A direct method for determining peptide or protein concentration in a sample, the method comprising (a) combining the sample with tartrate, copper sulfate, and a quantitation assay reagent composition to form a mixture, wherein the quantitation assay reagent composition comprises a complex containing the following general formula where each of R 1 and R 2 is independently alkyl including but not limited to a C 1 -C 6 straight or branched alkyl or a C 6 -C 20 aryl, alkylaryl, or arylalkyl such as methyl (-CH 3), ethyl (-CH 2 CH 3), propyl (-CH 2 CH 2 CH 3), butyl (-CH 2 CH 2 CH 2 CH 3) or phenyl (-C 6 H 5); each of R 3 and R 4 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and each of R 5 and R 6 is where R 7 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); with the proviso that at least one of R 3, R 4, and R 7 is not H; (b) incubating the mixture under conditions sufficient to form a colored complex; and (c) measuring absorbance of the colored complex at 450 nm to 500 nm as a direct indicator of peptide or protein concentration in the sample. (a) combining the sample with tartrate, copper sulfate, and a quantitation assay reagent composition to form a mixture, wherein the quantitation assay reagent composition comprises a complex containing the following general formula where each of R 1 and R 2 is independently alkyl including but not limited to a C 1 -C 6 straight or branched alkyl or a C 6 -C 20 aryl, alkylaryl, or arylalkyl such as methyl (-CH 3), ethyl (-CH 2 CH 3), propyl (-CH 2 CH 2 CH 3), butyl (-CH 2 CH 2 CH 2 CH 3) or phenyl (-C 6 H 5); each of R 3 and R 4 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and each of R 5 and R 6 is where R 7 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); with the proviso that at least one of R 3, R 4, and R 7 is not H; each of R 1 and R 2 is independently alkyl including but not limited to a C 1 -C 6 straight or branched alkyl or a C 6 -C 20 aryl, alkylaryl, or arylalkyl such as methyl (-CH 3), ethyl (-CH 2 CH 3), propyl (-CH 2 CH 2 CH 3), butyl (-CH 2 CH 2 CH 2 CH 3) or phenyl (-C 6 H 5); each of R 3 and R 4 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and each of R 5 and R 6 is where R 7 is independently selected from the group consisting of hydrogen, sulfonate (-SO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); phosphonate (-PO 3 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); and carboxylate (-CO 2 -) salt of sodium (Na +), potassium (K +) or lithium (Li +); with the proviso that at least one of R 3, R 4, and R 7 is not H; (b) incubating the mixture under conditions sufficient to form a colored complex; and (c) measuring absorbance of the colored complex at 450 nm to 500 nm as a direct indicator of peptide or protein concentration in the sample.