Supporting Information: Molecular Interactions between Graphene and Biological Molecules Xinguan Zou, *,1 Shuai Wei, *,1 Joshua Jasensky, 1 Minyu Xiao, 1 Qiuming Wang, 1 Charles L. Brooks III **,1,2 and Zhan Chen, **,1,2 1 Department of Chemistry, and 2 Department of Biophysics, University of Michigan, Ann Arbor, Michigan 4810, United States Method SFG signal intensity can be expressed as: 2 SFG VIS IR I I I (S1) where I and I are intensities of visible and IR beams, and can be written as 1 : VIS IR A NR IR i (S2) where is the nonresonant contribution, is the freuency of the IR beam, A, NR IR and are the strength, resonant freuency, and damping constant of the th vibrational mode, respectively. 2 he SFG spectra are normalized by the intensities of visible ( I ) and IR ( I ) beams to eliminate the effect of laser intensity fluctuations. VIS IR SFG spectra of ppp and ssp polarization combinations were fitted using 2 to obtain uantitative signal strengths. After considering the Fresnel coefficient, the / ratio can be calculated, and was used to deduce peptide orientation by zzz yyz relating the laboratory coordinate based susceptibility ijk ( i, j, k x, y, z) to the molecular coordinate based susceptibility ( l, m lmn, n a, b, c ) through the Euler angles. For the th vibrational mode, (S3) N ( iˆ lˆ )( ˆj mˆ )(kˆ n) ˆ ijk, lmn, l, m, n S1
Where N is the density or surface coverage of peptides and brackets denote averaging over the peptide orientation. he hyperpolarizability lmn, is an intrinsic property of a vibrational mode of a specific chemical group, and the value can be obtained from infrared and Raman characteristics of a certain vibrational mode by: lmn, lm n (S4) Q Q Where the superscript denotes the complex conjugate, lm Q and n Q are the Raman polarizability and infrared dipole derivatives with respect to the normal coordinate of the th vibrational mode, respectively. he relationship between laboratory coordinate system (x, y, z) and molecular coordinate system (a, b, c) can be specified by the Euler angles (,, ). For peptides adopting an α-helix secondary structure, the c-axis is parallel to the principal axis of the α-helix, and θ is the angle between the z-axis in the laboratory coordinate and the c-axis in molecular coordinate. is the azimuthal angle and we assume that the adsorbed peptides at graphene surface have azimuthal symmetry. here are three amide I vibrational modes (A, E 1, E 2 ) for an α-helical structure. he A mode is a symmetric stretching mode parallel to the principal axis of the α-helix, and the E 1 and E 2 modes are perpendicular to this principal axis. Since the E 2 mode is infrared inactive, SFG can only detect the A and E 1 modes that are both infrared and Raman active. For the A mode: 3,4 1 3 xxz, A yyz, A N [(1 r ) cos (1 r) cos ] ccc (S5) 2 3 zzz, A N[r cos (1 r) cos ] ccc (S6) Where r /. aac For the E 1 mode: 4 ccc,e,e N[ cos cos ] (S7) 3 xxz 1 yyz 1 aca S2
,E 2 N[ cos cos ] aca (S8) 3 zzz 1 he E 1 mode is a two-fold degenerate vibration in a plane perpendicular to the helix principal axis: one corresponds to the phase difference 100ºand the other corresponds to the phase difference -100º between adjacent peptide units. 3 So the SFG hyperpolarizability tensor aca of the E 1 mode can be expressed as: aca ac a ac a ( 100) ( 100)+ (- 100) (- 100) (S) Q Q Q Q E1 E1 E1 E1 In order to calculate the hyperpolarizability tensor values of aac, aca and ccc, we need to obtain the values of Raman tensor and IR dipole moment for both A and E 1 modes. he Raman tensor of the standing up α-helix segment of cecropin P1 (10 residues) can be calculated as: 4,5 A mode (all the residues vibrate in phase): cos( 100n/180) sin( 100n/180) 0 0.624 0 0.4 cos( 100n/180) sin( 100n/180) 0 (0 ) sin( 100n/180) cos( 100n/180) 0 0 0.05 0 sin( 100n/180) cos( 100n/180) 0 Q A n0 0 0 1 0.4 0 0.577 0 0 1 (S10) E 1 mode: (100 ) QE 1 n0 cos( 100n/180) sin( 100n/180) 0 0.624 0 0.4 cos( 100n/180) sin( 100n/180) 0 i sin( 100n/180) cos( 100n/180) 0 0 0.05 0 sin( 100 n/180) cos( 100n/ 180) 0 e 0 0 1 0.4 0 0.577 0 0 1 (S11) 100 n /180 ( 100 ) QE 1 n0 cos( 100n/180) sin( 100n/180) 0 0.624 0 0.4 cos( 100n/180) sin( 100n/180) 0 sin( 100n/180) cos( 100n/180) 0 0 0.05 0 sin( 100n/180) cos( 100n/180) 0 e 0 0 1 0.4 0 0.577 0 0 1 i100 n /180 (S12) he IR dipole moment of the standing up α-helical segment (10 residues) can be calculated as: 4,5 A mode (all the residues vibrate in phase): S3
cos( 100n/180) sin( 100n/180) 0 sin(42 /180) a ( 0)= sin( 100n/180) cos( 100n/180) 0 0 Q A n0 0 0 1 cos(42 /180) (S13) E 1 mode: cos( 100n/ 180) sin( 100n/ 180) 0 sin(42 / 180) e 0 0 1 cos(42 / 180) a(10 0)= sin( 100n/ 180) cos( 100n/ 180) 0 0 Q E n0 1 i100 n /180 (S14) cos( 100n/ 180) sin( 100n/ 180) 0 sin(42 / 180) e 0 0 1 cos(42 / 180) a(-10 0)= sin( 100n/ 180) cos( 100n/ 180) 0 0 Q E n0 1 (S15) i100 n /180 After obtaining the values of the hyperpolarizability tensor components and the zzz yyz ratio, the relationship between the zzz yyz ratio and the peptide tilt angle θ can be plotted by using the following formula. zzz zzz, A zzz,e1 yyz yyz, A yyz,e1 (S16) Figure S3 shows such a relationship. he SFG measured zzz yyz ratio is 1.58, corresponding to a small tilt angle of <10, which means that the cecropin P1 peptide segment stands almost perpendicularly to the graphene surface. More information about the graphene used in this study Graphene solution was purchased from Graphene Laboratories Inc. (Calverton, NY). Pristine monolayer graphene flakes (Carbon content:.%; Ultrapure: no oxidation, no surfactants) were dispersed in ethanol solution with a concentration of 1.0 mg/l. he graphene sheets were synthesized in the gas phase using a substrate-free, S4
atmospheric-pressure microwave plasma reactor, and characterized in by EM, EELS, Raman, SEM and electron diffraction. 7 he results showed that products were pure carbon, without detectable hydrogen, oxygen, and OH in EELS spectra. More details can be found in ref. 7. Figures and ables Figure S1. Graphene AFM image (3µm 3µm). Section analysis shows that the graphene thickness is around 0.7 nm. According to previously reported literature 6 this graphene is a monolayer. It is noted that the graphene layer is not continuous. We found that no SFG signals could be collected from the CaF 2 prism/peptide solution for both cecropin P1 and MSI-78(C1) (not shown). herefore the detected SFG signal from cecropin P1 is contributed by the cecropin P1 at the graphene interface. S5
Figure S2 AFM image of graphene sheets on a Mica substrate. In order to clearly show the graphene pattern and exclude the possibility of seeing residual water wetting spots instead of graphene, a diluted graphene solution (0.1mg/L) was used. he steps between graphene and Mica substrate could be clearly seen. (a) (b) Figure S3. Initial pose of peptide cecropin P1 (a) and MSI-78(C1) (b) above the graphene surface. he purple color indicates the α-helical structure in each peptide seuence. As shown, we initially put each peptide above the graphene surface with a distance of about 10 Å and parallel with the graphene surface. S6
Figure S4. / ratio as a function of helix tilt angle assuming a δ-distribution of zzz yyz orientation. Figure S5. (a) Orientation curve of / ratio as a function of helix tilt angle. (b) zzz yyz Orientation angle distributions of the mutant MSI78-78(C1) helical segment on graphene, the average tilt angle is 32.8. S7
able S1. Molecular dynamics simulation parameters applied in the surface model (as shown in Euation 1), which describe the residue-graphene surface interactions. hese parameters were generated in previous work against a large benchmark experimental data set, 8 which has been applied in many cases for simulation. -11 θ 1 θ 2 θ 3 θ s θ p 0.2340 0.436 0.1333 0.0067 0.0333 able S2. SFG spectral fitting parameters. Peak intensity Peak center (cm -1 ) Peak width (cm -1 ) (a.u.) ppp 8.1±5.5 1651.0±1.2 21.4±1.3 ssp 68.5±1.3 1655.0±0.4 21.2±0.5 S8
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