## 3 Results and Discussion
As the molecular weight of AngII is higher than those of the other compounds used in Figures 1-3, the number of AngII molecules is smaller as the concentration is the same (1 mg/mL). To explain the results in Figure 4, it was assumed that the distance between AngII and CdTe would be large, hindering efficient ionization. To confirm this assumption, AngII solution and CdTe dispersion were first mixed and then sonicated for 1 h to promote the mixing of those two compounds. The thus-obtained solution was subjected to MALDI MS and the spectrum is shown in Figure 4b. Contrary  
Figure 3: Mass spectra of (**a**) tyrosine (Tyr) and (**b**) stearic acid (Ste) with CuO nanoparticles as matrix (4 mg/mL). to the above expectation, the peak of [AngII\(-\)H]\({}^{-}\) could not be recognized clearly. Of course, it was possible that AngII was decomposed by the sonication. In fact, the intensities of the peaks around \(m/z\) = 220 and 863 became twofold higher after the sonication (not shown), although those of the other peaks below \(m/z\) = 1000 were almost unchanged. As another possibility, it was considered that AngII bound to CdTe strongly and the desorption of AngII to the gas phase was difficult. In a previous study of the interaction between CdS and hemoglobin, it was reported that electrostatic binding between those two molecules was induced [18]. Tellurium belongs to the same group as sulfur (S) on the periodic table. Although AngII does not have S in its structure, it has many oxygen (O) atoms and O belongs to the same group as S as well. Therefore, it might be possible to expect that electrostatic binding between AngII and CdTe was induced. The spectral feature observed in Figure 4 might also be explained by the HSAB (hard and soft acids and bases) theory. As CdTe is a semiconductor and charge density is expected to be approximately 0, CdTe is regarded as a soft acid. On the other hand, as AngII can easily polarize, AngII is regarded as a soft base. The soft acid CdTe and the soft base AngII attract each other and strong electrostatic binding between them is induced, making the desorption of AngII to the gas phase difficult. The above argument could be confirmed by measuring the diffuse reflectance spectra. Figure 5 shows the diffuse reflectance spectra of the mixed crystals of AngII and CdTe made in the same manner as that for MALDI MS measurements. The spectra obtained before and after stirring for 1 h are shown in the spectral region from 220 to 255 nm, and this region corresponds to the absorption by CdTe nanoparticles. After stirring for 1 h, the band around 225 to 230 nm decreased and a shoulder band newly appeared around 245 nm. In addition, the wavelength of the main peak, around 240 nm, was red-shifted by the stirring. These spectral changes clearly showed that CdTe interacted strongly with AngII and the energy of the electronic state of CdTe was changed. Therefore, it was verified that the strong electrostatic binding between CdTe and AngII prevented efficient desorption, and the peak intensity of AngII-related species became low. Figure 6a shows the mass spectrum of AngII measured with CuO (4 mg/mL). The peak intensity was very low and the deprotonated peak of AngII was not observed. In addition, another peptide, substance P (SubP), was subjected to MALDI MS using CdTe as the matrix (0.5 mg/mL) (Figure 6b). The peak of [SubP\(-\)H]\({}^{-}\) was observed at \(m/z\) = 1346; however, its intensity was also very low. When the concentration of CdTe was increased to 4 mg/mL, the peak of [SubP\(-\)H]\({}^{-}\) disappeared (not shown). By increasing the amount of CdTe, strong electrostatic binding occurred between CdTe and all SubP molecules so that the desorption of SubP was not possible anymore. The molecular weight of inorganic MALDI matrix, such as metal or semiconductor particles, is often higher than that of organic MALDI matrix.