Methyl substituent effects for methyltryptophans in 13C nmr

The ¹³C and ¹H nmr spectra of methyltryptophans 2–5 in 0.1 N sodium deuteroxide methanol-d₄ were assigned based on 1-D and 2-D nmr techniques, including COSY, inverse-detected direct (HMQC) and long-range (HMBC) correlation. Methyl substituent effects in chemical shifts (SCS) for the indole ring of tryptophan were calculated and compared with those of indole. The correlations were linear except for 4-methyltryptophan, which suggest structural changes in the indole ring of 4-methyltryptophan and 4-methylindole. The results of molecular modeling and NOE experiments supported that suggestion.