## 4 Conclusions  
In conclusion, assembly of trinucleotides on solid support with the starting nucleoside being linked to the polymer via a disulfide bridge is a straightforward strategy for the preparation of \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Phosphate**} & \multirow{2}{*}{**Trinucleotide**} & \multirow{2}{*}{**Mass Calculated**} & \multirow{2}{*}{**Mass Found**} & \multirow{2}{*}{**Cleavage Reagent**} \\
**Protecting Group** & & & & \\ \hline Me & ATT & 1260.14 & 1284.84 (+Na) & TCEP \\ Me & AAT & 1339.24 & 1385.82 (+2 Na) & TCEP \\ Me & TTT & 1180.34 & 1180.30 & TCEP \\ Me & AAA & 1402.50 & 1448.22 (+2 Na) & TCEP \\ Me & GGA & 1449.49 & 1472.49 (+Na) & DTT \\ Me & GGC & 1459.47 & 1482.34 (+Na) & DTT \\ Me & GGT & 1370.44 & 1393.38 (+Na) & DTT \\ \(\beta\)-cyanoethyl & TTT & 1258.37 & 1281.29 (+Na) & DTT \\ \(\beta\)-cyanoethyl & TTA & 1337.42 & 1360.34 (+Na) & DTT \\ \(\beta\)-cyanoethyl & TTC & 1347.39 & 1370.38 (+Na) & DTT \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Overview of synthesized trinucleotides. fully protected trimeric oligodeoxyribonucleotides. Upon cleavage of the disulfide under reducing conditions, the trimer with all protecting groups at the nucleobases, the phosphates, and the terminal 5'-OH-group remaining intact is released into solution. Following evaporation of the solvent, trimers are precipitated from water, dried, and stored until use in standard DNA synthesis, starting with phosphitylation as described [6]. The solid phase strategy nicely adds to our previously developed approach for the preparation of trinucleotide synthons in solution and to the more recently published methods for trinucleotide assembly on a precipitative soluble support [8; 23]. So far, syntheses were carried out at 1 umol scale to demonstrate the suitability of the method. In future work, fully protected trimers will be prepared at a larger scale to produce sufficient amounts for further evaluation of the trimers in gene library synthesis. The following are available online at [[http://www.mdpi.com/2076-3417/9/11/2199/s1](http://www.mdpi.com/2076-3417/9/11/2199/s1)]([http://www.mdpi.com/2076-3417/9/11/2199/s1](http://www.mdpi.com/2076-3417/9/11/2199/s1)), Figure S1. Standard curve of DMT-CI for calculation of loading density, Figure S2. Synthesis of 5'-O-dimethoxyyttrially-3'-O-dithionomethyl-N-benzoyl-2'-deoxycytidine with primary, secondary, or tertiary disulfide bridge (S-prim/sec/tert), Figure S3. Synthesized trinucleotides with primary, secondary, and tertiary disulfide bridge ATC-S\({}_{\text{prim}}\), CAC-S\({}_{\text{sec}}\), TTC-S\({}_{\text{fert}}\) (* calculated mass), Figure S4. Reductive cleavage of the 3'-disulfide bridge of trinucleotides ATC-S\({}_{\text{prim}}\), CAC-S\({}_{\text{sec}}\), and TTC-S\({}_{\text{fert}}\). Shown are the mass spectra of trinucleotides after 30 min and 9 h of treatment with DTT, Figure S5. MS-Spectra of trinucleotides after cleavage from the support, Figure S6. RP-HPLC of TTT-3'-OH after cleavage from solid support, buffer A: 5% acetonitrile, buffer B: 70% acetonitrile, VP 125/21 Nucleosil 120-10 C18, Figure S7. RP-HPLC CTA-3'-OH after cleavage from solid support, buffer A: 5% acetonitrile, buffer B: 70% acetonitrile, VP 125/21 Nucleosil 120-10 C18, Figure S8. RP-HPLC TTC-3'-OH after cleavage from solid support, buffer A: 5% acetonitrile, buffer B: 70% acetonitrile, VP 125/21 Nucleosil 120-10 C18, Figure S9. MS-Spectra of trinucleotides with methyl/protected phosphate after cleavage with DTT from the support. The difference of 14 indicates loss of a methyl group at the phosphate, Figure S10. MS-Spectra of trinucleotides with methyl-protected phosphate after cleavage with TCEP from the support, Table S1. Cleavage conditions, Table S2. 3'-OMT-S-prim/sec/tert yields and masses. Conceptualization, S.M. and B.A. ; methodology, M.J., R.S., B.A., and S.M. ; validation, M.J., R.S., and S.M. ; investigation, M.J. and R.S. ; resources, S.M. ; data curation, R.S. and S.M. ; writing--original draft preparation, R.S. and B.A.