Nitro-Phenylalanine: A Novel Sensor for Heat Transfer in Peptides

Mar 3, 2011 - To follow the energy and heat distribution in peptides or proteins one needs molecular reporting groups that can be incorporated in the ...
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Nitro-Phenylalanine: A Novel Sensor for Heat Transfer in Peptides Karin Haiser, Florian O. Koller, Markus Huber, Nadja Regner, Tobias E. Schrader,† Wolfgang J. Schreier,‡ and Wolfgang Zinth* BioMolekulare Optik and Center for Integrated Protein Science (Munich), Fakult€at f€ur Physik, Ludwig-Maximilians-Universit€at M€unchen, Oettingenstr. 67, 80538 M€unchen, Germany ABSTRACT: Femtosecond IR-pump-IR-probe experiments with independently tunable pulses are used to monitor the ultrafast response of selected IR absorption bands to vibrational excitation of other modes of Fmoc-nitrophenylalanine. The absorptions of both NO2-bands change rapidly within 96%). The sample was dissolved in DMSO-d6 at a concentration of about 40 mM (Sigma/Aldrich, purity 99,8%). DMSO-d6 was selected because of its high transparency in the investigated spectral range from 1300 to 1800 cm-1. Steady State Spectroscopy. Steady state IR spectra in Figure 1 were recorded with a FTIR spectrometer (model IFS 66 from Bruker, Ettlingen, Germany). In the steady state experiments as well as in the time-resolved measurements a flow cell system with CaF2 windows (d = 2 mm) and a sample thickness of about 90 μm was used. For the assignment of the vibrational bands of Fmoc-NPhe, the vibrational spectra were calculated using Gaussian98 with the functional B3-LYP-6-31þG* for gas-phase conditions.24,25 Additionally, steady-state spectra of Fmoc-Ala and Fmoc-Phe were recorded for comparison and testing of the band assignment. Time-Resolved Experiments. State of the art femtosecond IR sources supply broadband pulses (typical bandwidth 200 cm-1)26,27 that allow the monitoring of a broad spectral range with multichannel detection systems. In contrast to this, the bandwidth of the excitation pulses has to be adapted to the width of the specific mode of the sample molecule. A typical value for the width (fwhm) of vibrational absorption bands of molecules in solution is in the range of 20 cm-1. According to the Fourier limit a spectrally narrowed pump pulse allows a selective excitation but reduces the time resolution. One approach for spectral narrowing of fs mid-IR pump

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pulses includes the introduction of a Fabry-Perot filter into the beam path of the pump beam.28 Nevertheless, this is attained at the cost of pump energy. A different approach is based on difference frequency mixing of adequately chirped near IR pulses.29,30 This method allows an adjustable reduction of the bandwidth of the midinfrared pulses combined with a fast tunability of the wavelength. The reduction of the spectral bandwidth in a range of 20 to 50 cm-1 is obtained by the use of pair of silicon prisms. Details concerning the setup for spectral narrowing of the mid-IR pump pulses are given in ref 29. The femtosecond mid-IR pulses are generated according to the setup presented in refs 26 and 27. In brief, a 1 kHz Tisapphire laser-amplifier system (Spitfire Pro, Spectra Physics) with a central wavelength of ∼800 nm and pulse durations of ∼100 fs is used to generate a pair of near-IR pulses in a combination of a noncollinear optical parametric amplification (NOPA)31 and an OPA. The near-IR pulses are focused into an AgGaS2 crystal to generate tunable pulses in the mid-IR between 3 to 11 μm via difference frequency generation. For the experiments mid-IR pump pulses centered at 1346, 1513, 1545, and 1716 cm-1 with a spectral bandwidth of ∼25 cm-1 and an energy of ∼100 nJ were used. Both pump and probe pulses were focused to a diameter of ∼100 μm (fwhm) at the sample location. To avoid contributions due to rotational diffusion the measurements were performed under magic angle conditions. After passing the sample the mid-IR probe pulses were dispersed by two f = 300 mm spectrographs (Acton Research, 100 lines per mm grating) and detected by two MCT-diode arrays with 32 elements (Infrared Associates). The spectral resolution of the detection system was between 3 and 5 cm-1 depending on the wavelength of the probe pulses. The time resolution was measured via an independent experiment where the pump induced absorption changes in a thin Ge-plate (thickness = 0.5 μm) were recorded. The width of the crosscorrelation trace (fwhm) of ∼0.4 ps is determined essentially by the narrow-band pump pulses.29 Time zero of the various experimental traces was determined using the experiments on Ge to determine the cross-correlation function between pump and probe pulses. In order to improve the signal-to-noise ratio the absorption difference signals were recorded by chopping the pump light with a frequency of 500 Hz. During the experiments the excited volume in the flow cell system was exchanged by a peristaltic pump ensuring a complete sample exchange between two successive laser shots. For each delay position the signal was averaged over approximately 2000 probe pulses resulting in an overall sensitivity of about 30 μOD. For a quantitative comparison of the kinetics the experimental data was fitted with a sum of exponential functions. Thereby a global analysis (using a Levenberg-Marquart algorithm) was performed in the whole measured spectral range of each data set. In Table 1 the results for the time constants using biexponential modeling are given together with error ranges deduced from an exhaustive search algorithm.

3. RESULTS The stationary IR absorption spectrum of Fmoc-NPhe in DMSO-d6 (concentration ∼ 50 mM) is shown in Figure 1. The absorption spectrum in the MIR region between 1300 and 1800 cm-1 exhibits three distinct and strong vibrational bands. The bands at 1346 and 1513 cm-1 can be assigned to the symmetric and antisymmetric NO2 streching modes of the 2170

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Table 1. Time Constants with Error Ranges for Different Combinations of Excitation and Probe Frequencies probe (cm-1) pump (cm-1)

1346

1513

1545

(1.1 ( 0.3) ps

1346

1716

P

(4.3 ( 0.4) ps 1513

(2.4 ( 0.6) psP

11 ps (3-22 ps)

(4.8 ( 0.5) ps 1545

(3.2 ( 1.0) ps

(1.2 ( 0.6) psP (5.0 ( 0.8) ps

1716 P

(1.7 ( 0.5) psa

(1.2 ( 0.4) psa,b

(0.8 ( 0.4) psa

(7.4 ( 1.9) ps

(6.8 ( 1.4) ps

(4.5 ( 1.8) ps

b

Perturbed free induction decay signal before time zero. a Rise time of a signal build-up. b Data fitted only in the range of the induced aborption signal.

Figure 2. Time-resolved absorption changes of Fmoc-nitrophenylalanine in the region of the NO2 stretching modes after excitation at the indicated frequencies. Color coding: Positive absorption changes are shown in red and negative absorption changes in blue.

nitro group. The strong absorption band at 1716 cm-1 is due to the CdO stretching modes.32 The asymmetry of this band can be explained by the presence of two CdO groups. Another absorption feature visible in the time-resolved experiment is the broad band peaking at 1545 cm-1. Apparently, this band is due to the amide II mode. A comparison with absorption spectra of other amino acids in the same solvent supports this interpretation. To investigate the intramolecular transfer of vibrational excitation systematic IR-pump IR-probe experiments were performed. In a series of experiments, the vibrational modes at 1346, 1513, 1545, and 1716 cm-1 were excited with narrowband mid-IR pulses. The change in the IR absorption spectrum due to the excitation was observed in the region of the other modes covering different combinations of excitation and probing frequencies.

An overview of the results for probing the range 1300-1380 cm-1 of the symmetric NO2 and 1470-1590 cm-1, where the asymmetric NO2 mode and the amide II mode occur, is shown in Figure 2. For excitation of the NO2 (Figure 2a,b) and CdO (Figure 2c, d) stretch vibrational modes, the absorption changes are displayed in color coding (red, induced absorption; blue, absorption decrease) as a function of delay time and probing frequencies. All absorption difference spectra reflect rapid absorption decrease close to the positions of the original IR bands (or in its high frequency wing). These changes decay on the 10 picosecond time scale. Induced absorption is visible in the red wings of the original absorption bands evolving on the same time scale as the bleaching. These features are well-known from transient IR experiments where vibrational excitation leads to the (red-) shift 2171

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Figure 3. Transient absorption data in the region of the two NO2 stretching modes at the positions with the strongest absorption change. Excitation at 1513 (a), 1346 (b), 1716 (c and d). The excited and probed vibrational modes are indicated in the insets.

Figure 4. Transient absorption at the symmetric NO and CO stretching mode upon excitation in the amid II band at 1545 cm-1.

of absorption bands.33 In Figure 2a,b, one observes modulations of the absorption at and before time zero, which can be assigned to the perturbed free induction decay (PFID).34-38 PFID occurs when the polarization related with the probing process is rapidly (on the time scale of the inverse line width) modified by the excitation pulse. The modification may include an increase or decrease of the oscillator strength of the probed vibration or its frequency shift. This change may originate from direct anharmonic coupling between excited and probed modes. In this case, PFID gives a measure for the anharmonic coupling. In Figure 2d, one finds additional features: The bleach of the 1520 cm-1 band appears to be delayed as compared to the adjacent absorption increase. In addition one finds decreased absorption around 1545 cm-1 in the amide II range. Here the overlap of the absorption changes of the two modes has to be considered. The time dependencies of the absorption changes are shown in Figures 3 and 4 for different combinations of excitation and probing frequencies. Excitation of NO2 Bands. Before showing the effect of excitation in amide and CdO modes on single NO2 monitor

bands, we want to address the interplay between the two NO2 bands. Figures 2a and 3a refer to excitation of the high-frequency antisymmetric NO2 stretching mode at 1518 cm-1 and probing of the response in the region of the low-frequency symmetric stretching mode (1346 cm-1). At time zero, strong bleaching is observed at 1346 cm-1 (see Figure 2a). The bleach signal occurs instantaneously and decays subsequently on the 3 ps time scale. The absorption changes vanish completely after 20 ps. The neighboring induced absorption (1335 cm-1) shows the same kinetics. The kinetics at both frequencies can be fitted by a biexponential function (solid curves in Figure 3a) yielding decay time constants of τ1 = 2.4 and τ2 = 4.8 ps. Exciting the low frequency NO2 stretching mode of NPhe at 1346 cm-1 and probing the antisymmetric one obtains similar results but somewhat faster kinetics (see Figures 2b and 3b): the initial absorption change decays biexponentially with τ1 = 1.1 ps and τ2 = 4.3 ps. A direct comparison of the signal amplitudes (scaled to the same excitation energy) reveals that the excitation at higher frequencies leads to a 2-fold stronger peak amplitude of the probing signal. A closer inspection of Figure 2a,b indicates that (i) the 2172

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The Journal of Physical Chemistry A peaks of induced absorption signals shift with delay time to higher frequencies (by ∼5 cm-1 during the first 5 ps) and that (ii) excitation at 1346 cm-1 does not cause an absorption change in the range of the 1545 cm-1 band of the amide II mode. Excitation of the CdO Stretching Band at 1716 cm-1. Absorption changes induced by excitation of the CdO stretch vibrational modes are shown in Figure 2c,d. Weak contributions from PFID may exist at early times. When the symmetric NO2 band is probed after excitation of the CdO stretching band, one observes again a dispersive absorption difference spectrum. The kinetics in Figures 3c and 2c exhibit a delayed rise of the absorption signal followed by a decay on the 10 ps time scale. A biexponential fit yields time constants of 1.7 and 7.4 ps where the fast time constant is related with the initial signal rise. A second striking feature is related to the signal amplitudes and the position of the signal peaks. Upon excitation at 1716 cm-1 the signal amplitudes of the symmetric NO2 mode around 1340 cm-1 are smaller (by a factor of ca. 1/10) than found for excitation at 1513 cm-1. In addition, for the 1716 cm-1 excitation, the absorption features are shifted to higher frequencies, and only a weak shift of the absorption peak (by 0.5 cm-1 during 5 ps) is found. Probing the antisymmetric NO2 band around 1513 cm-1 (see Figures 2d and 3d) reveals some differences. The rise of the absorption increase at the low-frequency side of the absorption band is still delayed (see Figure 3d) but occurs faster (ca. 1.2 ps) than in the symmetric NO2 stretching band. The build-up of the absorption bleach recorded on the high-frequency side of the absorption band is much slower (see Figures 2d and 3d). In the range of the absorption band of the amide II modes (1545-1550 cm-1, see Figure 2d) one finds an absorption bleach that is formed with a short time constant of 0.8 ps and that decays with a time constant of 4.5 ps. A pronounced absorption increase on the low-frequency side of this band is absent. Apparently the overlap with the absorption bleach at 1521 cm-1 due to the antisymmetric NO2 band and the increase due to the amide II band lead to a partial cancellation. This overlap may also explain the different rise times of induced absorption and bleach signals in the range of the antisymmetric NO2 stretching mode. Excitation of the Amide II Band at 1545 cm-1. Figure 4a shows the changes in the symmetric NO2 stretching band after excitation at 1545 cm-1. There is a rapid change of the absorption properties (absorption decrease at the position of the NO2 band, 1346 cm-1, and increase on the low frequency side, 1335 cm-1), which peak at 1 ps after excitation. Subsequently, the absorption changes decay. The time dependence can be modeled by a biexponential function with the two time constants 1.2 (rise) and 5 ps (decay). For the range of the CdO absorption (see Figure 4b) the amplitudes of the absorption changes are much smaller. The signal rises within 1 ps and decays with a time constant of 3.2 ps. Experiments with excitation in the NO2 stretching bands and probing in the CdO range around 1710 cm-1 have also been performed. However, the signals observed under these conditions were very small and did not allow a detailed analysis (1518 cm-1 excitation) or were not detectable (1346 cm-1 excitation). The results from the different excitation and probing wavelengths are summarized in Table 1. Here the time constants deduced from a biexponential fit of the signal amplitudes are given. Time constants related with the build-up of the signal are marked with a. Situations where considerable amplitudes due to PFID appear are marked with a P.

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4. DISCUSSION The potential application of NPhe as a local thermometer, that is, as a sensor for vibrational excess energy, has been studied by IR-pump IR-probe experiments. For a detailed understanding of the experimental results we need a theoretical interpretation of the excitation induced changes in the vibrational spectrum. This can be accomplished using ref 33, where the absorption changes of polyatomic molecules induced by excitation of vibrational modes has been treated by using lowest order anharmonicities. For the special situation of NPhe, we consider the following processes, which may lead to absorption changes of a monitoring vibrational mode Ω: (i) Direct anharmonic coupling of the originally excited mode Α to the observed mode Ω. Here, the population of mode Α directly shifts the frequency of mode Ω. This situation will show-up via PFID at early times (tD < 0) and absorption changes afterward. The signal decays together with the population of the originally excited mode. (ii) Anharmonic frequency shifting of Ω by low frequency modes Λ, which are populated from the initially excited mode Α via intramolecular vibrational redistribution (IVR). (iii) Direct population of mode Ω by energy transfer from the originally excited mode Α in a vibrational redistribution process where anharmonic coupling, dipole interaction and collision induced processes are involved. Here the absorption change is determined by the diagonal anharmonic coupling.33 All mechanisms mentioned above lead to sigmoidal changes of the absorption spectrum after time zero. This is caused by a bleach at the position of the original absorption band of mode Ω and an induced absorption at the position determined by the anharmonic coupling and the population of the coupling modes. In general, the frequency of mode Ω is shifted to lower wavenumbers. The frequency shift depends on the population of the modes and smaller populations lead to smaller frequency shifts.33 In the following, we give explanations of the main observations: (i) Excitation and probing of the NO2 bands yields to an instantaneous response. This becomes visible on the one hand via the PFID and on the other hand by the absence of a subsequent signal rise (see Figure 5a,b). These observations point to a strong anharmonic coupling of the symmetric and the antisymmetric NO2 stretching modes. The observed initial decay of the signal amplitude in the 1 ps range can be assigned to vibrational redistribution between the two NO2 modes promoted by the strong anharmonic coupling. The decay on the longer time scale (4-5 ps) is characteristic for vibrational relaxation out of the NO2 modes and for the transfer of excess energy to the solvent surrounding.27,39 The rapid absorption exchange between the two NO2 modes indicates that both modes can be equally well used as a monitor. The rapid exchange will be of importance in special applications where one of the NO2 modes will be hidden in a strongly absorbing sample. (ii) When the CdO vibrational modes (among them is the amide I mode) are excited, we observe a delayed rise of the absorption changes of the NO2 modes with ∼1 ps and a decay with ∼8 ps. There are weak absorption changes before time zero (see Figure 2c,d). However, they can not be assigned unambigously to PFID pointing to a direct anharmonic coupling between the CdO and the NO2 modes. The delayed signal rise indicates that the excitation of the carbonyl groups is rapidly redistributed (e.g., via IVR) to other modes (low frequency modes Λ) with stronger anharmonic coupling to the NO2 modes. Finally, the excitation energy leaves the coupling modes on the 10 ps time scale, apparently via energy transfer to the surrounding 2173

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of the amide group. The short response time of the NO2 stretching modes of below 2 ps is faster than the intermolecular energy transfer to the solvent, which occurs on the time scale of 10 ps. Thus, the NO2 modes are able to monitor the intramolecular transfer of vibrational excitation, that is, NPhe can be used as a local and ultrafast thermometer in peptides and proteins. In a first demonstration, NPhe was applied as a local sensor to study the heat transfer in a polyproline system.40

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Current address: J€ulich Centre for Neutron Science, Lichtenbergstr. 1, 85747 Garching, Germany. ‡ Current address: Department of Chemistry and Biochemistry, 103 Chemistry and Biochemistry Building, P.O. Box 173400, Bozeman, MT 59717, U.S.A.

’ ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft through the DFG-Cluster of Excellence MunichCentre for Advanced Photonics and SFB 749 (A5). Figure 5. Transient IR absorption of Fmoc-nitrophenylalanine around delay time zero in the region of the two NO2 stretching modes (a) and (b) and the amide II mode (c). The step size of the color coding between 1 and -1 mOD is different from the absorption changes 1 mOD to visualize the PFID signal.

solvent (cooling). The weak signals and the smaller frequency shift of the positive absorption change found upon excitation of the CdO bands point to a weak anharmonicity (originally excited CdO modes) and low population densities of the involved vibrational modes. (iii) Excitation of the amide II band around 1545 cm-1 and probing the symmetric NO2 mode around 1340 cm-1 displays a PFID signal with medium amplitude (Figure 5c). This can be due to anharmonic coupling between the 1545 cm-1 mode and the symmetric NO2 stretching mode. However, we cannot rule out a certain spectral overlap between the excitation pulse centered at 1545 cm-1 and the antisymmetric NO2 mode at 1513 cm-1. The observation of a delayed signal rise with a time constant of 1.6 ps points to a rapid redistribution of the vibrational excitation out of the amide II mode to modes leading to stronger anharmonic frequency shifts.

5. CONCLUSION The experiments with different combinations of excitation and observation wavelengths have shown that both absorption bands of the NO2 stretching modes respond rapidly and with considerable amplitudes to vibrational excitation in the region of the backbone (amide groups). The experiments reveal that considerable anharmonic coupling exists in NPhe between the NO2 stretching modes with CdO stretching or amide II modes localized at different parts of the molecule. This property combined with the strong absorption of the NO2 stretching modes make NPhe a very sensitive marker for vibrational energy arriving in the region

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