551 TERPENE CONJUGATION hydroethanolic solution. The diffusion cells were placed into a 32°C water bath and the receptor phases were stirred during the study. After 48 h, the donor and receptor phases were pulled out and analyzed. The concentrations of peptide in the donor phase as well as in the receptor phase were measured by LC-MS. We performed the experiments were performed in triplicate. RESULTS AND DISCUSSION SYNTHESIS AND CHARACTERIZATION OF PEPTIDES Cit-KTTKS, Per-KTTKS, Pal-KTTKS (Figure 2), and KTTKS (Figure 1) were synthesized by the Fmoc strategy. Using mass spectrometry, synthesis of the peptides was confirmed so that singly charged ion peak of m/z 564.2, 802.1, 715.9, and 712.1 associated with doubly charged ion peak of m/z 282.7, 401.7, 358.9, and 356.6 confirmed synthesis of KTTKS, Pal- KTTKS, Cit-KTTKS, and Per-KTTKS respectively. STABILITY STUDIES Initial concentration of peptide solutions was 100 µg/mL. This value reached 98.00 ± 3.13 µg/mL for Cit-KTTKS and 98.35 ± 1.43 µg/mL for Per-KTTKS. These results are in agreement with the stability data for KTTKS and Pal-KTTKS obtained in previous work (99.3 ± 1.7 µg/mL for KTTKS, 97.5 ± 3.6 µg/mL for Pal-KTTKS) (14). Therefore, all peptides could be considered almost stable at least in the experimental condition at 32°C for 48 h. PERMEATION STUDIES Membrane model permeation study. As described earlier, we performed the peptide permeation studies using a membrane model of PVDF-filled n-hexadecane as a lipophilic barrier. Results (Table I) showed that KTTKS and Per-KTTKS were not detected in the receptor phases, but Pal-KTTKS and Cit-KTTKS permeated the membrane in detectable amounts and showed permeation of about 4% and 1% of applied doses, respectively. Amounts of permeated peptides per area over 24 h were then normalized to concentration to obtain estimated permeability coefficient (kp). Results showed estimated kp values of 17 × 10−4 cm/h and 2.9 × 10−4 cm/h for Pal-KTTKS and Cit-KTTKS respectively. The n-hexadecane, a 16-carbon alkane, was chosen to simulate the central part of the lipid bilayer matrix of human stratum corneum (SC), an important barrier to transdermal and topical delivery of hydrophilic compounds. This barrier was used to screen peptides according to their partition coefficient (as described by Moghimi et al.) for other hydrocarbons such as isooctane (13). Results showed that Pal-KTTKS, which has the most tendency to a lipophilic environment (clog P: 3.72), had the highest estimated kp and Cit-KTTKS (clog P: -0.08) was in second place. KTTKS and Per-KTTKS that possess cLogP of less than around −1 did not permeate this lipophilic barrier. These data agree with the general consensus of skin scientists that consider LogPs of around 0–4 good for skin permeation. These data also might show that PVDF-filled n-hexadecane might be a good model for the SC permeation studies.

Figure 2. The chemical structures of Cit-KTTKS (citronellic acid-KTTKS), Per-KTTKS (perillic acid-KTTKS), and Pal-KTTKS (palmitic acid-KTTKS). cLogPs of conjugates were calculated by ACD-ChemSketch freeware software (). 552 JOURNAL OF COSMETIC SCIENCE