and 13C liquid-state NMR signalenhancements obtained in varying glycerol volume content (%) in aqueous glassing matrix. Note: theNMR enhancements at 0 % glycerol content (pure water) were non-zero (89Y e = 387, 13C e = 141).c Growth of the relative 13C NMR signal as a function of microwave irradiation time of saturatedY-[13C]DOTA samples in varying glycerol content at 3.35 T and 1.4 K. The solid lines are fits to a singleexponential buildup equation. d The 13C polarization buildup time constants sbu extracted from c

76 L. Lumata et al.

123

can be polarized simultaneously at the same microwave irradiation frequency. In

this case, Ts(89Y) = Ts(

13C) = Ts(e-). After rearranging the terms, the 89Y

polarization P(89Y) can then be written as a function of 13C polarization P(13C):

P(89Y) = tanh[(89c/13c)tanh-1(P(13C))]. The solid curve in Fig. 5 illustrates that in

the thermal mixing DNP mechanism, nuclei with lower gyromagnetic ratio are

much harder to polarize to high levels. We have shown recently [6] that co-

polarization of 89Y and 13C spins with trityl OX063 is in agreement with the thermal

mixing prediction. The mirror axes in Fig. 5 show the corresponding NMR signal

enhancements at 9.4 T and 298 K in the ideal scenario where the nuclear

polarization of frozen polarized samples achieved in DNP at cryogenic conditions is

0 20 40 60 80 1000

20

40

60

80

100

0 2 4 6 8 100

2

4

6

8

10T1-corrected data (Figure 4)

T1-corrected data (Table 1)

thermal mixing

solideffe

ct

89Y

Po

lari

zati

on

(%

)

13C Polarization (%)

0 2 4 6 8 10 12

0

10

20

30

40

50

60

89Y en

han

cemen

t (x1000)

13C enhancement (x1000)

thermal mixing

solid

effe

ct89Y

Po

lari

zati

on

(%

)

13C Polarization (%)

0 20 40 60 80 100 120

0

100

200

300

400

500

600

89Y en

han

cemen

t (x1000)

13C enhancement (x1000)

Fig. 5 Simulated comparative plots of the 89Y (left axis) and 13C (right axis) polarization levels wherethe DNP process proceeds either by (i) thermal mixing or (ii) solid effect. The thermal mixing predictionis based on the equal spin temperature (EST) theory, where in this case both the nuclear Zeeman systems(89Y and 13C) acquire the same spin temperature as the electron spin–spin interaction/dipolar system[Ts(

89Y) = Ts(13C) = Ts(e

-)]. On the other hand, the ideal solid effect DNP mechanism predicts themaximum polarization that a spin-1/2 nuclear Zeeman system can attain is equal to the electron thermalpolarization [P(89Y) = P(13C) = P(e-)]. The mirror axes are the corresponding NMR signalenhancements that can be attained at 9.4 T and 298 K in the ideal case where the DNP-enhancedsolid-state nuclear polarization achieved in the polarizer is completely preserved in the liquid state afterdissolution of the frozen polarized samples. Inset: expanded view of the graph in the low-polarizationregion where the T1-corrected 13C and 89Y NMR enhancement data from Table 1 (solid squares) andFig. 4 (solid circles) are plotted

DNP Investigation of Y-[13C]DOTA 77

123

completely preserved after dissolution in the liquid state. In this ideal case, the

expected liquid-state 13C and 89Y signal enhancements in 89Y-[13C]DOTA at 9.4 T

and 298 K would be very nearly the same (see inset in Fig. 5). In practice, however,

T1 decay and the field gradients [18] experienced by the sample during the

dissolution and transfer from the polarizer result in a decrease of nuclear

polarization. The dissolution and transfer time in our commercial polarizer is about

8–10 s during which period a substantial amount of the 13C polarization decays via

T1 (14 s) relaxation, but only a negligible loss of the 89Y polarization occurs

(T1 = 480 s). In order to minimize these losses, efforts could be made such as

incorporating a rapid injection device [19] to the HyperSense commercial polarizer

which shortens the dissolution transfer time to close to 1 s.

4 Conclusion

In summary, we have characterized Y-[13C]DOTA via the fast dissolution DNP-

NMR technique and optimized both hyperpolarized 89Y and 13C NMR signals in

different glassing matrices. We have shown that the 89Y, 13C and 15N nuclei in this

complex can be copolarized at the same optimum microwave irradiation frequency,

a characteristic of the thermal mixing DNP mechanism. Higher 89Y and 13C NMR

enhancements were obtained in viscous glass formers such as glycerol and

propanediol. The optimum volume ratio of glycerol–water glassing matrix was 1:1

(v/v) because it afforded high 13C and 89Y NMR enhancements and a reasonable

nuclear spin count (concentration) of Y-[13C]DOTA. The T1-corrected 13C and 89Y

NMR enhancements reveal that the data fall within the thermal mixing DNP

prediction.

Acknowledgments We acknowledge the National Institutes of Health (grant no. R21EB009147) for the

financial support on this work.

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